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General Science 101 The atomic nature of Ma5er Chapters 11 and 32 The Atomic hypothesis Democritus and Leuccipus (5th century BCE) •  First developed concept of atom •  Atom is the smallest parEcle that could not be divided any further Aristotle (350 BCE) •  All ma5er is composed of combinaEons of 4 elements – Earth (solids) – air (gas) – water (liquids) – fire (state of flame) •  Persisted for more than 2000 years GSCI 101, Prof. M. Nikolic 2 The discovery of atom John Dalton (1766 -­‐ 1844) •  Set foundaEons of modern chemistry by proposing his atomic theory 1) All ma5er is made of atoms. Atoms are indivisible and indestrucEble. 2) All atoms of a given element are idenEcal in mass and properEes. 3) Compounds are formed by a combinaEon of two or more different kinds of atoms. 4) A chemical reacEon is a rearrangement of Basics of Dalton’s theory atoms. are sEll valid. •  No proof GSCI 101, Prof. M. Nikolic 3 The discovery of atom Robert Brown (1773 – 1858) •  Sco]sh botanist •  Observed that grains of pollen placed into a clear liquid moved strangely as if pushed with invisible parEcles – Brownian mo*on •  The first detecEon of atoms Albert Einstein (1879 – 1955) •  1905 – explained Brownian moEon and confirmed the existence of atoms •  Also got Nobel prize for this GSCI 101, Prof. M. Nikolic 4 Atomic structure Brown – All ma5er is made of atoms. Atoms are indivisible and indestrucEble. Atomic structure •  Atomic nucleus – filled with nucleons •  Protons – posiEvely charged nucleons •  Neutrons – neutral nucleons •  Electron shell – negaEvely charged GSCI 101, Prof. M. Nikolic 5 Discovery of atomic nucleus Ernest Rutherford (1907) Atom – small, massive and posiEvely charged core (nucleus) – large shell filled with light electrons GSCI 101, Prof. M. Nikolic Same charges repel and opposite charges a5ract each other. 6 Discovery of the electrons Joseph John Thomson (1897) Chapter 32 – The atom and quantum GSCI 101, Prof. M. Nikolic 7 Conceptual quesEon: Atomic structure Which of the following has the smallest mass? Proton. Neutron. Electron. ✓ Except for slight differences, all have about the same mass. Need more informaEon GSCI 101, Prof. M. Nikolic 8 The elements Substance composed of only one kind of atom is called an element. ScienEsts discovered 117 different kinds of elements – 90 occur in nature. – Others produced in laboratory are unstable. GSCI 101, Prof. M. Nikolic 9 The Elements 99.9 % of Earth is formed from a dozen of the elements ComposiEon of living things include these 5 elements: •  Oxygen (O) •  Carbon (C) •  Hydrogen (H) •  Nitrogen (N) •  Calcium (Ca) GSCI 101, Prof. M. Nikolic 10 Atomic structure ProperEes of elements and atoms 1.  Atomic number (Z) – number of protons in the nucleus of an atom – neutral atom: number of protons = number of electrons – all atoms in elements have the same Z Example: Hydrogen (H) Z = 1 → 1 proton Neutral hydrogen → 1 electron GSCI 101, Prof. M. Nikolic 11 Atomic structure ProperEes of elements and atoms 2. Mass number (A) – total number of protons and neutrons together Example: Hydrogen (H) Z = 1 → 1 proton Neutral hydrogen → 1 electron A = 1 → 0 neutrons Example: Lithium (Li) Z = 3 → 3 protons Neutral Li → 3 electrons A = 7 → 4 neutrons GSCI 101, Prof. M. Nikolic 12 Isotopes Atoms of the same element that have different numbers of neutrons are called isotopes. We idenEfy isotopes by their mass number A. Three isotopes of Carbon GSCI 101, Prof. M. Nikolic 13 Atomic structure ProperEes of elements and atoms 2.  