ATOMS are tiny particles made up of three even smaller subatomic particles called protons, electrons, and neutrons.
Atoms consist of a positively charged nucleus with negative charged particles moving around the nucleus. The negatively charged particles are the electrons and they are held in orbits (or energy levels) around the nucleus by electrostatic forces created from the positively charged particles in the nucleus called protons. The nucleus also contains the neutrons, but they have 0 charge. They are neutral.
The mass of proton is 1. The mass of a neutron is 1. The mass of an electron is 0. So, the mass of the atom is contained to the nucleus.
The electrons are ordered in the orbits, or energy levels, in a particular way. Below is a sodium atom.
There are different energy levels. Each energy level can hold a certain number of electrons. (Remember; 1st energy level can contain up to 2 electrons, 2nd energy level can contain up to 8 electrons and the third energy level also up to 8 electrons)
For example, a hydrogen atom has 1 electron. So, that's easy, the first energy level will have 1 electron and that's it.
An aluminium atom has 13 electrons. The first energy level will have 2 electrons, the second energy level 8 electrons and the third energy level 3 electrons.
This electron arrangement can also be represented as 2.8.3. This is called the ELECTRON STRUCTURE. So, the electron structure of aluminium is 2.8.3.
Atoms are neutral particles (number of electrons and number of protons balanced). During reactions and collisions atoms will lose or gain electrons. This can imbalance the proton: electron ratio resulting in the formation of positive or negative charged ions.
For example, when sodium reacts with fluorine the sodium will lose an electron to form a positive ion and fluorine will gain an electron to become a negative ion.
Positive ions are called cations. Negative ions are called anions.