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Chemical Bonding © Boardworks Ltd 2003 Elements • Elements are the simplest substances. There are about 100 different elements • Each elements has just one particular type of atom that is different to the atoms in any other element S Fe N O K • In some elements atoms bond to form small simple structures. • In other elements atoms bond into giant structures with millions of atoms. N N N • Even in elements atoms usually join (bond) together. N N C N N N © Boardworks Ltd 2003 Compounds are not just mixtures • Reacting is different to just mixing. • The physical and chemical properties of compounds are very different to their original elements. ELEMENTS Carbon C (black solid) + oxygen O O (colourless gas, that supports combustion) COMPOUND Carbon dioxide O C O (a colourless gas, that extinguishes fire) © Boardworks Ltd 2003 Activity Copy the Table and fill in the last column. Name Sodium chloride Nickel Copper sulphate Water Oxygen Rust Mercury Sand Formula Element or compound NaCl compound Ni element CuSO4 compound H2O compound O2 element Fe2O3 compound Hg element SiO2 compound Rust consists of iron combined with oxygen © Boardworks Ltd 2003 No charge on element atoms Two of the three particles in atoms have an electrical charge. Protons +1 Neutrons 0 Electrons -1 BUT atoms have equal numbers of protons and electrons. This means their overall charge = zero. For example 19 20 F Ne 9 10 Fluorine Neon Copy & Fill in the gaps 9 protons 9+ 10+ 10 protons ___ ___ 9 electrons 9- 10 electrons ___ 10___ 10 neutrons 0 0 10 ___neutrons ___ Total Charge 0 Total Charge ___0 © Boardworks Ltd 2003 Noble Gases: full shells The electrons around the outside of atoms are arranged in layers called shells st 2nd Shell: 8 electrons 1 Shell: 2 electrons 3rd Shell: 8 electrons Each shell has a maximum number of electrons that it can hold. In noble gases the shells are completely full rather than partially full. During bonding other atoms try to attain the “full electron shell” structure of the noble gases. © Boardworks Ltd 2003 Types of bonding Atoms can be joined together in 2 possible ways Metal and non-metal Ionic Bonding Non-metals only Covalent Bonding All three types involve changes in the electrons in the outermost electron shells of the atoms © Boardworks Ltd 2003 Ionic Bonding Involves the gain or loss of electrons to form charged atoms called ions. The positive ions and negative ions are attracted to each other and bond together. © Boardworks Ltd 2003 IONIC BONDING © Boardworks Ltd 2003 IONIC BONDING Most ionic compounds contain a metal and a non-metal. When metals react they lose outer shell electrons to leave a full electron shell. This produces a charged atom (ion) with a + charge. Lose 1 or more electrons (negative) Neutral atom n+ Positive ion When non-metals react with a metal they gain electrons to achieve a full electron shell. Gain 1 or more electrons Neutral atom n- Negative ion © Boardworks Ltd 2003 Opposites attract – lattices The oppositely charged ions are attracted into a lattice that gets bigger and bigger until it consists of millions of ions + + + + + - - - + - + - + - + attract - opposites + + - And ion, and ion, and ion! © Boardworks Ltd 2003 Ions and electron structures Metals lose electrons to form positive(+) ions called cations. Non-metals gain electrons to form negative (–) ions called anions. n+ n- We know that the atoms lose or gain electrons to achieve full electron shells. © Boardworks Ltd 2003 Formation of sodium chloride 1. Formation of sodium ions Sodium has 1 electron in its outer shell. If it loses this it will have no partially filled shells. Na Sodium atom (2.8.1) Loses 1 electron Na+ Sodium 1+ ion (2.8.0) This only happens if there is another atom able to accommodate the lost electron. © Boardworks Ltd 2003 Formation of sodium chloride 2.Formation of Chloride ions Chlorine has 7 electrons in its outer shell. If it gains 1 electron it can achieve a full outer electron shell. It is, therefore, going to be able to accept the electron that the sodium wants to lose. Cl Cl Gains 1 electron (from sodium) Chlorine atom (2.8.7) Chlorine I - ion (2.8.8) © Boardworks Ltd 2003 Formation of Sodium Chloride- the overall process. Sodium loses its outer electron. Chlorine gains it. The result is both end up with full shells Two oppositely charged ions are formed, which attract. Millions of such pairs form a giant lattice Na Na Cl Cl © Boardworks Ltd 2003 Simpler electron diagrams We can assume full inner electron shells. We can therefore sometimes shorten bonding diagrams by omitting to draw the inner electron shells. Na Cl Na Cl © Boardworks Ltd 2003 Bonding in Magnesium Oxide More than one electron may be transferred between atoms in ionic bonding. Mg 2.8.2. Magnesium atom O Magnesium Oxide 2.6 Oxygen atom O2- Mg2+ 2.8 2.8 © Boardworks Ltd 2003 Draw a simplified bonding diagram (omitting inner shells) for magnesium oxide. Mg O Magnesium Oxide Magnesium atom Oxygen atom Mg2+ O2- © Boardworks Ltd 2003 Copy the diagram and draw another box showing the electron configuration in sodium fluoride. Na 2.8.1. Sodium atom F Sodium Fluoride 2.7 Fluorine atom F- Na+ 2.8 2.8 © Boardworks Ltd 2003 Bonding in Aluminium Fluoride In aluminium fluoride it is the non-metal atoms that we need more of. F F F F Al F Al F © Boardworks Ltd 2003 Activity 1. 