Rules of Electron shells
1) Electrons always occupy shells
2) The lowest energy levels are always filled first - these are the closest to the nucleus.
3) Only a certain number of electrons are allowed in each shell:
1st shell: 2 2nd shell: 8 3rd shell: 8
4) Atoms are much happier when they have full electron shells - like noble gases in Group 0.
5) In most atoms the outer shell is not full and this makes the atom want to react to fill it.
Atoms join together to make compounds.
When different elements react, atoms from chemical bonds with other atoms to form compounds. It's usually difficult to separate the two original elements out again.
Making bonds involves giving away, taking or sharing electrons. Only the electrons are involved - it's nothing to do with the nuclei of the atoms at all.
A compound which is formed from a metal and a non-metal consists of ions. The metal atoms lose electrons to form positive ions and the non-metals gain electrons to form negative ions. The oppsite chargers (positive and negative) of the ions mean that they're strongly attracted to each other. This is called IONIC BONDING.
Another compound formed from non-metals consists of molecules. Each atom shares an electron with another atom - this is called a COVALENT BOND. Each atom has to make enough covalent bonds to fill up it's outer shell.
Atoms and Elements
The nucleus is in the middle of the atom, it comtains protons and neutrons.
Protons are positively charged. Neutrons have no charge (they're neutral). So the nucleus has a positive charge overall because of the protons.
The electrons move around the nucleus. They are negatively charged.
The number of protons equals the number of electrons.
Atoms have no charge overall. They are neutral. The charge on the electrons is the same size as the charge on the protons - but opposite. This means that the number of protons always equals the number of electrons in an atom. If some electrons are added or removed, the atom becomes charged and then is an ion.
Elements consist of one type of atom only.
Atoms have different number of protons, neutrons and electrons. It's the number of protons in the nucleus that decided what type if atom it is. E.g. An atom with one proton in it's nucleus is hydrogen and an atom with two protons is helium. If a substance contains only one type of atom, it's called an element.