* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chemistry can be defined as the study of the composition, structure
Organic chemistry wikipedia , lookup
X-ray fluorescence wikipedia , lookup
Abundance of the chemical elements wikipedia , lookup
Gas chromatography–mass spectrometry wikipedia , lookup
Livermorium wikipedia , lookup
Oxidation state wikipedia , lookup
Low-energy electron diffraction wikipedia , lookup
Isotopic labeling wikipedia , lookup
Hydrogen bond wikipedia , lookup
Artificial photosynthesis wikipedia , lookup
X-ray photoelectron spectroscopy wikipedia , lookup
Coordination complex wikipedia , lookup
Biochemistry wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Electrochemistry wikipedia , lookup
Bent's rule wikipedia , lookup
Chemical element wikipedia , lookup
Metastable inner-shell molecular state wikipedia , lookup
Electrolysis of water wikipedia , lookup
Inorganic chemistry wikipedia , lookup
Periodic table wikipedia , lookup
Hydrogen atom wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Atomic orbital wikipedia , lookup
Homoaromaticity wikipedia , lookup
Photosynthetic reaction centre wikipedia , lookup
Molecular orbital diagram wikipedia , lookup
Resonance (chemistry) wikipedia , lookup
Bond valence method wikipedia , lookup
History of chemistry wikipedia , lookup
Electronegativity wikipedia , lookup
Extended periodic table wikipedia , lookup
Atomic nucleus wikipedia , lookup
Chemistry: A Volatile History wikipedia , lookup
Metalloprotein wikipedia , lookup
Hypervalent molecule wikipedia , lookup
Metallic bonding wikipedia , lookup
IUPAC nomenclature of inorganic chemistry 2005 wikipedia , lookup
Chemical bond wikipedia , lookup
Electron configuration wikipedia , lookup
Scientific Principles (Chemistry, Biochemistry and Genetics). Textbooks • “Science in Nursing and Health Care”, Mark Foss. • “Science in nursing”, Laurie Cree. • “Chemistry for the Health Sciences” Sackman, Lehman. • “General Chemistry”, Ebbing, Gammon. • “Fundamentals of general, organic, and biological chemistry” John McMurry. Introduction to Chemistry (6 hours) – Dr. Scully States of Matter. Atoms, ions, elements, molecules, compounds, mixtures, chemical reactions. The atom and its structure – definitions, composition, electronically neutral atom, electronic shell, AMU, molecules. Chemical symbols and formulae. Basic introduction to the periodic table (chemical elements of the body, importance of these elements and their function). Basic principles of ionic and covalent bonding; cations, anions and covalent compounds. Hydrogen ions, acids and bases. The pH scale, buffers and indicators. Solutions, solution concentration, and its significance in nursing. Organic chemistry (3hours) Dr. McGlacken 1 Relevance of Chemistry to Nursing Your body is made of chemicals – keeps us alive. We breathe in oxygen and nitrogen to be used in different processes. Kreb's cycle which is a series of enzyme-catalysed chemical reactions of central importance in all living cells that use oxygen as part of cellular respiration. NSAIDs (nonsteroidal anti inflammatory drugs) for pain relief. Antibiotics for infections e.g. penicillin Cisplatin – cancer treatment. Blood work is analyzed in a lab using various chemical tests. We hyperventilate because our body's pH is off and breathing in CO2 (like breathing in a paper bag) helps stabilise the pH in our blood. Red blood cells contain haemoglobin which binds to oxygen and transports to around the body. Anaemia is defined as a qualitative or quantitative deficiency of hemoglobin. Oral rehydration therapy - effective treatment for dehydration – consists of a solution of electrolytes, especially sodium and potassium administered orally. Learning chemistry allows you to understand chemistry Language for its application in medicine Chemistry can be defined as the study of the composition, structure and properties of matter and the reactions that matter undergoes. Physical Chemistry is concerned with structure of matter, energy changes. Analytical Chemistry is concerned with the identification, separation and quantitative determination of the composition of different substances. Organic Chemistry deals with the synthesis and reactions of the compounds of carbon. Inorganic Chemistry is concerned with the chemistry of elements other than carbon and their compounds. 