Protons, Neutrons and Electrons

This table easily identifies protons, neutrons and electrons in terms of their relative charges and relative masses.

	Location	Relative mass	Relative charge
Proton	Inside the nucleus	1	1
Neutron	Inside the nucleus	1	0
Electron	Outside the nucleus	1/1836	-1

Different atoms are distinguished by their numbers of protons and neutrons. We write the symbols using the following notation:

A is called the nucleon number, or the mass number. It is the total number of nucleons.
Z is the proton number or the atomic number, which is the number of protons. The number of protons determines the element.

We can determine the number of neutrons simply by subtracting the proton number from the nucleon number. ( No of neutrons = A – Z) Atomic particles are always in whole numbers.

When you add the number of Protons and Neutrons together, we get the nucleon number


Isotopes

Isotopes are different types of atoms of the same chemical element, each having a different number of neutrons. In a corresponding manner, isotopes differ in mass number (or number of nucleons) but not in atomic number.



Deducing the number of protons, neutrons and electrons from proton number and nucleon number

To find the number of protons in an atom or ion, we look at the proton number.

To find the number of neutrons in an atom or ion, we subtract the proton number from the nucleon number.

To find the number of electrons in an atom or ion, we look at the proton number, then add to the subtract the charge. (E.G. Ca2+ - 20-2=18 )


Distribution of mass and charges within an atom

The mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called a Dalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66 × 10−27 kg.

Electrons have an electric charge of −1.602×10−19 coulomb, which is used as a standard unit of charge for subatomic particles. Within the limits of experimental accuracy, the electron charge is identical to the charge of a proton, but with the opposite sign. As the symbol e is used for the elementary charge, the electron is commonly symbolized by e−, where the minus sign indicates the negative charge. The positron is symbolized by e+ because it has the same properties as the electron but with a positive rather than negative charge.