Atomic mass (A) – total mass of an atom a)  Electrons – small mass → negligible contribuEon b)  Protons c)  Neutrons Atoms have so small mass that it is hard to express their mass in grams and kilograms → atomic mass unit (amu) -­‐ 1nucleon (proton or neutron) → mass of 1 amu Atomic mass unit is defined as 1/12 of the mass of a carbon-­‐12 (12C) GSCI 101, Prof. M. Nikolic 14 Conceptual quesEon: Atomic structure The atomic number of an element matches the number of Protons in the nucleus of an atom. Electrons in a neutral atom. All of the above. ✓ None of the above. GSCI 101, Prof. M. Nikolic 15 Conceptual quesEon: Atomic structure A nucleus with an atomic number of 44 and a mass number of 100 must have 44 neutrons. 56 neutrons. ✓ Z = 44 → 44 protons A = 100 → 100 – 44 = 56 neutrons 100 neutrons. None of the above. GSCI 101, Prof. M. Nikolic 16 Electron configuraEon Niels Bohr (1885 – 1962) •  Postulated the idea of atomic structure based on Rutherford’s experiments •  Electrons populate shells with specific energies around nucleus 1.  Electrons in the same shell have the same energy. 2.  Electrons can only jump between shells. GSCI 101, Prof. M. Nikolic 17 Why don’t electrons fall onto the nucleus? Electrons are not just parEcles, they are waves too! Even more, they standing waves è These waves close in on themselves. Electron’s mass and charge are spread out over the wave. Even the smallest orbit of electrons sEll has to close on itself GSCI 101, Prof. M. Nikolic Not an orbit for an electron. è  Electrons cannot fall, they have to circle. 18 Why don’t electrons fall onto the nucleus? Electrons are not just parEcles, they are waves too! GSCI 101, Prof. M. Nikolic 19 Why don’t electrons fall onto the nucleus? Modern quantum mechanics – electron wave is spread out in 3 dimensions forming an electron “cloud” of probability. We can only calculate the probability of finding the electron in the cloud (Schrodinger’s wave equa3on). Why? Because of the Heisenberg uncertainty principle. We ether know where the electron is or how fast it is. GSCI 101, Prof. M. Nikolic 20 Electron configuraEon Niels Bohr (1885 – 1962) •  Postulated the idea of atomic structure based on Rutherford’s experiments •  Electrons populate shells with specific energies around nucleus 1.  Electrons in the same shell have the same energy. 2.  Electrons can only jump between shells. GSCI 101, Prof. M. Nikolic 21 Electron configuraEon The arrangement of electrons in shells is called electron configura*on. Elements may have up to 7 shells also called energy levels a)  n = 1, shell K b)  n = 2, shell L c)  n = 3, shell M, … Each shell may have up to 4 subshells also called orbitals a) 
b) 
c) 
d) 
s = 2 electrons p = 8 d = 10 f = 14 GSCI 101, Prof. M. Nikolic 22 Electron configuraEon Number of orbitals in a shell ≤ n (number of the shell). a)  n = 1, only s orbital and up to 2 electrons → 1s2 b)  n = 2, s and p orbitals → 2s2 2p6 c)  n = 3, s, p, and d orbitals → 3s2 3p6 3d10 d) n = 4, s, p, d, and f orbitals → 4s2 4p6 4d10 4f14 e) n = 5, s, p, d, and f orbitals → 5s2 5p6 5d10 5f14 Example: Chlorine 17Cl Z = 17 → 17 protons → 17 electrons GSCI 101, Prof. M. Nikolic n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p6 (8 electrons) n = 3 → 3s2 3p5 (7 electrons) 23 Electron configuraEon -­‐ rules 4s orbital comes before 3d orbital! Exercise : Find the electron configuraEon copper 29Cu Z = 29 → 29 electrons GSCI 101, Prof. M. Nikolic n = 1 → 1s2 (2 electrons) n = 2 → 2s2 2p6 (8 electrons) n = 3 → 3s2 3p6 (8 electrons) – total of 18 electrons n = 4 → 4s2 (2 electrons) – total of 20 electrons n = 3 → 3d9 Before going to 3d orbital check