Atomic radii can be defined as one-half the distance between nuclei of identical atoms that are bonded together. The trend to smaller atoms across a period is caused by the increasing positive charge of the nucleus. As the atomic number increases, the proton number in the nucleus increases causing a stronger pull on the electrons on the outer orbitals. This decreases the size since the electrons are being pulled in closer to the center of the atom. The atomic radii will increase down a group. This is because as you go down the group an outer orbital is added so that the atom can hold more electrons.
Ionization energy is defined, as the energy required removing one electron from a neutral atom of an element. Across a period, ionization energies of main-group elements increase. This increase is caused by increasing nuclear charge because a higher charge attracts electrons in the same energy level more strongly. Ionization energies decrease down a group because as more orbitals are added causing an electron shielding effect. The decrease is caused because electrons are further from the nucleus decreasing the pull that the nucleus has on the electrons.
Electron affinity is the energy change that occurs when an electron is acquired by a neutral atom. Across a period, electron affinities become increasingly negative as electrons are added to the orbitals. Down a group, electron