Particles in the atom and atomic radius — common mistakes

When asked to explain trends in atomic or ionic radius, you must refer to three key factors for full marks: 1. Nuclear charge (number of protons), 2. Electron shielding (repulsion from inner shells), and 3. The number of occupied principal electron shells.

The mass on the periodic table is the relative atomic mass ($A_r$), which is the weighted average mass of all the naturally occurring isotopes of an element, relative to 1/12th the mass of a carbon-12 atom. Since most elements exist as a mixture of isotopes, the average is not a whole number.

Electron shielding (or screening) is the effect where electrons in inner shells repel the outermost (valence) electrons. This repulsion partially cancels out the attractive force of the positive nucleus, reducing the 'effective nuclear charge' experienced by the valence electrons.

They are extremely similar in mass, and for A-Level Chemistry, their relative masses are both taken as 1. In reality, a neutron is very slightly heavier than a proton, but this difference is negligible for most chemical calculations.