Chemical Combinations – SS1 Chemistry Lesson Note (First Term)

Chemical combination happens when atoms join through bonds. Ionic bonds form by the transfer of electrons from metal to non-metal atoms, covalent bonds by the sharing of electron pairs, and metallic bonds by a cloud of free electrons. Weaker forces, such as hydrogen bonds and van der Waals’ forces, act between molecules and affect the states of matter.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe the formation of ionic, covalent, coordinate covalent, hydrogen and metallic bonds;
• state the properties of ionic and covalent compounds;
• explain van der Waals’ forces and the importance of the hydrogen bond;
• explain melting, boiling, evaporation, condensation and freezing;
• state the assumptions of the kinetic theory of gases and the phenomena that support it.

What is an electrovalent (ionic) bond?

An electrovalent bond is characterised by the transfer of electrons from metallic atoms to non-metallic atoms during the reaction. The metallic atom that donates electrons becomes positively charged, while the non-metallic atom that accepts electrons becomes negatively charged. The strong electrostatic attraction that holds the oppositely charged ions together is called an ionic bond.

Electron dot representation of the formation of ionic compounds

Formation of sodium chloride and calcium oxide:

Properties of some ionic compounds

1. They are solids at room temperature.
2. They contain oppositely charged ions.
3. They readily dissolve in water and other polar solvents like ethanol.
4. They have high melting and boiling points.
5. They are good conductors of electricity when in molten or aqueous form.

What is a covalent bond?

A covalent bond involves the sharing of a pair of electrons between two reacting atoms. The shared electrons are each contributed by the reacting atoms and are called shared pairs. A shared pair of electrons in a covalent bond is represented by a short horizontal line (-) between the two atoms.

Electron dot representation of the formation of covalent compounds

Formation of hydrogen molecule and carbon(IV) oxide:

Properties of covalent compounds

1. They consist of molecules with a definite shape.
2. They are gases or volatile liquids.
3. They readily dissolve in non-polar organic solvents.
4. They have low melting and boiling points.
5. They do not conduct electricity because the molecules do not contain charged particles.

What is a coordinate covalent (dative) bond?

In a coordinate covalent bond, the shared pair of electrons is supplied by one of the combining atoms. A coordinate covalent bond is often formed in molecules that have a lone pair of electrons, that is, a pair of electrons not directly concerned with an existing bond.

Electron dot representation of the formation of a dative bond

Formation of hydroxonium ion (H₃O⁺) and ammonium ion (NH₄⁺):

Compounds containing coordinate covalent bonds are similar in properties to purely covalent compounds. Neither type conducts electricity, but the presence of a coordinate covalent bond tends to make a compound less volatile.

What is a hydrogen bond?

A hydrogen bond is a dipole-dipole intermolecular force of attraction which exists when hydrogen is covalently bonded to a highly electronegative element of small atomic size. The electronegative element can be N, O or F.

The highly electronegative element has a very strong affinity for electrons. Hence, it attracts the shared pair of electrons in the covalent bond towards itself. This forms a dipole, with a partial positive charge on the hydrogen atom and a partial negative charge on the electronegative atom. An electrostatic attraction between two dipoles is set up when the positive pole of one molecule attracts the negative pole of the other. This attractive force is known as a hydrogen bond.

Importance of the hydrogen bond

1. It accounts for the solubilities of some compounds containing O, N and F in certain hydrogen-containing solvents such as water.
2. The crystalline shape of solid water (ice) is due to hydrogen bonds.

What is a metallic bond?

Metal atoms are held together in a solid crystal lattice by metallic bonds. Each metallic atom contributes its outer (valence) electron to the electron cloud, thus becoming positively charged. The resulting positively charged metallic ions tend to repel each other but are held together by the moving electron cloud and overlapping residual electron orbits. Thus, a metallic bond is a force of attraction between the positive metal ions and the free mobile electrons.

What are van der Waals’ forces?

The attractive forces which make it possible for non-polar molecules like nitrogen and CO₂ to form liquids and solids are called van der Waals’ forces. These forces are very weak when compared to ionic and covalent bonds. They are important in the liquefaction of gases and in the formation of molecular lattices, as in iodine and naphthalene crystals.

What are the states of matter?

The three states of matter, solid, liquid and gaseous, can be distinguished by the motion of the particles they are made of and the attractive force between their particles.

How does matter change state?

Melting

Melting is the physical process where a substance changes from a solid to a liquid. When a solid is heated, the particles acquire greater kinetic energy and move violently. A point is reached when the forces of vibration overcome the cohesive forces holding the solid particles together, and the crystalline structure collapses. The particles are no longer held in fixed positions but are free to move about, and the liquid state is reached. The temperature at which this occurs is called the melting point of the solid.

Boiling

When a liquid is heated, the rate of evaporation increases until the saturated vapour pressure equals the prevailing atmospheric pressure. When this happens, the liquid is said to boil, and the temperature at which this happens is known as the boiling point of the liquid.

The boiling point of a liquid changes with a change in atmospheric pressure. If the pressure is raised, the boiling point increases, and if the pressure is lowered, the boiling point decreases. The presence of impurities also increases the boiling point of a liquid.

Evaporation

Evaporation is the process of vaporisation of liquids at all temperatures. When the surface of a liquid is exposed, the molecules near the surface acquire extra kinetic energy, large enough to enable them to break away from the cohesive force binding them to the neighbouring particles. Once free, they escape from the liquid surface to become molecules in the vapour state.

Evaporation results in a decrease in the volume of liquid and lowers the temperature of the liquid, so it causes cooling. It occurs at all temperatures but increases with an increase in temperature. In addition, it is slower in electrovalent liquids than in covalent liquids.

Differences between evaporation and boiling

Condensation and freezing

Condensation is a process whereby a vapour loses some of its kinetic energy to a colder body and changes into a liquid.

When a liquid cools, it loses heat energy to its surroundings, causing its temperature to drop. If the cooling continues, the temperature keeps dropping until it reaches the freezing point of the liquid. At this temperature, the liquid changes into a solid.

What is the kinetic theory of gases?

The theory postulates the following for an ideal or perfect gas:

1. Gas molecules are in constant, rapid, straight motion, colliding with one another and with the walls of the container.
2. The collision of gas molecules is perfectly elastic.
3. The total volume of the gas molecules is negligible compared to the volume of the container.
4. The force of attraction between the gas molecules is negligible.
5. The average kinetic energy of the molecules is a measure of the temperature of the gas.

Phenomena supporting the kinetic theory of gases

1. Brownian motion: This is the constant, irregular movement of particles in a liquid or gas. It shows that gas molecules are in constant motion.
2. Diffusion: Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. It is common in gases and results from the random movement of the particles of a gas.

Evaluation

1. Explain how an ionic bond forms between a metal atom and a non-metal atom.
2. State three properties of covalent compounds.
3. How does a coordinate covalent bond differ from an ordinary covalent bond?
4. State two differences between evaporation and boiling.
5. List the assumptions of the kinetic theory of gases.

Assignment

1. Define a metallic bond and a hydrogen bond.
2. State two ways in which ionic compounds differ from covalent compounds.
3. Explain why the boiling point of a liquid changes when atmospheric pressure changes.