Atomic Structure

Atomic Structure

An atom is the smallest unit of an element, having properties of that element. All atoms are made up of just 3 particles - the proton, the neutron and the electron. The following table shows the properties of these 3 particles:

                                   relative mass                charge

Proton    :                        1                                 +1

Electron :                        1                                    0

Neutrons:                        1/1836                        -1

The center of the atom is called the nucleus, and it contains the protons and neutrons. The nucleus takes up less than 1% of the volume of an atom. The rest of the over 99% of the atom is empty space occupied by moving electrons. Electrons occupy shells at different distances from the nucleus.

The mass of an atom is the total mass of its protons, neutrons and electrons. However, since the mass of electrons are about 1836 times less than protons or neutrons, it is very insignificant, and the mass of an atom is calculated based on the total mass of its protons and neutrons only.

An atom always has the same number of protons and electrons, so that the positive charges on the protons balance the negative charges on the electrons, and the atom is overall neutral.

Atomic Number and Mass Number

Atomic number (Z) → number of protons in an element

Mass number (A) → number of protons + number of neutrons in an element

In a periodic table, the symbol of an element is always written with the element's atomic number and mass number.