C2 Revision

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C2 Revision:

Relative Formula Mass:

The relative formula mass (Mr) of a compound is all the relative atomic masses in its formula added together.

Example:

MgCl2 = Mg + (2xCl) = 24.3 + (2x35.5) = 95.3

It's that simple!

Covalent Bonding:

Covalent bonds = sharing electrons.

When non-metal atoms combine together they form covalent bonds by sharing pairs of electrons. This way, both atoms have a full outer shell and they become stable.

Covalent bonds are strong because there is a strong electrostatic bond between the positive nuclei of atoms and the negative electrons in each pair.

Usually, each atom involved makes enough covalent bonds to fill up its outer shell and become stable. You can use dot and cross diagrams to represent this:

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Ionic Bonding:

Simple ions form when atoms lose or gain electrons. Ions are charged particles and when atoms lose or gain electrons to form ions, they are trying to get a full outer shell.

When metals form ions, they lose electrons to form positive ions.

When non-metals form ions, they gain electrons to form negative ions.

The number of electrons lost or gained is the same as the charge on the ion: losing two = 2+, gaining three = 3-

Which groups are most likely to form ions?

Groups 1 and 2 and 6 and 7 are the most likely to form ions. Group 1 and 2 are metals, so lose electrons to form positive ions. Group 6 and 7 are non-metals, so gain electrons to form negative ions.

Elements in the same group all have the same number of outer electrons, so they have to lose or gain the same number to get a full outer shell, meaning that they form ions with the same charge! Group 1 = 1+, group 2 = 2+, group 6 = 2- and group 7 = 1-

As you go down each group, you add electrons to the outer shell and they get further from the nucleus. For group 1 and 2, this means that it gets easier to remove the outer electrons to form ions as you go down the group. Elements get more reactive as you go down groups 1 and 2 because they do not have strong bonds with the nucleus as they are further away.

However, for groups 6 and 7, it gets harder to gain electrons to form ions because the elements get less reactive as you go down groups 6 and 7.

Ions with opposite charges form ionic bonds:

When a metal and a non-metal react together, the metal loses electrons to form a positively charged ion and the non-metal gains electrons to form a negatively charged ion. These oppositely charged ions are then strongly attracted to one another by electrostatic forces and form an ionic bond.

To find the formula of an ionic compound, you balance out the positive and negative charges:

Sodium Chloride = Na+ + Cl- --> NaCl

Magnesium Chloride = Mg2+ + 2Cl- --> MgCl2

Potassium Oxide = 2K+ + O2- --> K2O

Alike to covalent bonding, you can show ionic bonding using dot and cross diagram. They show the arrangement of electrons in an atom or ions but also show what happens to the electrons when atoms react with each other.

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Structure and Bonding of Ionic and Covalent Compounds:

Ionic compounds:

Ionic compounds always have giant ionic lattice structures. The ions form a closely packed regular lattice. There are very strong electrostatic forces of attraction between oppositely charged ions.

A single crystal of sodium chloride (salt) is one giant ionic lattice. The Na+ and Cl- ions are held together in a regular lattice.

Ionic Compound Properties:

Ionic compounds have high melting and boiling points due to the strong attraction between the ions. It takes a large amount of energy to overcome this attraction.

Solid ionic compounds don't conduct electricity because the ions are fixed in place and can't move. When an ionic compound melts, the ions are free to move and will carry an electric charge.

A lot also dissolves easily in water. The ions separate and are all free to move in the solution so they carry an electric charge.

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Covalent Compounds:

Covalent bonds are simple molecular substances. They have low melting and boiling points e.g. H20 and CO2. The atoms within the molecules are held together by very strong covalent bonds. However, the forces of attraction between these molecules are very weak, so are easy to overcome. This makes the melting and boiling points very low.

Giant Covalent Structures:

Giant covalent structures are similar to giant ionic lattices except that there are no charged ions.

The atoms are bonded to each other by strong covalent bonds meaning they have very high melting and boiling points.

They do not conduct electricity, except for graphite, graphene and fullerenes.

Carbon can form lots of different types of molecules because carbon atoms can form up to four covalent bonds and bond easily to other carbon atoms to make chains and rings.

Diamonds:

Pure diamonds are sparkly and colourless, ideal for jewellery.

Each carbon atom forms four covalent bonds in very rigid giant covalent structures which makes diamond really hard. This makes diamonds ideal as cutting tools.

All those strong covalent bonds take a lot of energy to break and give a diamond a very high melting point, which is another reason diamond is a good cutting tool.

It doesn't conduct electricity because it has no free electrons.

Graphite and Graphene:

Graphite is black and opaque but still slightly shiny.

Each carbon atom only forms three covalent bonds, creating sheets of carbon atoms that are free to slide over each other.

The layers are held together weakly so they are slippery and can be rubbed off onto paper to leave a black mark, that is how a pencil works. This also makes graphite an ideal lubricating material.

Graphite also has a high melting point, the covalent bonds need lots of energy to break.

Since only three out of each of carbon's four outer electrons are used in bonds, there are lots of delocalised electrons that can move. This means that graphite conducts electricity.

A single sheet of graphite is called graphene. Graphene's covalent bonds make it extremely strong yet a sheet of graphene is so thin that it is transparent and incredibly light. Its delocalised electrons are completely free to move about which makes it even better at conducting electricity than graphite!

Fullerenes:

Fullerenes are another form of carbon. They aren't giant covalent structures, they're large molecules shaped like hollow balls or tubes. Different fullerenes contain different numbers of carbon atoms.

The carbon atoms in fullerenes are arranged in rings, similar to those in graphite. They have delocalised electrons so they can conduct electricity.

Their melting and boiling points aren't anything like as high as those of diamond and graphite but they're high for molecular substances because they're big molecules so have more intermolecular forces.