2 What is Matter? Matter is the general term for the material things around us. It can defined as whatever occupies space and can be perceived by our senses. There are two ways of classifying matter – by its physical status (solid, liquid or gas) or by its chemical status (element, compound or mixture). Physical Classification of Matter: A solid is a form of matter which is rigid and has a definite shape and volume. A liquid is a form of matter which flows and has no fixed shape but has a definite volume. A gas is a form of matter which flows and has no fixed shape or volume. Example: Water can exist as a solid, liquid or a gas. It exists as a solid in the form of ice; as a liquid as liquid water; as a gas as steam (gaseous water). Chemical Classification of Matter: A substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. While a mixture is a kind of matter that that can be separated by physical means. 3 All matter is composed of indivisible (cannot be decomposed further) atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions. Basic diagram of an atom. An element is a type of matter composed of only one kind of atom. Today 111 elements are known and these are listed in the periodic table. Examples of elements would be sodium, oxygen, chloride, iodine, gold, silver… Basic diagram of an element, which is composed of only one kind of atom. These atoms are not chemically bonded together. A compound is a type of matter composed of two or more elements chemically combined in fixed proportions. Examples of compounds are sodium chloride (the element sodium and the element chloride are chemically bound together. This is commonly known as salt). Basic diagram of a compound composed of two ore more elements chemically bound together. Example: Both sodium and chlorine are elements. Therefore, sodium is only composed of sodium atoms, and chlorine is only composed of chlorine atoms. Both of these elements can chemically combine together in fixed proportions to form a compound. This compound is called sodium chloride (salt). This compound is a substance which cannot be separated by physical means. A mixture is a material that can be separated by physical means into two or more substances. Example: salt in water which can separated by distillation – boiling off the water. Mixtures can be classified into two types: 4 A homogenous mixture (solution) is a mixture that is uniform in its properties throughout the samples. Cannot be easily separated. Example: Air is a gaseous mixture of nitrogen and oxygen, which are physically combined but not chemically. A heterogeneous mixture is mixture that consists of physically distinct parts, each with different properties. Can be easily separated. Example: a mixture of salt and sugar stirred together. If you looked closely you would see the separate crystals of sugar and salt. Chemical Symbols and formulae: It is convenient to use symbols for the atoms of the different elements. An atomic symbol is a one or two letter notation used to represent an atom corresponding to a particular element. Typically the atomic symbol consists of the first letter in capitals from the name of the element, sometimes with an additional letter from the name in lower case. Examples: Chlorine has the atomic symbol of Cl. Oxygen has the atomic symbol of O. Hydrogen has the atomic symbol of H. Sodium has the atomic symbol of Na (from its latin name). Z Name Symbol protons neutrons electrons 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorous H He Li Be B C N O F Ne Na Mg Al Si P 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 2 4 5 6 6 7 8 10 10 12 12 14 14 16 1 2 3 4 5 6 7 8 9 10 11 12 1 14 15 16 17 18 19 Sulphur Chlorine Argon Potassium S Cl Ar K 16 17 18 19 16 18 22 20 16 17 18 19 5 Molecules: Some atoms do not normally naturally occur singly but as molecules. A molecule may consist of two or more identical atoms such as oxygen (O2 – contains two atoms of atoms chemically bonded together). Gases often like to exist as two atoms chemically bonded together. i.e. N2 (nitrogen gas), H2 (hydrogen gas). A molecular formula gives the exact number of atoms of an element in a molecule. Example: H2O is the molecular formula of water (contains two atoms of hydrogen and one atom of oxygen chemically bonded together)., CO2 is the molecular formula for carbon dioxide (contains one atom of carbon and two atoms of oxygen), NaCl is the molecular formula for sodium chloride, in which it contains one atom of sodium and one atom of chlorine. The molecules in a substance are so small that if a glass of water was enlarged to the size of the earth, the water molecules would be about the size of a golf ball. Chemical reactions: Molecules or elements can undergo a chemical reaction to produce a new type of compound: 2H2 + O2 = 2H2O Two molecules of hydrogen gas (2 X H2) and one molecule of oxygen gas (O2) gives two molecules of water (H2O). Na + Cl2 = NaCl One molecule of the element sodium (Na) and one molecule of chlorine gas (Cl2) gives one molecule of sodium chloride (NaCl – salt). 