Chemical Bonding 2

COVALENT BONDING

Sharing electrons

When two non-metals react together, they both need to gain electrons to complete full outer shells. The only way this can be achieved is if they share their outer electrons

Hydrogen: Each hydrogen atom has only one electron and needs one more to complete its first shell. When two hydrogen atoms get close together their shells can overlap and then they can share their electrons.

Since, electrons are being shared, there is a strong force of attraction between them. This force is a covalent bond.

The bonded atoms form molecules. Hydrogen's molecular formula is H₂.

Chlorine: A chlorine atom needs a share of one other electron to obtain a full outer shell. If two chlorine atoms are placed together the result is as shown below:

Oxygen: Each oxygen atom requires a share of two electrons.

Since each oxygen atom has a share of two pairs of electrons, we call this a double covalent bond.

Covalent compounds

There is a vast number of compounds that exist as molecules.

Water: In each molecule, H₂O, one oxygen atom shares electrons with two hydrogen atoms.

Ammonia: In each molecule, NH₃, one nitrogen atom shares its electrons with three hydrogen atoms, so that they all reach full shells.

Methane: Its formula is CH₄. One carbon atom shares its electrons with four hydrogen atoms.

2. TYPES OF SOLID

The four types that we shall study in this quick learn are:

1. Metals

2. Ionic

3. Molecular

4. Giant molecular

The metals

In a metal, the atoms are very tightly packed, leaving little space between them. Due to this tight packing, the outer electrons become delocalised from their atoms. This results in a 'sea' of electrons around a lattice of ions or 'pseudo' cations.

Properties of metals

Here are some general properties, but remember there are always exceptions!

1. They are hard.
2. They are tough.
3. They are not easily compressed.
4. High tensile strength - not easily stretched.
5. Malleable - can be bent or hammered into a shape.
6. Ductile - can be drawn into wires.
7. Good conductors of heat and electricity because of sea of electrons that can move around the lattice carrying heat energy or charge.
8. Usually high melting points.

Ionic solids

Ionic solids are made up of a lattice composed of oppositely charged ions. One of the most common ionic solids is sodium chloride. Sodium chloride is made up of sodium and chloride ions packed - a lattice. The ions are held by electrostatic charges in an ionic bond.

Properties of ionic solids

1. High melting points and boiling points due to strong ionic bonds. Most are solids at room temp.
2. They are brittle - will shatter with a hammer.
3. Usually soluble in water. Insoluble in non-polar solvents.
4. Do not conduct electricity in solid state. They do conduct when molten or dissolved in water since the ions are free to carry the charges as the ionic bonds do not hold them firmly in the liquid state.

Molecular solids

In a molecular solid, the molecules are held together by weak Van Der Waal's force, but packed in a regular pattern. Iodine is an example of a molecular solid. Each iodine molecule is made up of 2 iodine atoms, held together by a strong covalent bond. Each iodine molecule is held to another by weak Van Der Waal's forces.

Properties of molecular solids

1. Low melting and boiling point due to weak forces between molecules.
2. They are brittle.
3. Insoluble in water but soluble in non-polar solvents such as tetrachloromethane and petrol.
4. Do not conduct electricity. Molecules do not carry a charge so even when melted, molecular solids cannot conduct.

Giant molecular solids

Diamond and graphite

Diamond: Is made up of a lattice of carbon atoms. Each carbon atom can make 4 covalent bonds to 4 other carbon atoms. Each outer atom then bonds to 3 more and so on. Eventually millions of carbon atoms are bonded to form a giant lattice.

Properties of diamond

1. Very hard - hardest known substance. Each atom held to 4 others by strong covalent bonds - this explains the high melting point.
2. Does not conduct electricity due to no ions or free electrons to carry charge.

Graphite: Graphite is made up of flat sheets of carbon atoms.

Each carbon atom makes three covalent bonds to other carbon atoms. T his gives rings of 6 atoms. The flat sheets that lie on top of each other are held by weak forces - Van Der Waal's.

Properties of graphite

1. It is soft and slippery due to sheets of atoms been able to slide over one another because of weak forces between them.
2. A good conductor of electricity. This is due to each atom only using 3 out of 4 outer electrons in bonding. The fourth electron of each atom becomes delocalised throughout the lattice, enabling graphite to carry charge.
3. High melting point due to strong covalent bonds holding atoms of carbon together in the rings.