Question

Why are cations always smaller than the atoms from which they are formed?

Short answer

Cations are always smaller than the atoms from which they are formed due to the loss of one or more electrons from the outer energy level. This electron loss results in an increase in the net positive nuclear charge due to the unchanged number of protons, and a decrease in electron shielding as fewer electrons remain. These changes cause electrons to be drawn more strongly towards the nucleus, reducing the overall size of the cation compared to the original neutral atom.

Step-by-step solution

Understanding atoms and cations

A neutral atom consists of a nucleus with protons and neutrons, surrounded by electrons in various energy levels. When an atom loses electrons, it becomes a positively charged ion, called a cation.

Electron loss

When a cation is formed, it loses one or more electrons from the outer energy level, while the number of protons in atoms and cations remain the same.

Increase in nuclear charge

With the loss of electrons, the positive charge of the nucleus (number of protons) remains the same, while the negative charge (number of electrons) decreases. As a result, the net positive charge experienced by the electrons increases.

Decrease in electron shielding

When an element loses outermost electrons, the remaining electrons experience less shielding from the inner electrons. With less shielding, the remaining electrons are drawn closer to the nucleus.

Smaller cation size

The increased nuclear charge and decreased electron shielding cause the remaining electrons to be attracted more strongly towards the nucleus. As a result, the average distance between the electrons and the nucleus decreases, making the cation smaller than the original neutral atom. To sum up, cations are always smaller than the neutral atoms from which they are formed because of electron loss, increased nuclear charge, and decreased electron shielding.