Particles in the atom and atomic radius

The atom is overwhelmingly empty space surrounding a tiny, dense nucleus that holds the protons and neutrons.

Almost all the mass and all the positive charge of the atom are concentrated in this nucleus, while the electrons occupy shells in the surrounding space.

• Protons carry a charge of +1 and have a relative mass of 1.
• Neutrons are neutral (charge 0) and have a relative mass of 1.
• Electrons carry a charge of −1 and have negligible mass (about 1/1840 of a proton).

The proton number Z identifies the element, while the nucleon number A gives the total number of protons and neutrons.

The number of neutrons follows directly as A − Z, and the formation of ions changes only the electron count, leaving the proton and neutron numbers unaffected.

When beams of these particles move at equal velocity through an electric field, the sign of the charge sets the direction of deflection while the charge-to-mass ratio sets the magnitude of deflection.

• Electrons curve sharply towards the positive plate, deflecting the most because their very small mass gives them a very high charge-to-mass ratio.
• Protons curve modestly towards the negative plate, deflecting far less because their much greater mass gives a low charge-to-mass ratio.
• Neutrons travel straight through, undeflected, because they carry no charge.

The same logic of nuclear charge and shielding governs the trends in atomic radius:

• Across a period, the nuclear charge increases while shielding stays roughly constant (electrons are added to the same outer shell), so the outer shell is pulled in and the atomic radius decreases.
• Down a group, additional electron shells and greater shielding outweigh the rise in nuclear charge, so the atomic radius increases.

Cations are smaller than their parent atoms because they often lose the entire outermost shell, and the remaining electrons are held more tightly by the relatively greater effective nuclear charge. Anions are larger than their parent atoms because the added electrons increase electron–electron repulsion in the outer shell.

Across an isoelectronic series (species with the same number of electrons), the increasing nuclear charge pulls the fixed number of electrons in more strongly, so the radius progressively decreases.

At a Glance