6 The Structure of the Atom: Remember that an atom is an extremely small particle of matter that retains its identity during chemical reactions. An atom consists of protons, electrons and neutrons. A proton is a particle that has a mass of approx. 1 amu (atomic mass unit) and is positively charged. An electron is a tiny particle which has a mass of approx. 0 amu and is negatively charged. A neutron is a particle that has a mass of 1 amu and is neutrally charged. The protons and neutrons exist in the atom’s central core which is called a nucleus. This is positively charged due to the protons. The electrons exist around the atom’s central core of the nucleus in a region called the electron cloud. Since electrons are negatively charged they balance the positively charged protons in the nucleus, so that the overall charge of the atom is neutral. Basic diagram of the atom. Particle Mass (amu) Charge Proton ~1.0 +1.0 Neutron ~1.0 Electron ~0 0 -1.0 Location in Atom Nucleus Nucleus Electron cloud 7 The electrons exist in areas within the electron cloud in regions known as electronic shells. These electronic shells can only contain a certain number of electrons. Electronic Shell No. of Electrons 1 2 2 8 3 8 Therefore the first electronic shell can only contain two electrons. The second electronic shell can only contain 8 electrons…. The Atomic Number (Z) of an element is the total number of protons or electrons in the nucleus of an atom. Due to the fact that the overall charge of the atom is neutral then the number of protons is equal to the number of electrons. The Mass Number (M) of an element is the total number of protons plus neutrons in the nucleus of an atom. (Note: the mass number is bigger than the atomic number). The difference between the mass number and the atomic number is equal to the number of neutrons in the nucleus. Therefore each element has its own atomic number and mass number which is represented as the mass number in subscript and the atomic number in superscript. The atomic symbol, atomic number and mass number of the elements are listed in the periodic table. Mass Number = no. of neutrons + protons/electrons. Atomic number = no. of electrons/protons. Mass number – Atomic number = no. of neutrons. Example: Oxygen is represented as 16O. The atomic number is 8. The mass number is 16. From the atomic number, oxygen has 8 electrons. It also tells us that oxygen has 8 protons. From the mass number, it has 16 neutrons and protons. Since we know it has 8 protons (from the atomic number), we know that oxygen has 8 neutrons. Overall, oxygen has 8 protons (8 positive charges), 8 electrons (8 negative charges) and 8 neutrons (neutral)… so overall charge of oxygen atom is neutral. Carbon is written as 12C. Therefore, carbon has 6 protons, 6 electrons and 6 neutrons. 8 How to Draw an Atom: Hydrogen is written as 1H. Therefore, hydrogen has 1 proton, 1 electron and 0 neutrons. It has 2 electrons in its valence (outer shell). Carbon is written as 6C. Therefore, carbon has 6 protons (p+), 6 electrons (e-) and 6 neutrons (n). There are two electrons in the first electronic shell, and the remaining four electrons go into the second electronic shell. This is the outer most shell which is known as the valence shell. Oxygen is written as 8O. Therefore, oxygen has 8 protons, 8 electrons and 8 neutrons. It has 6 electrons in its valence (outer shell). 9 Fluorine is written as 9F. Therefore, fluorine has 9 protons, 9 electrons and 10 neutrons. It has 7 electrons in its valence (outer shell). 