2. 3. 4. 5. Following the previously shown slides draw bonding diagrams for the compounds formed from the following atoms: Lithium (2.1) and fluorine (2.7) Sodium (2.8.1) and sulphur (2.8.6) Magnesium (2.8.2) and sulphur (2.8.6) Magnesium (2.8.2) and fluorine (2.7) Aluminium (2.8.3) and nitrogen (2.5) Remember that the total number of electrons lost by the metal must equal the total number of electrons gained by the non-metal © Boardworks Ltd 2003 Electrons Lost = Electrons Gained Both sodium chloride and magnesium oxide are simple ionic compounds. In both cases the metal and non-metal need to lose or gain the same number of electrons respectively. Na 1 electron Mg Cl Na+ Cl- O Mg2+ O2- 2 electrons This is not always the case. © Boardworks Ltd 2003 Different numbers of electrons Consider the reaction of sodium with oxygen. – Sodium (2.8.1) needs to lose 1 electron. – Oxygen (2.6) needs to gain 2 electrons This can only happen if there are two sodium ions formed for every one oxygen ion formed. Na Na+ 1 electron per sodium O2- O Na Na+ The formula will be Na2O. © Boardworks Ltd 2003 Different numbers of electrons Consider the reaction of magnesium with chlorine. – Magnesium (2.8.2) needs to lose 2 electron. – Chlorine (2.8.7) needs to gain 1 electrons We need 2 chlorines for every one magnesium. Cl Mg ClMg2+ 2 electrons Cl Cl- The formula will be MgCl2 © Boardworks Ltd 2003 Charges on ions • When atoms form ions they aim to attain electron shells that are either completely full or completely empty. • If we know the electron configuration of an atom we can usually work out how many electrons it must lose or gain to achieve a noble gas configuration. • This will tell us the charge on its ion. © Boardworks Ltd 2003 • Octet Rule: atoms will bond with other atoms to gain, lose or share electrons in order to achieve a full outer shell (i.e. 8) • Ion = charged atom • Ionic bond: is formed between positive ions and negative ions which are attracted to each other and bond together. • Most ionic compounds contain a metal and a non-metal. • Need to know MgO and NaCl © Boardworks Ltd 2003 COVALENT BONDING © Boardworks Ltd 2003 Covalent Bonding Involves the sharing of electrons between atoms to form molecules © Boardworks Ltd 2003 Covalent compounds • Covalent compounds are generally formed when nonmetal atoms react together. • Sharing of electrons © Boardworks Ltd 2003 Covalent bonds • • • Figure 1 Covalent compounds are held together by this sharing of electrons. A covalent bond is when a pair of electrons are shared It is sometimes represented in full bonding diagrams (see figure 1). Often these bonds are just shown as a pair of electrons (xx) or even just a line (see figure 2). Figure 2 F XX F - F F © Boardworks Ltd 2003 Small covalent structures • Sometimes just a few atoms join together in this way. • . covalent bonds © Boardworks Ltd 2003 Covalent bonding and electron structures H Cl Cl H C H H H N H Cl H H © Boardworks Ltd 2003 Covalent bonding in chlorine Chlorine (2.8.7) needs 1 more electron to attain a full electron shell. Cl Cl (2,8,7) (2,8,7) Cl-Cl Cl (2,8,8) Cl (2,8,8) © Boardworks Ltd 2003 Both fluorine and chlorine needs 1 more electron to attain a full electron shell. F (2,7) Cl Copy this diagram and add the electron arrangements that could exist in fluorine chloride (FCl). (2,8,7) F (2,8) Cl (2,8,8) © Boardworks Ltd 2003 Covalent bonding in hydrogen chloride Both hydrogen (1) and chlorine (2.8.7) needs 1 more electron to attain a full outer shell. H Cl (1) (2,8,7) H-Cl H Cl (2) (2,8,8) © Boardworks Ltd 2003 Covalent bonding in water Hydrogen (1) needs 1 more electron but oxygen (2.6) needs 2 more. Therefore, we need 2 hydrogens. H O H O H H O H H © Boardworks Ltd 2003 • Hydrogen (1) needs 1 more electron. • How many does nitrogen (2.5) need? 3 • How many hydrogens per 1 nitrogen? 3 • Draw bonding diagrams for ammonia. H N H H H N H H © Boardworks Ltd 2003 • Hydrogen (1) needs 1 more electron. • How many does carbon (2.4) need? 4 • How many hydrogens per 1 carbon? 4 • Draw bonding diagrams for methane. H H H C H H H C H H © Boardworks Ltd 2003 Covalent bonding in oxygen Oxygen (2.8.6) needs 2 more electrons to attain a full electron shell. O O 4 electrons O O=O O © Boardworks Ltd 2003 Nitrogen (2.8.5) needs 3 more electrons to attain a full electron shell and forms a triple bond. Draw a bonding diagram of nitrogen. 6 electrons N N=N N N N © Boardworks Ltd 2003 Draw ‘dot and cross’ type bonding diagrams for Bohr Models each of the following: 1. Hydrogen fluoride (HF) 2. Hydrogen sulphide (H2S) 3. Ethane (C2H6 and the carbons are joined by a single covalent bond) 4. Carbon dioxide (CO2 and the carbon oxygen bonds are double bonds) H F H H S H H H C C H H H O C O © Boardworks Ltd 2003 Giant covalent structures: diamond • One form of carbon is diamond. Diamond strong covalent bonds • Each diamond consists of millions of carbon atoms bonded into a single giant structure. • It is very hard. carbon atoms © Boardworks Ltd 2003 BONDING AND PHYSICAL PROPERTIES © Boardworks Ltd 2003 Bonding and physical properties The type of structure that substances have has a huge effect upon physical properties. These are things such as: • • • • Density Conductivity Malleability/ brittleness Melting point The next few slides illustrate just a few of the general patterns. © Boardworks Ltd 2003