Atomic radii decreases when moving down a period, as the number of protons increases, which thence increases the effective nuclear charge.
Atomic radii increases when moving down a group, as the number of shells increases, which thence increases shielding.
The downward-sloping pattern on the above graph shows the decrease in atomic radii down a period, and the sudden jumps upward indicate the start of a new period, as a new shell is added.
First Ionization Energy (kJ/mol) of the First 20 Elements:
Ionization energy increases when moving down a period, as more energy is required to take an electron from the element when it has a higher number of protons, and thus a higher effective nuclear charge and stronger pull on its electrons. Ionization energy decreases when moving down a group, as more shells are added and shielding increases. Because the electrons are more loosely held, less energy is required to remove them.
The upward sloping pattern on the above graph shows the increase in the first ionization energy down a period, and the sudden jumps down indicate the start of a new period, as a new shell is added.
Electronegativity (Pauling Scale) of the First 20 Elements:
Electronegativity increases down a period, as the ability of the element to attract an electron increases. Because there are more protons on the right side of the periodic table, there is a higher effective nuclear charge, enabling the element to more easily attract electrons.
Electronegativity decreases when going down a group, as more shells are added, causing the effective nuclear charge to decrease. This causes a weaker pull on the electrons, which thence are not attracted to the atom.
The upward-sloping pattern on the above graph shows the increase in electronegativity down a period, and the sudden jumps down indicate first the noble gases, and then the start of a new period, as a new shell is added. The noble gases are the elements that cause the biggest leap down, as they do not attract any electrons because they already have a full outer shell.
Melting point (°C) of the First 20 Elements:
The melting point increases when moving down a period, as more protons are added, causing for higher effective nuclear charge, which requires more energy to break down and melt. The increase continues until group 15, when there is a sudden decrease in melting point. This decrease remains until a new period commences, as the end of the period is comprised of diatomic molecules, which only have weak Van der Waals forces to hold them together. As melting points depends entirely on intermolecular forces, which in these cases are weak, the melting points here are quite low.
The melting point decreases when moving down a group, as there are more shells, and thus a weaker electrostatic force and effective nuclear charge. This leads to less tightly bound electrons, which require less energy to break down and melt.