Chemical Combinations – SS1 Chemistry Lesson Note (First Term)

Chemical combination is the joining of atoms through bonds to form compounds.

• Ionic bond: forms by the transfer of electrons from metal atoms to non-metal atoms.
• Covalent bond: forms by the sharing of electron pairs.
• Metallic bond: forms between metal ions and a cloud of free electrons.
• Weaker forces: 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:

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

What are the types of chemical bonds and forces?

There are four types of chemical bond (ionic, covalent, coordinate covalent and metallic) and two weak intermolecular forces (hydrogen bonds and van der Waals’ forces).

What is an electrovalent (ionic) bond?

An ionic bond is the strong electrostatic attraction between oppositely charged ions formed by the transfer of electrons from a metal to a non-metal.

• Metal atom: donates electrons and becomes a positive ion (cation).
• Non-metal atom: accepts electrons and becomes a negative ion (anion).
• Examples: sodium chloride (NaCl) and calcium oxide (CaO).

How are ionic compounds formed (electron dot representation)?

Sodium chloride and calcium oxide form when a metal atom gives its outer electrons to a non-metal atom.

1. Sodium (2,8,1) loses one electron to form Na⁺ (2,8).
2. Chlorine (2,8,7) gains that electron to form Cl^– (2,8,8).
3. The oppositely charged ions attract to form NaCl.
4. Calcium (2,8,8,2) loses two electrons to form Ca²⁺.
5. Oxygen (2,6) gains two electrons to form O^2-.
6. The two ions attract to form CaO.

What are the properties of ionic compounds?

1. Physical state: they are solids at room temperature.
2. Particles: they contain oppositely charged ions.
3. Solubility: they readily dissolve in water and other polar solvents like ethanol.
4. Melting and boiling points: they are high.
5. Conductivity: they conduct electricity when molten or in aqueous solution.

What is a covalent bond?

A covalent bond is the sharing of a pair of electrons between two reacting atoms.

• Shared pair: each atom contributes one electron to the pair.
• Representation: a shared pair is drawn as a short line (-) between the two atoms.
• Single bond: one shared pair, as in H₂.
• Double bond: two shared pairs, as in CO₂.
• Triple bond: three shared pairs, as in N₂.

How are covalent compounds formed (electron dot representation)?

Hydrogen and carbon(IV) oxide form when atoms share electrons to complete their outer shells.

1. Each hydrogen atom contributes one electron to make a shared pair, H-H.
2. Carbon shares two electrons with each oxygen atom, giving two double bonds, O=C=O.

What are the properties of covalent compounds?

1. Structure: they consist of molecules with a definite shape.
2. Physical state: they are usually gases or volatile liquids.
3. Solubility: they readily dissolve in non-polar organic solvents.
4. Melting and boiling points: they are low.
5. Conductivity: they do not conduct electricity because the molecules contain no charged particles.

How do ionic and covalent compounds differ?

Ionic compounds are made of ions held by electrostatic attraction, while covalent compounds are made of molecules held by shared electron pairs.

What is a coordinate covalent (dative) bond?

A coordinate covalent bond is a covalent bond in which one of the combining atoms supplies the whole shared pair of electrons.

• Lone pair: a pair of electrons not directly concerned with an existing bond.
• Donor: the atom with the lone pair supplies both shared electrons.
• Acceptor: the other species, such as H⁺, shares the pair without supplying any electron.

How is a dative bond formed (electron dot representation)?

The hydroxonium and ammonium ions form when a lone pair on a molecule is shared with a hydrogen ion.

• Hydroxonium ion (H₃O⁺): a water molecule shares a lone pair on oxygen with H⁺.
• Ammonium ion (NH₄⁺): an ammonia molecule shares the lone pair on nitrogen with H⁺.

What are the properties of coordinate covalent compounds?

• Similarity: they are similar in properties to purely covalent compounds.
• Conductivity: they do not conduct electricity.
• Volatility: 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 that exists when hydrogen is covalently bonded to a small, highly electronegative element: nitrogen, oxygen or fluorine.

