Structure and bonding
YOUR SHOULD be able to:
■ Explain the formation of ionic and covalent bonds.
■ Predict the formation of ionic and covalent bonds based on atomic structure.
■ Write formulae to represent ions and molecules.
Only electrons are involved in bonding; protons remain unchanged in the nucleus.
Atoms bond in order to have a full shell (or an octet of electrons) which makes them stable. When atoms have a full shell, we say they have attained a noble gas configuration. Atoms can bond in two main ways. They can share electrons or they may transfer electrons from one atom to another. Losing or gaining electrons results in the formation of an ionic or electrovalent bond, and occurs between metals and non-metals. It is the outer, or valence, electrons only that are involved in bonding.
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.
Ionic or electrovalent bonding involves the transfer of electrons between two atoms. One atom loses electrons (gives up) and the other atom gains (accepts) electrons. This type of bonding usually occurs between a metal and a non-metal. The metal loses electrons to form cations and the non-metal gains electrons to form anions.
Cations are positively charged ions. Anions are negatively charged ions. In an ionic compound, the negative and positive charges cancel out each other. The ions in the magnesium oxide forms an ionic lattice of negative and positive ions which attract each other.
Covalent bonding is shown in substances such as methane, ammonia and water.
QUICK CHECK
What are the differences in properties between an ionic and a covalent compound?
FURTHER PRACTICE
Draw diagrams to show bonding between the following: i. Carbon and oxygen ii. Calcium and chlorine