Chemistry, Third edition

48 4 · BONDING BETWEEN ATOMS

(see Box 4.1). The chlorine atom that has accepted an electron now has 17 protons

and 18 electrons, so it has an overall charge of 1, is written as Cland is called a

chloride ion.

Note that once a sodium atom has given an electron to a chlorine atom, that electron is

indistinguishable from the other electrons present in the outer shell of the chloride ion. All

eight electrons now ‘belong’ to the chloride ion. Crosses and dots are used to represent the

electrons, simply to show which atoms they originated from. The use of a cross or a dot,

does not imply that the electrons are different.

These changes are shown below:

Before

Na atom Cl atom

electrons = 11 (1) electrons = 17 (1)

protons = 11 (+1) protons = 17 (+1)

Neutral Neutral

After

Na+ion Clion

electrons = 10 (1) electrons = 18 (1)

protons = 11 (+1) protons = 17 (+1)

Charge = +1 Charge =  1

Sodium chloride is therefore composed of Na+and Clions in a 1:1 ratio. The com-

pound is called an ioniccompound (because it is composed of ions), and the bond-

ing in the compound is termed ionic. Ionic compounds are formed whenever one

atom (or group of atoms) transfers electrons to another. Note that, when sodium

and chlorine react, they both end up with the electron configuration of a noble gas

but they do not change into noble gases; they are still the same elements. This is because

the identity of an atom depends on the number of protons in its nucleus and, as you

can see, these remain the same for sodium and chlorine both before and after the

reaction.

The ‘ordinary’ chemical equation for this reaction tells us that the product of the

reaction is sodium chloride, but gives no information about the bonding in the com-

pound formed:

2Na(s) + Cl 2 (g)2NaCl(s)

For this reason, it is better written as

2Na(s) + Cl 2 (g)2Na+, 2Cl(s)

BOX 4.1

Ions

Ionsare atoms (or groups of atoms) that carry electrical charges. Examples of ions are K+, F,

CO 32 , N^3 , Ca2+and Al3+.

Positively charged ions (e.g. K+, Ca2+and Al3+) are called cations, whereas negatively charged

ions (e.g. F, CO 32 and N^3 ) are called anions.