3.1.1 Atomic Structure  Protons, neutrons and electrons
Students should

Mass number
The atomic mass number is represented by the symbol (letter) 'A'. This is not to be confused with the relative atomic mass Ar.
The mass number gives the integral number of nucleons, protons and neutrons found in the nucleus of an atom.
The relative mass is a value that is not necessarily integral that compares a mass to the mass of a carbon isotope, assigned a value of exactly 12.0000 units.
Atomic number
This is represented by the symbol (letter) 'Z'. It shows us the number of protons in an atom (and the number of electrons in a neutral atom.
Example: How many protons and electrons does an atom of iron contain? The atomic number of iron is 26 therefore it contains 26 protons The number of electrons = number of protons therefore there are 26 electrons 
AZE
Any isotope of any element can be defined by using the A value, the Z value and the element symbol.
Using the values of A and Z it is possible to calculate the number of subatomic particles within any specific isotope of an element.
Example: Determine the number and type of subatomic particles in the following atom: The atomic number is 1 therefore there is 1 proton and 1 electron The mass number is 3 therefore there are (31) = 2 neutrons 
Example: Determine the number and type of subatomic particles in the following atom: The atomic number is 5 therefore there are 5 protons and 5 electrons The mass number is 11 therefore there are (115) = 6 neutrons 
Ions
The system can be extended to cover ions simply by adding the charge onto the element symbol. It is important to remember that a positive ion has LOST electrons.
Example: Determine the number of electrons in the following ion: Mg^{2+} The atomic number is 12 therefore in a neutral atom there would be 12 protons and 12 electrons. However, the charge is 2+ therefore the atom has LOST 2 electrons The remaining electrons then = 10 electrons 