Video Transcript
Why does the atomic radius of atoms
get bigger as you move down a group in the periodic table? (A) Because the masses of the atoms
increase. (B) Because the number of neutrons
in the atoms increases. (C) Because the electronegativity
of the atoms increases. (D) Because the ionization energy
of the atoms increases. Or (E) because the energy levels of
the atoms increase.
As we descend a group in the
periodic table, the atomic number and mass number increase significantly, as does
the atomic radius. However, going across a period also
means an increase in both the atomic number and the mass number, but in this case
the atomic radius decreases. Therefore, it is not a simple
matter of more protons, neutrons, and electrons always leading to a larger atom.
The number of protons increases
along a period. The protons attract the electrons
towards the nucleus. The number of electrons increases
as the number of protons increases. But the electrons fill the same
energy level. So the effective nuclear charge is
larger, which attracts the electrons to the nucleus and causes the atomic radius to
decrease. The increase in atomic radius down
a group is therefore due to fitting in the increasing numbers of electrons into
different shells.
Along a period, the additional
electrons go into the same energy level shell. This means no change in energy
level for the extra electrons. Down a group, however, the extra
electrons need an extra shell at a higher energy level. This is what makes the atomic radii
increase down a group. So the atomic radius of atoms get
bigger as you move down a group in the periodic table because the energy levels of
the atoms increase, or answer choice (E).