10 Isotopes Some elements which have the same atomic number may differ in the number of neutrons in the nucleus (i.e. their mass number). These atoms which have identical atomic numbers but different mass numbers are known as isotopes of the same element. In other words the nuclei have the same number of protons but different numbers of neutrons. Isotopes of the same element have the same chemical properties. The atomic weight of carbon (C) is actually 12.011. This is because there are traces of heavier carbon atoms found naturally 13C, 14C. These are termed isotopes of carbon as they have the same atomic number (6) but differ in their mass number (differ in the number of neutrons in the nucleas). Isotope 12 6C 13 6C 14 6C Abundance 98.89% 1.1% trace Periodic table: In 1869 a Russian chemist Dmitri Mendeleev developed the first version of the periodic table. The modern version, which all chemists use today, arranges the elements by atomic number. Each entry lists the atomic number, atomic symbol and atomic weight of an element. The basic structure of the periodic table is its division into rows and columns, or periods and groups. A period consists of the elements in any one horizontal row of the periodic table. Each time the outermost electron shell is filled and a further electron is then added a new period is begun. The first period of elements only consists of hydrogen (H) and helium (He). The second period has 8 elements, beginning with lithium (Li) and ending with neon (Ne). The third period starts with sodium (Na) and ends with argon (Ar). A group consists of the elements in any one column of the periodic table. All the elements in a group have the same number of electrons in their outermost shell (valence shell). 11 Elements of the same group have similar chemical properties. Example: the elements in the Group I are called the Alkali metals and are all metals (with the exception of hydrogen which is a gas). Group II = alkaline earth metals. Group 17 = halogens. Group 18 = Noble gases. Group Name no. Valence Properties electrons Element example I Alkali metals 1 Metals (except for H Na which is a gas); Highly reactive II alkaline earth metals 2 Similar to alkali metals – Mg not as reactive. 17 halogens 7 Variable physical Br properties - range from solid (I2) to liquid (Br2) to gaseous (F2 and Cl2); Particularly reactive with alkali metals. 18 Noble gases 8 (full) Non reactive due to full Xe valence shell; Gases. 12 Metals, non-metals: The elements of the periodic table are divided by a heavy staircase line into metals on the left and nonmetals on the right. A metal is a substance or mixture that has a characteristic luster, or shine and is generally a good conductor of heat and electricity. A non-metal is an element that does not exhibit the characteristics of a metal. Most of the nonmetals are gases. Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Period 1 2 3 4 5 6 7 1 2 H He 3 4 5 6 7 8 9 10 Li Be B C N O F Ne 11 12 13 14 15 16 17 18 Na Mg Al Si P S Cl Ar 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te 55 56 Cs Ba 87 88 Fr Ra * ** * Lanthanides ** Actinides I Xe 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Key Metals Metalloids Nonmetals Periodic table showing position of metals and nonmetals. 13 Chemical elements of the body and importance of these elements and their function: Element % mass in body Oxygen (O) 65 Function Toxic effects Cellular respiration Damage to eyes, nervous system and lungs at high pressure. Carbon (C) 18 Component of organic Inhaled as compounds. Particles - lung disease. (carbon monoxide poisoning) Hydrogen (H) 10 Component of organic acidosis, burns. compounds. Nitrogen (N) 3 Component of proteins, harmful in oxide Amino acids and cell form. membranes. Calcium (Ca) 1.5 Bone and teeth Nausea. Phorphorus (P) 1.0 Call membranes Liver damage. The human body also contains amounts of sulphur (S), sodium (Na), Chlorine (Cl), and trace amounts of Iron (Fe), Fluorine (F), Copper (Cu). Elements of Biological Importance: Oxygen (O): It may seem obvious that people need to breathe oxygen to survive, but plants need this element too. Many people think plants "breathe" carbon dioxide and "exhale" oxygen. But in reality, plants also "breathe" oxygen at certain times. Without oxygen, plants could not survive. Without plants, we wouldn’t have food to eat. It is also worth mentioning that water is a compound of hydrogen and oxygen (H2O) and that water is absolutely necessary for virtually all life as we know it. Water is 14 incredibly important in our bodies. In fact, more than 50% of our bodies are made of water. It dissolves other life-supporting substances and transports them to fluids in and around our cells. It is also a place in which important reactions take place in our bodies. Many people