Arrange the following elements in order of increasing (from smallest to largest) atomic size: H, Br, Cl, O. Explain the reasoning behind your chosen order.

Atomic size is defined as the distance between the nucleus and valence shell of an atom. Valence shell is the outermost shell of an atom.

When we move across a period in a periodic table then there will occur a decrease in atomic size of the atoms. But when we move across a group in a periodic table then there will occur an increase in atomic size of the atom.

Hydrogen is a period 1 element, oxygen is a period 2 element, chlorine is a period 3 element and bromine is a period 4 element.

Therefore, given elements are arranged in order of increasing atomic size as follows.

H < O < Cl < Br