WAEC - Chemistry (2017 - No. 10)
The reason for the decrease in the atomic size of elements across a period is that
nuclear charge increases while the outermost electrons are drawn closer to the nucleus
nuclear charge decreases while the outermost electrons are drawn closer to the nucleus
valence electrons increase across the period while the valence shell remains constant
nuclear charge decreases while the distance of the valence shell from the nucleus is increasing
Maelezo
This is caused by the increase in the number of protons and electrons across a period.