Video Transcript
Which of the following is correct
as you move from left to right across a period in the periodic table? (A) The atoms get smaller due to an
increase in the effective nuclear charge. (B) The atoms get smaller due to an
increase in the ionization energy. (C) The atoms get smaller due to an
increase in the atomic mass. (D) The atoms get smaller due to an
increase in the electronegativity. Or (E) the atoms get smaller due to
an increase in the metallic properties.
As we look across a period, the
atomic mass gradually increases as more protons are added to the nucleus. We can see this if we compare an
atom of lithium with an atom of beryllium. Lithium from group one has three
protons, whereas beryllium from group two has four protons in its nucleus. However, the reason that the atoms
get smaller, not larger, is due to the effect of adding these protons, combined with
the effect of the additional electrons that are added to the same valence shell.
The additional electrons are added
into the same energy level as we cross a period. This means that the number of
shielding electrons stays the same for all of the elements in a period. The shielding electrons are any
electrons in a shell closer to the nucleus than the outer shell. The electrostatic interactions
between the protons and electrons cause the protons to attract electrons towards the
nucleus. Shielding essentially blocks some
of the effective nuclear charge of the protons from affecting the outer
electrons. The three protons in the atom of
lithium create less nuclear pull than the four protons in beryllium. However, both of the atoms have
outer electrons protected by the same shielding effect created by two electrons in
the inner shell.
So, across the period, while the
shielding stays the same, an extra proton is added to the nucleus, increasing the
total nuclear pull. The electrons in the outer shell of
each element therefore feel an increase in effective nuclear charge. They feel a greater pull from the
nucleus. The overall pull on the outer
electrons is greater in the beryllium atom than in the lithium atom. The increased effective nuclear
charge pulls the outer shell electrons increasingly close to the nucleus, leading to
smaller atoms.
So the key reason for the atoms
getting smaller as you move across a period is (A). The atoms get smaller due to an
increase in the effective nuclear charge.
