Structure and bonding
Francine Taylor-Campbell, Contributor

Why do atoms bond?

Atoms participate in bonding in order to obtain a filled outer electron shell, similar to that of noble gases in Group 8/O. To bond, atoms may either lose, gain or share electrons. Losing or gaining electrons results in the formation of an ionic or electrovalent bond and occurs between metals and non-metals. The sharing of electrons results in the formation of a covalent bond, and this occurs between non-metals.

Metals with one, two or three electrons in their outer shell will readily give up or lose their electrons (and form positively charged ions called cations) in order to be stable or to have a filled outer shell. Non-metals with five, six or seven electrons in their outer shell will readily accept or gain electrons (and form negatively charged ions called anions) to acquire a noble gas configuration. Two or more non-metals with four to seven electrons in their outer shell may share their electrons in order to have a stable arrangement.

How would magnesium and oxygen bond?

Take a look at magnesium: E.C. = 2:8:2 (atomic # = 12)

This means that magnesium has 12 electrons with two in the outer shell. Magnesium is, therefore, in Group 2, but since the outer shell can hold a maximum of eight electrons for it to be filled, then magnesium is not stable and either needs to gain six more electrons (which is harder to do) or lose the two outer electrons to be stable (this is more favourable).

When magnesium loses the two electrons, its electronic configuration will be: E.C. = 2:8 (# electrons =10 and atomic number/# protons = 12).

Remember, only electrons are involved in bonding; protons remain unchanged in the nucleus.

Since magnesium has 10 electrons (with negative charge -10) and 12 protons (with positive charge +12), then the overall charge on the magnesium ion is +2 (+12 - 10 = +2). In other words, Mg has two more protons than electrons - Mg2+.

Positive ions generally exist with negative ions around it, so Mg would have to give up those two electrons to another element that needed it to be stable. For example: oxygen has an electronic configuration E.C. = 2:6 (atomic # =8). This means that oxygen is in Group 6 and needs two more electrons in order to have eight in the outer shell and become stable.

When O gains or accepts two electrons, it will now have 10 electrons (-10) and 8 protons (+8). The overall charge on O is now -2 (+8 - 10 = -2). O² has an E.C. = 2:8.

An ionic or electrovalent bond is formed between Mg and O.

Mg^xx --------> Mg²⁺ + 2e- (lost 2 electrons)

12p, 12e ------ 12p, 10e

xx

x O x + 2e- O² (gained 2 electrons)

xx

8p, 8e --------- 8p, 10e

So, the formula of the compound formed between magnesium and oxygen is Mg²⁺ O² = MgO (+2-2 = 0 the charges cancel out) magnesium oxide.

What type of bond exists between carbon and oxygen?

Consider the bond between carbon and oxygen. It is difficult and energetically unfavourable for C 2:4 or O 2:6 to give up any electrons. It is easier for them to share these outer electrons. O needs two and C needs four; hence, two oxygen atoms can share with one carbon atom.

C 2:4 + O 2:6 = OCO ( THE MOLECULE IS CO₂)

This sharing of electrons between two non-metals results in the formation of a covalent bond.

During our tour of the science lab, 'Youthlink' caught this student writing down the results of her experiment. - FILE

Francine Taylor-Campbell is an independent contributor.