consider water to be the "blood of life". When you consider the full importance of oxygen, it becomes clear that this versatile element is the single most important substance to life. Sodium (Na): Since there no reserve store of sodium ions in the animal body, losses above the amount of intake come from the functional supply of cells and tissues. Salt (sodium chloride) is important in many ways. It is an essential part of the diet of both humans and animals and is a part of most animal fluids, such as blood, sweat, and tears. It aids digestion by providing chlorine for hydrochloric acid, a small but essential part of human digestive fluid. Persons with hypertensive heart disease often must restrict the amount of salt in their diet. 0.9% sodium chloride in water is called a physiological solution because it is isotonic with blood plasma. It is known medically as normal saline. Physiological solution is the mainstay of fluid replacement therapy that is widely used in medicine in prevention or treatment of dehydration. Calcium (Ca): Approximately 99% of total body calcium is in the skeleton and teeth and 1% in blood and soft tissues. Calcium has four major biological functions: 1. structural as stores in the skeleton 2. electrophysiological - carries charge during an action potential across membranes 3. intracellular regulator, and 4) as a cofactor for extracellular enzymes and regulatory proteins. Calcium builds and maintains bones and teeth; regulates heart rhythm; eases insomnia; helps regulate the passage of nutrients in & out of the cell walls; assists in normal blood clotting; helps maintain proper nerve and muscle function; lowers blood pressure; important to normal kidney function and in current medical research reduces the incidence of colon cancer, and reduces blood cholesterol levels. Calcium 15 deficiency may result in arm and leg muscles spasms, softening of bones, back and leg cramps, brittle bones, rickets, poor growth, osteoporosis (a deterioration of the bones), tooth decay, depression. Phosphorus (P): Phosphorous is one of the most abundant minerals in the human body, second only to calcium. This essential mineral is required for the healthy formation of bones and teeth, and is necessary for our bodies to process many of the foods that we eat. It is also a part of the body's energy storage system, and helps with maintaining healthy blood sugar levels. Phosphorus is also found in substantial amounts in the nervous system. The regular contractions of the heart are dependant upon phosphorus, as are normal cell growth and repair. Since phosphorus is found in almost all plant and animal food sources, a deficiency of this mineral is rarely seen. However, phosphorus deficiency can and does occur, particularly in people who take certain types of antacids for many years. Since phosphorus is important in maintaining the bodys energy system and proper blood sugar levels, it should seem logical that not getting enough of this mineral will affect the energy level in the entire body. Indeed, feeling easily fatigued, weak and having a decreased attention span can be symptoms of mild phosphate deficiency. The human body must maintain a balance between magnesium phosphorus, and calcium. Excess intake of phosphorus can occur in people with diets high in processed foods, soft drinks, and meats, leading to osteoporosis. 16 Summary: Be familiar with most common elements and their atomic symbols e.g. oxygen, carbon, nitrogen, sodium. Atomic number Z = number of protons Atomic Mass number A = number of protons + number of neutrons For a neutral atom Number of electrons = Number of protons Symbol 12 6C 23 11Na 1 Name Carbon Sodium Hydrogen Chlorine Mass Number (A) 12 23 1 35 Atomic Number (Z) 6 11 1 17 Protons 6 11 1 17 Neutrons 6 12 0 18 Electrons 6 11 1 17 1H 35 17Cl 17 Chemical Bonding: Octet Rule: A fundamental property of nature is that all atoms strive to have a full outer electronic shell of electrons. In other words, all atoms want a full valence shell. When they can achieve a full outer shell, the atoms are stable. This is a major driving force in chemical bonding. Many atoms gain or lose electrons in order to have a full valence shell (in order to become stable). This loss or gain of electrons resulted in the formation of charged atoms called ions. An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons. The numbers of electrons lost or gained is kept to a minimum – an atom will prefer to gain 2 electrons rather than lose 6 e (or to lose 1e rather than gain 7e). An atom that picks up an extra electron becomes a negatively charges ion called an anion. (Note: remember that an electron is negatively charged so the addition of an electron to an atom results in a negatively charged atom – anion). An atom that loses an electron becomes a positively charged ion called a cation. (Note: remember that an electron is negatively charged so the loss of an electron to an atom results in a positively charged atom – cation). 