what happens with 4s 1s2 2s2 2p6 3s2 3p6 4s2 3d9 24 Periodic table of elements GSCI 101, Prof. M. Nikolic 25 Periodic table of the elements Group A – how many electrons are in the last shell a)  Elements are classified by their atomic number (Z) b)  Groups A – representaEve elements B – transiEon elements Periods – how many shells (n) are there in an atom GSCI 101, Prof. M. Nikolic 26 Periodic table of the elements Group A a)  I A – 1 electron – alkali metals (shiny, sot metals) b)  II A – 2 electrons – alkaline earth metals c)  VII A – 7 electrons – halogens (very reacEve) d)  VIII A – 8 electrons – filled outer shell – inert gases (colorless, odorless, nonreacEve gases) GSCI 101, Prof. M. Nikolic 27 Exercise: Periodic pracEce Write down the electron configuraEon of silicon (14Si) and then idenEfy its period and group. Silicone 14Si Z = 14 → 14 protons → 14 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p6 (8 electrons) n = 3 → 3s2 3p2 (4 electrons) Silicon belongs to 3rd period, IV A group What about iron (26Fe)? Iron 26Fe Z = 26 → 26 protons → 26 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p6 (8 electrons) n = 3 → 3s2 3p6 (8 electrons) n = 4 → 4s2 (2 electrons) -­‐ total of 20 electrons n = 3 → 3d6 GSCI 101, Prof. M. Nikolic Iron belongs to 4rd period, VIII B group (you add up 2e-­‐from 4s and 6e-­‐ from 3d) 28 Ions and neutrals Neutral atom è number of protons = number of electrons If an atom loses/gains a proton è it becomes a different atom (element). If an atom loses/gains a neutron è it becomes a different isotope of the same element. GSCI 101, Prof. M. Nikolic 29 Compounds and mixtures When atoms of different elements bond to one another, they make a compound. – A compound is different from the elements from which it is made. – It can only be separated into its consEtuent elements by chemical means. Example: Salt (a compound of sodium and chlorine) A substance that is mixed together without chemically bonding is called a mixture. Example: Air (a mixture of several gases)
GSCI 101, Prof. M. Nikolic 30 Molecules Molecules •  Two or more atoms bonded together Example: •  NH3 (ammonia) •  3 atoms of hydrogen and 1 atom of nitrogen Why do atoms bond into molecules? GSCI 101, Prof. M. Nikolic 31 Valence electrons Electrons populaEng the last electron shell are called valence electrons. Carbon 6C Z = 6 → 6 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p2 (4 electrons) •  Groups in the periodic table contain an informaEon on how many valence electrons each element has. •  VIII A group – 8 electrons filled shell – most stable elements Octet rule – atoms a5empt to acquire an outer shell with 8 electrons. GSCI 101, Prof. M. Nikolic 32 Halogen elements Group VII A – 7 valence electrons Fluorine 9F Z = 9 → 9 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p5 (7 electrons) Fluorine is one electron short of having a completely full 2p orbital. •  To saEsfy the octet rule and have stable configuraEon with 8 valence electrons, fluorine is pracEcally begging to gain 1 electron •  On the other hand, fluorine (and the other halogens) isn’t too keen on accepEng any more than one electron. GSCI 101, Prof. M. Nikolic 33 Other elements Group VI A – 6 valence electrons Oxygen 8O Z = 8 → 8 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p4 (6 electrons) Oxygen has two vacancies in the last shell. Group V A – 5 valence electrons Nitrogen 7N Z = 7 → 7 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p3 (5 electrons) Nitrogen has three vacancies in the last shell. GSCI 101, Prof. M. Nikolic 34 On the other side of periodic table Group I A – 1 valence electron Sodium 11Na Z = 11 → 11 electrons n =1 → 1s2 (2 electrons) n = 2 → 2s2 2p6 (8 electrons) n = 3 → 2s1 (1 electron) Only one electron. If sodium gets rid of that electron, then the configuraEon of the remaining electrons is very stable. GSCI 101, Prof. M. Nikolic 35 Metals These elements are trying to lose an electron, or two. GSCI 101, Prof. M. Nikolic Non-­‐metals These elements are trying to gain an electron, or two (even three). We are not going to discuss these, they are too complicated. 