(Chem) 關於Periodic Table的問題

Because the attraction between the nucleus (+ charge) and electrons (- charge) are increasing from left to right (due to more protons in the nucleus). Every electron is feeling a stronger effective nuclear charge (http://hk.knowledge.yahoo.com/question/question?qid=7008061001319).
Electrons are pulled closer to the nucleus. So when measuring the size or atomic radius, we found that the sizes of the atoms decreased on the same row from left to right.

The reason for this is because elements have more electrons as you move from left to right of the periodic table. The increase in the number of electrons will increase the overall atomic charge of the element. Eventhough that there will be more shells of electrons so you may think that the size will increase but it is actually the opposite. The greater the charge the higher the attraction. Therefore, the size decrease.

Think in a sequence...

Across a period, number of outermost shell electrons increases

However, electrons enter the same quantum shell

leading to poor shielding effect ( Repulsive forces between electrons in
different shells )

Overall attraction of nucleus increases

Effective nuclear chanrge increases

Size decreases...

( Effect of attraction caused by increasing nuclear charge across the period
outweighs the effect of repulsion caused by poor shielding effect coz electrons enter the same quantum shell )

Please remind that the attraction between the nucleus (+ charge) and electrons (- charge) are increasing from left to right (more proton in nucleus). So in simple words, the sizes of the atom will decrease on the same row from left to right.