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I always receive two type of explanation for the decreasement of atomic size for elements across the period.

Explanation according to proton number

This explanation is from my teacher.

  1. The proton number of elements increases across the period.
  2. The force of attraction between the proton in the nucleus and the electrons is stronger across the period as the proton will pull the electrons towards the nucleus.
  3. So, the atomic size of the elements will decrease across the period

Explanation according to number of valance electron

This explanation is from my friends

  1. The valance electron of elements increases across the period.
  2. The force of attraction between the valance electrons and the proton in the nucleus is stronger across the period between the valance electrons is pulled towards to the nucleus
  3. So, the atomic size of the elements will decrease across the period.

My question is which explanation is more suitable for the statement

the atomic size of the elements decrease across the period.

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    $\begingroup$ iirc, the real explanation is too complicated and needs lots of computing, but the two you explained above are just parts of the real explanation. They're not wrong, but they're not complete either. $\endgroup$ – M.A.R. May 2 '15 at 8:53
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Both of those explanations include part of the puzzle, but are not really complete on their own. To get a better understanding, you need to combine them, as well as understand something called Coulomb's law.

The basics of Coulomb's law are this:

  • The closer two oppositely charged things are, the stronger the attraction
  • The stronger the charge on either of the things, the stronger the attraction

With those points in mind, let's combine it all. I'll keep my explanation similar to the way you phrased yours:

  1. Each step across a row on the periodic table adds both one nuclear proton and one valence electron. So, that is increasing both the total positive charge of the nucleus and the total negative charge of the valence shell, but not the distance between them (because electrons are added to the same shell).
  2. The force of attraction between two charged objects depends on how strong each of them are (Coulomb's law). Since both the nucleus and the valence shell are stronger than in the previous step, and the distance between them has not increased (the electrons are added to the same shell) there is a stronger force of attraction between them, and the two are pulled closer together.
  3. So, the atomic size of elements decreases across a period.
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