Question

1. the trend in atomic size can be explained by

increasing nuclear force
increasing number of protons
decreasing atomic mass
increasing electron shielding

Explanation:

As the number of protons in the nucleus increases across a period, the nuclear charge increases. This stronger positive - charge pulls the electrons closer to the nucleus, decreasing the atomic size. So, the trend in atomic size can be explained by the increasing number of protons.

Answer:

Increasing number of protons