18 Example of a Cation: A sodium (Na) atom has 11 protons, 11 electrons and 12 neutrons. It has one electron in its valence (outer most shell – electronic shell number 3). Therefore in order to become stable the sodium atom wants to lose one electron so it can than have a full valence shell (electronic shell number 2). When the sodium atom loses this electron it forms a positively charged atom called a sodium cation. Example of an Anion: A chlorine (Cl) atom has 17 electrons, 17 protons and 18 neutrons. It has seven electrons in its valence shell (electronic shell number 3). In order to become stable, the chlorine atom needs to gain one electron in order to have a full outer shell. When the chlorine atom gains an electron it becomes negatively charged atom called a chlorine anion (Cl-). When you are drawing the structure of an ion you have to put the structure of the atom in square brackets and the charge of the ion (positive or negative) in the top right had corner. The notation that one uses to signify an ion is the atomic symbol of the element and the charge in superscript (Na+). Na – e = Na+ Cl + e = Cl19 Ca -2e = Ca+2 O + 2e = O-2 Ions have different physical and chemical properties to the atoms from which they are derived. Sodium and lithium (metal atoms) react violently with water in a reaction which can become explosive. Sodium ion, however, is found in table salt and is an important constituent of human body fluids. Lithium ion is administered for the treatment of anti-depressive conditions. Chlorine atoms are too reactive to exist alone and in the absence of anything else to react with they will react with themselves to form chlorine molecules (Cl2) which forma greenish, toxic gas with a pungent odour. Chloride ion is found in table salt and is the usual counter ion found widely distributed throughout the body to balance the charges of the cations (Na+, K+). The sodium and chlorine ions are singly charged but they can also be doubly charged (Ca+2, O-2). This leads us to chemical bonding.. The properties of a substance such as sodium chloride are determined in apart by the chemical bonds that hold the atoms together. A chemical bond is a strong attractive force that exists between certain atoms in a substance. There are two types of chemical bonds – ionic (involves the losing or gaining of electrons) and covalent (involves the sharing of electrons). Ionic Bonding: An ionic bond is a chemical bond formed by the electrostatic attraction between positive and negative ions (particles with opposite electrical charges). The bond forms between two atoms when one or more electrons are transferred from the valence shell of one atom to the valence shell of another atom. These two chemically bound atoms are then termed a compound (remember the definition: a compound is a type of matter composed of two or more elements chemically combined in fixed proportions). Stable compounds are formed when the outermost shells of the atoms involved are full. 20 The number of ions of each atom involved in ionic bonding is such that the positive charge of the positive ions equal the negative charge on the negative ions. Example: sodium chloride (NaCl) involves the sodium cation (Na+) and the chlorine anion (Cl-). The sodium wants to lose one electron in order to have a full valence shell, so it gives its electron to chlorine which wants an electron in order to have a full valence shell. The overall charge on the compound is neutral. Diagram of ionic bonding between sodium and chlorine. Therefore, an ionic compound is a compound composed of cations and anions. The strong attraction between positive and negative charges hold the ions together in a regular pattern in space. For examples, in sodium chloride, each Na+ ion is surrounded by six Cl- ions and each Cl- ion is surrounded by six Na+ ions. The result is a crystal, which is a kind of solid having a 3-dimensional arrangement of atoms, molecules or (in the case of NaCl) ions. 21 Example: LiF. Li has 1 valence electron. F has 7 valence electrons. Therefore, Li loses an electron so it can have a full outer shell. F gains an electron so it can have a full outer shell. 