Question

as you move across the periodic table from left to right, the atomic radius decreases. this is because - the number of protons increases, so attraction to electrons increases the number of energy levels increases the number of electrons increases the atomic mass increases

Explanation:

To determine why atomic radius decreases across a period (left - to - right on the periodic table), we analyze each option:

1. Option 1: the number of protons increases, so attraction to electrons increases

• As we move from left to right across a period, the atomic number (number of protons) increases. The electrons are added to the same valence shell (energy level) across a period. The increased positive charge from more protons in the nucleus exerts a stronger attractive force on the electrons in the same energy level. This pulls the electrons closer to the nucleus, reducing the atomic radius.

1. Option 2: the number of energy levels increases

• As we move down a group (top - to - bottom) in the periodic table, the number of energy levels (shells) increases. But across a period (left - to - right), the number of energy levels remains the same. So this option is incorrect for the left - to - right trend.

1. Option 3: the number of electrons increases

• While the number of electrons does increase across a period, the electrons are added to the same energy level. The key factor for the decrease in atomic radius is the increase in nuclear charge (due to more protons) which pulls the electrons closer. Just the increase in the number of electrons alone does not explain the decrease in radius. For example, if there were no increase in nuclear charge, adding electrons would increase electron - electron repulsion and potentially increase the radius.

1. Option 4: the atomic mass increases

• Atomic mass is the sum of protons and neutrons. An increase in atomic mass does not directly relate to the change in atomic radius. The radius is mainly determined by the electron - nucleus interaction and the number of energy levels. So this option is incorrect.

Answer:

A. the number of protons increases, so attraction to electrons increases