2. TOPIC 4 How Atoms Combine

3. Covalent Bonding  When Atoms join they do so by a Chemical Bond  When non - metal atoms join they form a Covalent Bond  When Atoms join it is their outer electrons that are directly affected  Remember that atoms join so as to try and achieve their nearest most stable electron arrangement.  To try to do this non-metal atoms overlap and share Electron Clouds

4. Covalent Bonding  Let us consider two Chlorine atoms joining Cl ** * * * * * Each Chlorine atom has seven electrons in its outer energy level Chlorine atoms overlap and electrons are shared equally It is the attraction of the outer unpaired electrons for the nearby atoms positive nucleus that pulls the atoms together Cl -Cl

5. Ionic Bonding  Ionic Bonding occurs between Metal and a Non-Metal atoms.  They do so this to get to their nearest most stable Electron Arrangement ie. 2,2,8 or 2,8,8 etc. Consider Sodium and Chlorine joining together NaCl Electron arrangement of each atom 2,8,1( 11p, 11e) 2,8,7( 17p,17e) Na wants to get to an EA of 2,8 Cl wants to get to an EA of 2,8,8 In order to do this an electron is transferred from the metal to the non-metal e This leaves the Na with one less electron and there fore positively charged and the and the Cl atom with one more and therefore negatively charged. Opposite charges attract to give Na + -Cl -

6. Valency  The Valency is the Combining Power of an atom.  It is calculated form the number of electrons that an atom wishes to gain or lose so as to get to its most stable Electron Arrangement  Group 1, 2 and 3 metals have a valency of 1+,2+ and 3+ respectively. Groups 7, 6 and 5 have valencies of 1-,2- and 3- since they are more likely to receive electrons in order for them to get to their most stable Electron Arrangement.  Group 0 elements have valency of zero since they are already in their most stable EA.  Valencies are used to write down Chemical Formulae

7. Chemical Formulae  Rules for Writing Chemical Formulae  1. Write it down in WORDS  2. Put in the chemical symbols.  3. Write in the valencies  4. Balance the valencies by cross multiplying.  5. Write down the formula Write down the chemical formula for Aluminium Chloride Aluminium Chloride Al Cl AlCl 3+ - Al 1 Cl 3 Al 3+ Cl - () 3

8. Valency of Transition Metals  Transition Metals can have more than one valency. Valencies of 1,2,3,4,5,6 & 7+ are not uncommon. When writing a chemical formula you will be told the valency to use. It will be in the form of a Roman Numeral  eg Zinc( II )Chloride – here the valency of Zinc is two ie Zn 2+ (Cl - ) 2 Iron( III )Oxide - here the valency of Iron is three ie Fe 3+ 2 O 2 - 3

9. Complex ions  A complex ion is formed when atoms collect together to form one ion.  egSulphate – SO 4 2 - Nitrate - NO 3 - Carbonate – CO 3 2 - Phosphate- PO 4 3 - When writing formulae with complex ions the complex ion should be treated just the same as a single ion Na + SO 4 2 - ( ) 2 eg Cu 2+ NO 3 - ( ) 2