22 Covalent Bonding: Many compounds do not contain ions. These compounds consist of atoms bonded tightly together in molecules that result when atoms share electrons instead of transferring electrons from one atom to another. These are known as covalent compounds. The bonds that hold atoms together in such molecules are covalent bonds. Non-metals form covalent bonds with each other. Diatomic molecules such as H2, O2, Cl2 are covalent molecules. Compounds containing more than one non-metal are covalent compounds – NH3, CO2, Therefore for covalent bonding there is sharing of electrons between atoms, so each atom can achieve a full valence shell (octet rule). Stable compounds are formed when the outermost shells of the atoms involved are full. Example: Hydrogen gas (H2). Each hydrogen atom has 1 electron in its valence shell (electronic shell number 1). In order for the hydrogen atom to be stable, the atom must have two electrons in its valence shell (octet rule and the first electronic shell can only have a maximum of two electrons). Therefore, there is a sharing of electrons between the two hydrogen atoms. Only one electron from each atom is being shared so this is termed a single bond. Both hydrogen atoms have one electron in their valence shell. 23 Each atom shares one electron each to form a H2 molecule. Both atoms are now stable as they have full valence shells (electronic shell 1 can have a max. of 2 electrons). In cases where there are two electrons being shared from each atom, this is termed a double bond. Example: Oxygen gas O2 In cases where there are three electrons being shared from each atom, this is termed a triple bond. There can also be covalent bonding between many atoms. Example: Ammonia NH3 N has 5 valence electrons, and each hydrogen atom has 1 valence electrons. Therefore, N shares one of its electrons with three H atoms, so N can have full valence shell. 24 For shorthand bonding between atoms can be shortened to a line or lines between atoms. A single bond is represented by a single line; a double bond is represented by two lines and a triple bond is represented by three lines. H-H (H2), O-O (O2), O=C=O (CO2), N≡N (N2). Double and triple bonds are classified as multiple bonds. Multiple bonds are shorter and stronger than single bonds. Organic Chemistry Example: Organic chemistry involves the chemistry of the element carbon. Carbon never forms ions (to lose or gain 4 electrons in one go would cost too much in energy terms). Carbon always shares its 4 electrons but it is capable of multiple bonds. The carbon atom can single bond to another carbon atom, or have double or triple bond to another carbon. ethene (contains a carbon carbon double bond, C=C). ethane (contains a carbon carbon single bond, C-C). ethyne (contains a carbon carbon triple bond). acetone (contains a carbon oxygen double bond). ethanol (contains a carbon carbon single bond and a carbon oxygen bond). 25 Structure of Compounds: A molecule has: Fixed ratio of atoms always joined in the same way. A definite shape in 3D space. Particular bond lengths and bond angles. Bond length is the distance between the nuclei of neighbouring atoms. O C O Linear H N H H Trigonal planar. H C H H H Tetrahedral 26 Valency: Valency is simply the combining power of atoms or ions when forming compounds and it is thus this which determines the ratios in which these combine together. In ionic bonding the valency is simply the same as the charge on the ion. Thus, sodium (Na) which has one electron in the outermost (valence) shell which it needs to lose to form a singly charged positive ion Na+ has a valency of 1. Calcium (Ca) has two electrons in its valence shell, and so needs to lose 2 electrons to form a calcium cation (Ca+2) and so has a valency of 2. Fluorine (F) has 7 electrons in its valence shell and so needs to gain another electron in order to become stable (F-). Therefore, it has a valency of 1. Oxygen has 6 electrons in its valence shell and so needs to gain 2 electrons in order to become stable (O-2). In ionic compounds all valencies must be satisfied, thus sodium chloride is NaCl (each has a valency of 1) whereas calcium chloride is CaCl2 (here we require 2 chloride ions to satisfy the valency of 2 for calcium). In covalent bonding the valency is the number of electrons actually used in bonding (shared electrons). Carbon (C) never form ions (to lose or gain 4 electrons in one go would cost too much in energy terms) and always shares its 4 electrons, therefore having a valency of 4. Nitrogen (N) and Phosphorus (P) both have 5 electrons in their valence shell and so may have a valency of either 3 or 5. The valency of elements is shown on the periodic table: All elements in group I – valency of 1. (H, Li, Na) All elements in group II – valency of 2. (Be, Mg) All elements in group 13– valency of 3. (B, Al) All elements in group 14 – valency of 4. (C, Si) All elements in group 15 – valency of 3/5. (N, P) All elements in group 16 – valency of 2. (O, S) All elements in group 17 – valency of 1. (F, Cl) All elements in group 18 – valency of 0. (He, Ne, Ar – full valence shell). 27 Therefore can predict how many atoms will combine together to form a compound. Example: NaCl Na has a valency of 1, Cl has a valency of 1. You can switch these valencies to get the molecular formula – NaCl. CaCl2 Ca has a valency of 2. Cl has a valency of 1. When you switch these, you get the molecular formula of CaCl2 CH4 C has a valency of 4. H has a valency of 1. Switch these and you get the molecular formula of CH4 BF3 NH3 Therefore valency is a way of determining the combining ratios of atoms and ions in molecules. How to predict if a compound will undergo ionic or covalent bonding? A convenient way of predicting the type of bonding involved between reacting atoms is to use their electronegativity values. Electronegativity is a measure of the ability of an atom in a molecule to draw bonding electrons to itself. Rule: If the electronegativity difference between reacting atoms is large the sharing of electrons between them will be so uneven that the “shared” electrons will effectively spend all their localised on the atom of greater electronegativity, thus a charged species (ion) will be formed. If the electronegativity difference is ≥ 1.98, ionic bonding takes place. If the electronegativity difference is < 1.98, covalent bonding takes place. Example: LiF (remember that both lithium (Li) and fluorine (F) both have a valency of 1). Li = electroneg. of 0.98 28 F = electroneg. of 3.98. Difference is 3.00 – Ionic Bonding. CN C = electroneg. of 2.55. N = electroneg. of 3.04. Difference is 0.49 – Covalent Bonding. Polarity and solubility: In the case of covalent bonding the atom of greater electronegativity acquires a partial negative charge δ-, and the atom of lower electronegativity an equal partial positive charge, δ+. The bond then is said to be polar, ad the greater the difference in electronegativity (but still less than 1.98), the greater the polarity. It is the polarity of water, which allows ionic molecules to dissolve NaCl. Ions are solvated and can move around with a greater degree of freedom. Organic, non-polar substances do not dissolve in water. Amino acids (building blocks of proteins) which have basic structure of +H3N-CHRCOO- - lots of polar bonds like O-H, N-H. Since they are polar, they are soluble in water and can move around the body in the bloodstream. 29 Hydrogen Bonds When H is covalently bonded to an electronegative atom such as O or N (not C) in an organic compound, e.g. H O C N C H H The bond will be polarised and the hydrogen will become slightly positively charged (δ+). This slight positive charge can cause it to be attracted to another, slightly negatively charged, electronegative atom (denoted as X) in another molecule. Thus weak forces of attraction can be formed between molecules. These are known as hydrogen bonds and are important in the chemistry of life. Indeed it is hydrogen bonds which are the reason that water is a liquid, when the corresponding compounds of atoms close to oxygen in the periodic table, combined with hydrogen, are all gases. Whilst, water may be expected to be a gas by comparison with its neighbours, it is overall strength of many weak hydrogen bonds which constrain the water molecules to remain close together as a liquid, at room temperature, rather than being free to float away as a gas. H O H H H O O H H H H O O H H Of course if energy is supplied in the form of heat, this will break these weak forces of attraction and the water molecules can escape by boiling off as water vapour. 30