Rapid Review

Chemical Periodicity

Relationship between Electronic Structure and the Periodic Table
(Only the first four rows are shown.)

H 1s1

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He 1s2

Li 2s1

Be 2s2

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B 2p1

C 2p2

N 2p3

O 2p4

F 2p5

Ne 2p6

Na 3s1

Mg 3s2

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Al 3p1

Si 3p2

P 3p3

S 3p4

Cl 3p5

Ar 3p6

K 4s1

Ca 4s2

Sc 3d1

Ti 3d2

V 3d3

Cr 3d4

Mn 3d5

Fe 3d6

Co 3d7

Ni 3d8

Cu 3d9

Zn 3d10

Ga 4p1

Ge 4p2

As 4p3

Se 4p4

Br 4p5

Kr 4p6

Definitions

First ionization energy - The energy required to remove the outermost electron from a neutral atom in the gas phase.

Electron affinity - The energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion.

Electronegativity - The tendency of an atom to draw the electrons in a bond towards it.

The first ionization energies, electron affinities, and electronegativities of elements generally increase as one moves closer to the top right-hand corner of the periodic table. They increase as one moves across a row from left to right because the force of attraction between the nucleus and an electron increases as the number of protons in the nucleus increases. They decrease as one moves down a group in the periodic table because the electrons in inner orbitals tend to shield the outer electrons from the attractive force of the nucleus.

Group I elements have large second ionization energies compared with their first ionization energies, while group II elements have second ionization energies not much higher than their first ionization energies. This can be explained by their respective electronic configurations.

Examples:

Na has a small first ionization energy but a large second ionization energy. Cl has a large electron affinity and electronegativity. As a result, when Na and Cl₂ are placed in the same container, they react: Cl readily gains an electron to become Cl^-, while Na readily loses an electron to become Na⁺. The difference in electronegativities between Na and Cl means that the bond between them is ionic instead of covalent. The large second ionization energy of Na is the reason why NaCl₂ is not formed.

Mg has small first and second ionization energies, but a large third ionization energy. Thus, magnesium chloride has the formula MgCl₂. Similarly, the difference in electronegativities between Mg and Cl means that the bond between them is ionic instead of covalent.

Formal charge is the charge on an atom in its Lewis structure, or the overall charge on an ion or molecule.

Steps in determining formal charge:
1. Divide the electrons in each covalent bond between the atoms in the bond.
2. Determine whether the atom of interest has more electrons than protons (negative formal charge), less (positive formal charge), or the same number (zero formal charge).