Atoms consist of subatomic particles such as Protons, Electrons and Neutrons.
Electrons have a -1 charge and whizz around the orbitals of the atom. The orbitals take up most of the volume of the atom.
Protons and neutrons are found in the nucleus and it is very SMALL. The NUCLEUS takes up MOST of the mass of the ATOM.
All neutral atoms have no overall charge and have the number of Electrons is the same as the number of Protons.
Ions do have a charge, and the number of electrons are DIFFERENT to the number of protons.
NEGATIVE ions have more electrons. One more electron is added to a negative ion.
(e.g. Li3 has three electrons, add 1 more electron it now has 4 electrons and becomes Li-.
POSITIVE ions have fewer electrons. (e.g. Mg has 12 electrons, it loses 2 to have 10 electrons, so it is now Mg2+).
Isotopes - of an element are atoms with the SAME number protons but different numbers of neutrons.
For example, Cl-35 and Cl-37. Cl-37 has 2 more neutrons than Cl-35, but the proton number is still the same.
The NUMBER and ARRANGEMENT of electrons decides the the chemical properties of an element, this is because all elements have the same ELECTRONIC CONFIGURATION.
The physical properties of each isotope is DIFFERENT, e..g. different densities, rates of diffusion. This is because it is the MASS that determines the physical properties (the more isotopes there are means that each one has an extra neutron(s), but the number of protons is still the same for each isotope of an element.)