How does a hydrogen bond form?

1. The electronegative atom (N, O or F) strongly attracts the shared pair in the covalent bond.
2. A dipole forms: the hydrogen atom is partly positive and the electronegative atom is partly negative.
3. The positive pole of one molecule attracts the negative pole of a neighbouring molecule.
4. This electrostatic attraction between dipoles is the hydrogen bond.

What is the importance of the hydrogen bond?

• Solubility: it explains why some compounds containing O, N and F dissolve in hydrogen-containing solvents such as water.
• Ice: the crystalline shape of solid water is due to hydrogen bonds.

What is a metallic bond?

A metallic bond is the force of attraction between positive metal ions and free mobile electrons.

• Lattice: metal atoms are held together in a solid crystal lattice.
• Electron cloud: each atom gives its outer (valence) electron to a shared cloud and becomes a positive ion.
• Stability: the positive ions repel one another, but the moving electron cloud and overlapping residual electron orbits hold them together.

What are van der Waals’ forces?

Van der Waals’ forces are the weak attractive forces that allow non-polar molecules such as nitrogen and CO₂ to form liquids and solids.

• Strength: they are very weak compared with ionic and covalent bonds.
• Liquefaction: they make it possible to liquefy gases.
• Molecular lattices: they hold molecules together in iodine and naphthalene crystals.

What are the states of matter?

The three states of matter are solid, liquid and gas, and they differ in particle motion and the attractive force between particles.

How does matter change state?

Matter changes state when it gains or loses heat energy.

What is melting?

Melting is the change of a substance from a solid to a liquid, and the temperature at which it happens is the melting point.

1. Heating gives the solid particles more kinetic energy, so they vibrate more strongly.
2. The vibrations overcome the cohesive forces holding the particles together.
3. The crystalline structure collapses.
4. The particles are free to move about, and the liquid state is reached.

What is boiling?

Boiling is the change of a liquid to vapour throughout its volume when its saturated vapour pressure equals the atmospheric pressure.

• Boiling point: the temperature at which a liquid boils.
• Heating: as a liquid is heated, its rate of evaporation increases until boiling begins.
• Effect of pressure: raising the pressure raises the boiling point, and lowering the pressure lowers it.
• Effect of impurities: impurities increase the boiling point of a liquid.

What is evaporation?

Evaporation is the change of a liquid to vapour at its surface at any temperature.

1. Molecules near the exposed surface gain extra kinetic energy.
2. They break away from the cohesive force of neighbouring particles.
3. They escape from the surface as vapour.

• Effect on the liquid: its volume decreases and its temperature falls, so evaporation causes cooling.
• Temperature: it occurs at all temperatures but is faster at higher temperatures.
• Type of liquid: it is slower in electrovalent liquids than in covalent liquids.

What are the differences between evaporation and boiling?

Evaporation is surface vaporisation at any temperature, while boiling is vaporisation throughout the liquid at its boiling point.

What is condensation?

Condensation is the change of a vapour into a liquid when it loses kinetic energy to a colder body.

What is freezing?

Freezing is the change of a liquid into a solid at its freezing point.

1. The liquid loses heat energy to its surroundings, so its temperature drops.
2. Cooling continues until the temperature reaches the freezing point.
3. At this temperature, the liquid changes into a solid.

What is the kinetic theory of gases?

The kinetic theory of gases explains the behaviour of an ideal (perfect) gas in terms of its moving molecules.

What are the assumptions of the kinetic theory?

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

What phenomena support the kinetic theory of gases?

• Brownian motion: the constant, irregular movement of tiny particles suspended in a liquid or gas, which shows that the molecules around them are in constant motion.
• Diffusion: the movement of particles from a region of higher concentration to a region of lower concentration, common in gases and due to random movement of the particles.

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

In your notebook, answer all three parts:

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.