C1aL3 Bonding
Key Words
Atoms - the smallest particle that make up an element.
Attraction - pulling together.
Compound - substance made up of two or more elements chemically combined.
Covalent bond - bond where electrons are shared.
Electron - tiny particle of negative charge.
Element - substance that contains one type of atom.
Ionic bond - a bond where the attraction between oppositely charged ions holds the elements together.
Ion - a charged atom.
Molecule - a particle made up of two or more atoms, either of the same element, or different elements.
Stable - unlikely to change.
Grade E
There are many elements that do not exist naturally as single atoms. Gases like oxygen or chlorine exist as molecules of two atoms (O2 and Cl2 respectively). Oxygen can exist as ozone (O3) where there are three atoms per molecule. The molecules are held together by covalent bonds, where electrons are shared. The Noble gases (helium, neon, etc.) exist as single atoms.
Compounds consist of two or more atoms of different elements, like magnesium oxide (MgO) or methane (CH4). While methane forms molecules, magnesium loses two electrons to form a positive ion, while oxygen gains two electrons to make a negative ion. The attraction between the positive and negative ions forms an ionic bond.
Grade C
The Noble gases are stable, because they have full outer electron shells. Other atoms want to achieve that stability. They can do this by:
- Sharing the electrons;
- One atom losing electrons to form a positive ion, while another atom gains the electrons to make a negative ion.
We will look at how electrons are shared in covalent bonds. First look at the carbon atom (electronic configuration 2, 4), and the hydrogen atom (electronic configuration 1).
The first shell in the hydrogen atom has only one electron. To make it more stable, we need to have a second electron. The second shell in the carbon has four electrons. To make it stable, we need to have eight, so we need four more. Note that the dots and crosses are just for convenience to show us to which atom the electrons belong.
To make everything stable, the carbon and hydrogen share electrons as covalent bonds. In the picture above, you can see the electrons from the hydrogen making up the full outer shell of the carbon. The shared electrons from the carbon fill in the shells of the four hydrogen atoms.
This diagram is often called a Lewis diagram.
In ionic bonding, the positive ion donates an electron to the negative ion, as shown in the picture of sodium chloride.
The outer shell electron for the sodium goes across to the chlorine atom.
Which gives us a chloride ion.
Grade A
To draw Lewis diagrams for covalent bonds would be very tedious, even for small molecules. Instead, chemists use a dash between the symbols to represent a covalent bond. For example, chlorine is a molecular element, Cl2, with a single bond between the chlorine atoms.
Carbon atoms can be joined by not only single bonds, but also double bonds (as in ethene shown below), or even triple bonds.
The way that carbon bonds gives rise to organic chemistry, an entire discipline in its own right.