TOPIC 4

How Atoms Combine

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

Covalent Bonding

Let us consider two Chlorine atoms joiningCl Cl

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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 It is the attraction of the outer unpaired electrons outer unpaired electrons

for the nearby atoms for the nearby atoms positive nucleus that pulls positive nucleus that pulls

the atoms togetherthe atoms together

Cl -Cl

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

Na ClElectron 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-

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

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

Al Cl3+ -

Al1

Cl3

Al 3+ Cl -( )3

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 Zn2+(Cl-)2

Iron(III)Oxide - here the valency of Iron is three ie Fe3+

2O2-3

Complex ions A complex ion is formed when atoms collect together

to form one ion.

eg Sulphate – SO42- Nitrate - NO3

-

Carbonate – CO 3 2 -

Phosphate- PO4 3-

When writing formulae with complex ions the complex ion should be treated just the same as a single ion

Na+

SO42-( )

2 eg

Cu2+

NO3-( )

2