Chemistry Unit 1 - Atoms, Compounds and The Periodic Table

Atoms are made up of a small nucleus surrounded by electrons. If a substance only contains one type of atom it is called an element.

The nucleus is in the middle of the atom and it tiny compared to the rest of the atom. It contains protons and neutrons. 
Protons have a charge of +1 and neutrons have a charge of 0 (they're neutral). Therefore the nucleus has a positive overall charge becuase of the protons.

Electrons move around the nucleus and have a charge of -1. They occupy shells around the nucleus.
In an atom, the number of protons equals the number of electrons giving that atom no overal charge.

All atoms of a particular element have the same number of protons. For example, all atoms that contain one proton are hydrogen atoms and all atoms with two protons are helium atoms.

Atomic number = number of protons
Mass number = number of protons + number of neutrons.

Elements with similar properties form columns.

The columns are called groups.

All the elements in a group have the same number of electrons in their outer shell.

This is why elements in the same group have similar properties.

The Group 1 elements are Li, Na, K, Rb, Cs, Fr. They're all metals and they react the same way. So, they all react with water to form an alkaline solution and hydrogen gas and they all react with oxygen to form an oxide.

The elements in the final column (Group 0) are the noble gases. They all have eight electrons in their outer shell, apart from Helium (which has two). This makes them stable and unreactive because they have full outer electron shells.

Electrons always occupy shells (sometimes called energy levels).

The lowest energy levels are always filled first - these are the ones closest to the nucleus.

Only a certain number of electrons are allowed in each shell:

• 1st shell - 2
• 2nd shell - 8
• 3rd shell - 8

Atoms are much happier if they have full electron shells - like the noble gases do.

In most atoms the outer electron shell (or highest energy level) is not full and this makes the atom want to react to fill it.

The electronic structure of an atom can be worked out using the number of protons. For example, nitrogen has 7 protons so it must have 7 electrons. The first shell can only take 2 electrons, so there are 5 electrons left. The second shell can take up to 8 electrons, so all the remaining electrons go on the second shell. Therefore, the electronic structure for nitrogen is 2,5.

Compounds are made up of two or more elements bonded together.

When different elements react, atoms form chemical bonds with other atoms to form compounds. It's usually difficult to separate the two original elements out again.

Making bonds involves atoms giving away, taking or sharing electrons. Only electrons are involved in bonding.

A compound which is formed from a metal and non-metal consist of ions. The metal atoms lose electrons to form positive ions and the non-metal atoms gain electrons to form negative ions. The opposite charges of the ions attract each other. This is called IONIC bonding.

A compound formed from non-metals consist of molecules. Each atom shares an electron with another atom - this is called COVALENT bonding. Each atom has to make enough covalent bonds to fill its outer shell

The properties of a compound differ from the properties of the original elements.

Compounds can be small molecules like water, or lattices like sodium chloride.

Atoms aren't lost or made in chemical reactions.

The same number of atoms are present at the end of chemical reactions as where present at the start. They are just arranged in different ways.

Balanced symbol equations show the atoms at the start (the reactants) and the atoms at the end (the products) and how they're arranged. 

Because atoms aren't gained or lost:
THE MASS OF THE REACTANTS = THE MASS OF THE PRODUCTS
So, if you react 6g of Mg with 4g of O, you end up with 10g of MgO.