In the middle of an atom is a small nucleus. This has the two particles: protons and neutrons. A third type of particle orbits the nucleus called electrons.
Protons have a positive charge, neutrons have no charge. The nucleus overall has a positive charge. Electrons orbiting the nucleus are negatively charged.
Any atom has the same amount of electrons as protons, therefore the overall charge is exactly zero.
Atoms are arranged in the periodic table in order of their atomic number (aka proton number).
The Arrangement Of Electrons In Atoms
The electrons in an atom are arranged in energy levels or shells. Each shell represents a different energy level. The lowest energy level is shown by the shell which is nearest to the nucleus.
Atoms with the same number of electrons in their outer shell belong in the same group of the periodic table.
The number of electrons in the outer shell of an atom determines the way that the atom behaves in a chemical reaction.
The lowest energy level (closest to the nucleus) holds 2 electrons, the second holds 8, the third also holds 8 etc.
Separating mixtures is usually quite easy, but separating substances once they have reacted can be quite difficult.
When two or more atoms react they make a compound, which is different to the two original atoms.
When two or more atoms are mixed, they don't change - they stay as their original atoms
Elements react to form compounds by gaining or losing electrons or by sharing electrons.
The elements in Group 1 react with the elements in Group 7 because Group 1 elements can lose an electron to gain a full outer shell. This electron can be given to an atom from Group 7, which then also gains a full outer shell.
Ionic compounds are held together by strong forces between the oppositely charged ions.
Other elements that can form ionic compounds include those in groups 2 and 6.