36 Ionic bonds In an ionic bond, a metal gives up one or more electrons and a non-­‐metal accepts one or more electrons. What happens when a sodium atom meets a chlorine atom? Na loses electron è posiEve ion Cl gains electron è negaEve ion The oppositely charged ions stay bound together, forming the molecule NaCl (table salt) GSCI 101, Prof. M. Nikolic 37 Ionic bonds What happens if magnesium meets up with chlorine? One of magnesium’s extra electrons goes to a chlorine atom, but that chlorine atom won’t accept more. Cl Mg Cl So another chlorine atom appears and takes the other electron magnesium wants to give up. In this molecule, for every one magnesium we have two chlorines: MgCl2. GSCI 101, Prof. M. Nikolic 38 Covalent bonds A covalent bond is a chemical bond formed by the SHARING of at least a pair of electrons. •  Each of these fluorine atoms is willing to share one electron with the other. •  Each fluorine atom is then fooled into thinking it has the same configuraEon as neon. The resulEng molecule is F2. GSCI 101, Prof. M. Nikolic 39 Covalent bonds •  Similar processes occur to form H2, Cl2, Br2 and I2. •  What about oxygen? •  Each atom of oxygen is willing to share two electrons. •  Each oxygen shares two of its electrons with another oxygen. •  Each oxygen atom is then fooled into thinking it has the same configuraEon as neon. The resulEng molecule is O2. GSCI 101, Prof. M. Nikolic 40 Covalent bonds Naturally, non-­‐metal atoms can join up with non-­‐metal atoms of other elements in covalent bonds. The red circles, represenEng the electrons, are ge]ng shared •  Each hydrogen atom “has” 2 electrons •  Oxygen atom “has” 8 electrons GSCI 101, Prof. M. Nikolic 41 Molar mass Molar mass of a molecule is the sum of the atomic masses (amu) of all the atoms in a molecule. Exercise : What is the molar mass of water, which has the formula H2O? Atoms Atomic mass unit (amu) 2 of H 2 x 1 amu = 2 amu 1 of O 1 X 16 amu = 16 amu Molar mass (M) = 18 amu GSCI 101, Prof. M. Nikolic 42 Exercise: Molar mass What is the molar mass of ethyl alcohol, which has the formula C2H5OH? Atoms Atomic mass unit (amu) 2 of C 2 x 12 amu = 24 amu 6 of H 6 x 1 amu = 6 amu 1 of O 1 X 16 amu = 16 amu Molar mass (M) = 46 amu GSCI 101, Prof. M. Nikolic 43 Units of measurement Atomic mass unit (amu) is based on 12 g of isotope Carbon-­‐12. In 12 g of Carbon-­‐12 there is 6.0 x 1023 atoms è Avogadro’s number (Na) The amount of substance that contains Avogadro’s number of parEcles is defined as a MOLE of the substance. 1 mole è Na = 6.0 x 1023 parEcles è 12 g of Carbon-­‐12 What about other elements? Hydrogen: 1 mole è 6.0 x 1023 parEcles è 1 g of Hydrogen-­‐1 Oxygen: 1 mole è 6.0 x 1023 parEcles è 16 g of Hydrogen-­‐16 Magnesium: 1 mole è 6.0 x 1023 parEcles è 24 g of Magnesium-­‐24 GSCI 101, Prof. M. Nikolic 44 Conceptual quesEon: Mole and Avogadro’s number How many atoms is in 1 mole of Phosphorus (P)? 1. One mole of any element is always 6.0 x 1023 atoms. 10. 6.0x1023 ✓ 15x6.02x1023 31x6.02x1023 Three moles of any element are always 3 x 6.0 x 1023 atoms = 18 x 1023 atoms, etc. Or, by applying mathemaEcal formula: N = n ⋅ Na
