Periodic Properties of the Elements

In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons.
The nuclear charge that an electron experiences, Zeff, depends on both factors.
Zeff = Z-S * Z is the atomic number * Zeff is the effective nuclear charge * S is the screening constant
The inner electrons partially screen the outer electrons from attraction to the nucleus
The value of S is usually close to the number of core electrons
As distance from the nucleus increases, the value of S increases
Zeff increases ACROSS any row; the core electrons stay the same but the actual nuclear charge increases
Zeff changes far less going DOWN a column than across a row; increases slightly.

The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei.
Radius (Å) increases going down a column due to the increase in the principal quantum number, n. * Outer electrons are further from the nucleus.
Radius decreases from left to right across a row (period). * Valence electrons are drawn closer to the nucleus and the size decreases.
Ionic radii can be determined experimentally from interatomic distances in ionic compounds.
Ionic size depends on nuclear charge, number of electrons, and the orbitals involved
Cations are smaller than parent atom since electron(s) have been removed and there is less electron-electron repulsion
Anions are larger than the parent atom; extra electrons cause more electron-electron repulsion and the atom is spread out
For ions of the same charge, size increases going down a column.

Ionization energy: the amount of energy required to remove an electron from the ground state of a gaseous atom or ion. * First ionization energy, I1: energy required to remove the first electron. * Second ionization energy, I2: energy required to remove the second electron, etc.
It requires more energy to remove each successive electron.
Largest ionization energies involve removal of core electrons
I1 increases from left to right across a row. * Zeff is increasing and radius is decreasing; attraction between electron & nucleus increases. * I1 has larger value…harder to pull off electron
I1 decreases going down a column. * Radius increases and Zeff changes only slightly; attraction between electron & nucleus decreases. * I1 has smaller value…easier to pull off electron

Electron affinity: the energy change accompanying the addition of an electron to a gaseous atom
In general, electron affinity becomes more exothermic as you go from left to right across a row.
Group 7A has the most negative electron affinity values; adding an electron to this group results in an octet configuration (stable)
Group 8A has positive electron affinity values; this implies electrons will NOT add
Group 2A has anomalous values (less negative than expected); this can be understood by looking at the electron configurations; adding an electron requires a large energy input to get to the 2p energy level