Classwork Series and Exercises {Chemistry – SS1}: Chemical Combination

Chemistry SS 1 Week 5

Topic: Chemical Combination

Introduction

A chemical bond is the result of an attraction between atoms or ions. Chemical compounds are formed by the joining of two or more atoms. The types of bonds that a molecule contains will determine its physical properties, such as melting point, hardness, electrical and thermal conductivity, and solubility.

The two extreme cases of chemical bonds are:

• Covalent bond: bond in which one or more pairs of electrons are shared by two atoms.
• Ionic bond: bond in which one or more electrons from one atom are removed and attached to another atom, resulting in positive and negative ions which attract each other.

Other types of bonds include co-ordinate covalent, metallic bonds and hydrogen bonding. The attractive forces between molecules in a liquid can be characterized as van der Waals bonds.

Covalent Bonds

Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.

Hydrogen gas forms the simplest covalent bond in the diatomic hydrogen molecule. The halogens such as chlorine also exist as diatomic gases by forming covalent bonds. The nitrogen and oxygen which makes up the bulk of the atmosphere also exhibits covalent bonding in forming diatomic molecules.

Examples of covalent bonding

Chlorine

For example, two chlorine atoms could both achieve stable structures by sharing their single unpaired electron as in the diagram.

The two chlorine atoms are said to be joined by a covalent bond. The reason that the two chlorine atoms stick together is that the shared pair of electrons is attracted to the nucleus of both chlorine atoms.

Note: The fact that one chlorine has been drawn with electrons marked as crosses and the other as dots is simply to show where all the electrons come from. In reality there is no difference between them.

Hydrogen

Hydrogen atoms only need two electrons in their outer level to reach the noble gas structure of helium. Once again, the covalent bond holds the two atoms together because the pair of electrons is attracted to both nuclei.

Hydrogen chloride

The hydrogen has a helium structure, and the chlorine an argon structure

Other examples:

Even with a more complicated molecule like PCl₃, there’s no problem. In this case, only the outer electrons are shown for simplicity. Each atom in this structure has inner layers of electrons of 2,8. Again, everything present has a noble gas structure.

Ionic Bonds (Electrovalent Bonds)

In an ionic bond, an electron is actually transferred from the less electronegative atom (metals) to the more electronegative element (non-metals). Ionic bonds are the result of an electrostatic attraction between ions that have opposite charges; in other words, cations and anions. Ionic bonds usually form between metals and nonmetals. Ionic bonds are very strong, so compounds that contain these types of bonds have high melting points and exist in a solid state under standard conditions.

Ionic bonding in sodium chloride

Sodium (2,8,1) has 1 electron more than a stable noble gas structure (2,8). If it gave away that electron it would become more stable.

Chlorine (2,8,7) has 1 electron short of a stable noble gas structure (2,8,8). If it could gain an electron from somewhere it too would become more stable.

The answer is obvious. If a sodium atom gives an electron to a chlorine atom, both become more stable.

The sodium has lost an electron, so it no longer has equal numbers of electrons and protons. Because it has one more proton than electron, it has a charge of 1+. If electrons are lost from an atom, positive ions are formed. Positive ions are sometimes called cations.

The chlorine has gained an electron, so it now has one more electron than proton. It therefore has a charge of 1-. If electrons are gained by an atom, negative ions are formed. A negative ion is sometimes called an anion.

The sodium ions and chloride ions are held together by the strong electrostatic attractions between the positive and negative charges. One sodium atom to provide the extra electron for one chlorine atom, so they combine together 1:1. The formula is therefore NaCl.

Some other examples of ionic bonding

Magnesium Oxide

Again, noble gas structures are formed, and the magnesium oxide is held together by very strong attractions between the ions. The ionic bonding is stronger than in sodium chloride because this time you have 2+ ions attracting 2- ions. The greater the charge, the greater the attraction.

The formula of magnesium oxide is MgO.

Calcium Chloride

This time you need two chlorines to use up the two outer electrons in the calcium. The formula of calcium chloride is therefore CaCl₂.

Potassium Oxide

Again, noble gas structures are formed. It takes two potassium to supply the electrons the oxygen needs. The formula of potassium oxide is K₂O.