Atoms are the building blocks of all basic materials in the universe. Every object is made up of a number of ‘elements’and those elements are made up of atoms. Every atom contains a number of protons neutrons and electrons. The neutrons and protons stick together to form the “nucleus” or core of the atom, while the electrons orbit the atom in ‘shells’.
Electron shells are numbered from 1-4, starting from the shell closest to the nucleus. Each shell can contain 2*(n^2) electrons. Where n is the shell number.
This means the first shell can contain a total of [2*(1^2)] = 2 electrons. The second shell can contain a total of [2*(2^2)] = 4 electrons. And so on.
Shells will fill starting from shell one, then when shell one is full; they will begin filling shell two, and so on.
Electron shells can be further broken into ‘Subshells’.
The outer most shell of an electron is known as the ‘Valance shell’, and it determines the chemical properties of an atom.
Each neutron has no charge. Each proton has a positive charge.([+] 0.16 * 10-18 coulombs) Each electron has a negative charge. ([-] 0.16 * 10-18 coulombs) The charge of an electron and a proton are equal but opposite.
Neutrons and Protons have approximately the same size and mass, while electrons are much smaller, and weigh around 1840 times less. As such, when calculating the weight of an atom it is normal to ignore the mass of the electrons.
An atom is characterized by its number of protons, the number of protons and atom has is called its atomic number. For example a hydrogen atom will have one proton (atomic number of 1), and a helium atom has two protons (atomic number of 2). Hydrogen and helium are elements, and can be identified using the ‘periodic table’.
A standard atom has an identical number of protons, neutrons and electrons.
When an atom has more or less neutrons then its protons it is called an isotope. Isotopes will affect the atoms mass but not its charge as neutrons have no charge.
When an atom has more or less electrons then its protons it is call an Ion. Ions will not affect the atoms apparent mass, but will effect it’s charge.
The difference in an atoms charge can be referred to as its valance. For example, if an atom has an additional electron, it has a valance of negative one; this is often used in calculations.
For example, a standard lithium atom has three protons (+3), three neutrons (+0) and three electrons (-3), thus it will have no charge (3+0-3=0), thus it can be said the atom has a valance of 0. If the atom gained an electron, it will now have a charge of (3+0-4) -1, thus it can be said that the atom has a valance of -1. An atom with additional electrons will be negatively charged, this is called an anion. An atom which loses electrons will become positively charged, this is called a cation.
For more information see, Electron Shells and Subshells, The Periodic Table or Ionic Bonding.