NotaEon for mole is: n (small le5er n). GSCI 101, Prof. M. Nikolic N – number of parEcles n – number of moles Na – Avogadro’s number 45 Mass of elements and molecules Mass of an element/molecule is given with the formula: m = n⋅M
m – mass of the molecule given in grams n – number of moles M – molar mass given in amu If we are looking for the mass of an element than use atomic mass of the element. Example: What is the mass of 2 moles of water? First, find the molar mass of the water M(H2O): M(H2O) = 2xA(H) + 1xA(O) = 2x1 + 1x16 = 18 amu Then, find the mass using the formula: m(H2O) = n x M(H2O) = 2 moles x 18 amu = 36 g GSCI 101, Prof. M. Nikolic 46 Conceptual quesEon: Mole and Avogadro’s number What is the mass of 2 moles of Chlorine atom? 36 amu. 36 g. Atomic mass of chlorine A(Cl) = 36 amu m(Cl) = n x A(Cl) = 2 moles x 36 amu = 72 g 72 amu. What about chlorine molecule Cl2? 72 g. ✓ The same rules apply for molecules just instead of atomic mass unit you should use molar mass (M). Need more info. The molar mass: M(Cl2) = 2 x A(Cl) = 2x36 = 72 amu m(Cl2) = n x M(Cl2) = 2 moles x 72 amu = 144 g GSCI 101, Prof. M. Nikolic 47 Exercise: Mole and mass I What is the mass of 4 moles of iron oxide, Fe2O3? First, find the molar mass of the iron oxide M(Fe2O3): M(Fe2O3) = 2xA(Fe) + 3xA(O) = 2x56 + 3x16 = 112 + 48 = 160 amu m = n x M(Fe2O3) m = 4 moles x M(F2O3) = 4 moles x 160 amu = 640 g = 0.64 kg GSCI 101, Prof. M. Nikolic 48 Exercise: Mole and mass II How many atoms are in 96 g of O2? To find number of atoms we need number of moles in O2 Since: m = n x M(O2) If we find molar mass of O2 we could solve for number of moles. Molar mass of O2: M(O2) = 2xA(O) = 2x16 = 32 amu Number of moles of O2: 96 g= n x 32 amu è n = 96/32 = 3 moles Number of atoms of O2: N = n x Na N = 3 moles x 6x1023 = 18x1023 atoms GSCI 101, Prof. M. Nikolic 49 AnEma5er Ma5er è Composed of atoms with posiEve nuclei and nega3ve electrons An*maYer è Composed of atoms with nega*ve nuclei and posi%ve electrons (positrons) GSCI 101, Prof. M. Nikolic 50 AnEma5er •  Positron was discovered in 1932 •  The first arEficial anE-­‐atom was constructed in 1995 Every charged parEcle has an anEparEcle of the same mass and opposite charge. Even neutrons have anE-­‐neutrons. GSCI 101, Prof. M. Nikolic 51 AnEma5er •  GravitaEonal force does not disEnguish between ma5er and anEma5er •  Both ma5er and anEma5er emit the same light There could be galaxies made of anEma5er and we would not know! But when anEma5er meets the ma5er they mutually annihilate each other converEng the equal amounts of ma5er and anEma5er to energy (E=mc2) GSCI 101, Prof. M. Nikolic 52 Conceptual quesEon: AnEma5er If 1.0 gram of anEma5er meets with 4.0 grams of ma5er, the energy released would correspond to the energy equivalent of 1 gram. 2 grams. ✓ 4 grams. When ma5er meets the anEma5er they annihilate the equal amounts of masses. Since we have only 1 gram of anEma5er è this is the most mass that anEma5er can annihilate + Much more than 4 grams. The same amount of mass from the ma5er (1 g) GSCI 101, Prof. M. Nikolic è Total energy released corresponds to the energy equivalent of 1 g + 1 g = 2 g 53 Dark ma5er Dark ma5er is NOT an anEma5er! Dark ma5er is the strange and unknown ma5er that cannot be seen, since it does not reflect, absorb or emit light. ScienEsts predicted dark ma5er because gravity laws were not adding up for most of the galaxies in our universe. Dark energy is the property of space – it counterparts the gravity and causes the universe to expand. GSCI 101, Prof. M. Nikolic 54