Atomic Number and Mass Number
The nucleus of atom contains positively charged particles called protons and neutral particles called neutrons. The number of protons in an atom is called the atomic number and is denoted by the symbol Z. All atoms of an element have the same atomic number.
The electrons occupy the space outside the nucleus. In order to account for the electrically neutral nature of the atom, the number of protons in the nucleus is exactly equal to the number of electrons. Thus,
Atomic number = number of protons = number of electrons
According to Dalton's theory, the atoms of different elements are different from each other. This difference is due to difference in the numbers of protons present in the nucleus of the element. In other words, different elements differ in terms of their atomic number. For example, the atoms of hydrogen and helium are different because hydrogen has one proton in its nucleus whereas the nucleus of helium atom contains two protons. Their atomic numbers are 1 and 2, respectively.
In the Rutherford’s model, the mass of the atom is concentrated in its nucleus. This is due to the presence of two heavy particles namely protons and neutrons in the nucleus. These particles are called nucleons. The number of nucleons in the nucleus of an atom is called its mass number. It is denoted by A and is equal to the total number of protons and neutrons present in the nucleus of an element. Thus,
Mass number (A) = number of protons (Z) + number of neutrons (n)
Atomic number and mass number are represented on the symbol of an element. An element, X with an atomic number, Z and the mass number, A is denoted as:
AZX
For example, 126C means that the carbon has an atomic number of 6 and the mass number of 12. This can be used to compute the number of different fundamental particles in the atom. As the atomic number is 6, this means:
Number of protons = number of electrons = 6
As Mass number = number of protons + number of neutrons
⇒ 12 = 6 + number of neutrons
⇒ number of neutrons = 12 - 6 = 6
Thus, an atom of 126C has 6 protons, 6 electrons and 6 neutrons.