Periodic Table Trends

Electron configuration and periodic properties

By Josiah B. and Carter B.

Atomic Radii

Period Trends- The trend to smaller atoms across a period is caused by the increasing positive charge of the nucleus.

Group Trends- In general the atomic radii of the main-group elements increase down a group.

Ionization Energy

An ion is an atom or group of bonded atoms that has a positive or negative charge.

Period Trends- In general, ionization energies of the main-group elements increase across each period.

Group Trends-Among the main-group elements, ionization energies generally decrease down the groups.

Removing Electrons from Positive Ions- each successive electron removed from an ion feels an increasingly stronger effective nuclear charge (the nuclear charge minus the electron shielding).

Electron Affinity

The energy change that occurs when an electron is acquired by a neutral atom is called the atom’s electron affinity.

Period Trends- Among all of the elements on the Periodic Table, the halogens gain electrons most readily. The ease with which halogen atoms gain electrons is a major reason for the high reactivities of the Group 17 elements. As electrons add to the same p sublevel of atoms with increasing nuclear charge, electron affinities become more negative across each period within the p block. An exception to this trend is groups 14 and 15.

Group Trends- A general rule, electrons add with greater difficulty down a group. This pattern is a result of two competing factors. The first, a slight increase in effective nuclear charge down a group, which increases electron affinities and the second, an increase in atomic radius down a group, which decreases electron affinities.

Adding Electrons to Negative Ions- For an isolated ion in the gas phase, it is always more difficult to add a second electron to an already negatively charged ion. Second electron affinities are all positive.

Ionic Radii

A positive ion is known as a cation, and a negative ion is known as an anion.

Period Trends- Cationic radii decrease across a period because the electron cloud shrinks due to the increasing nuclear charge acting on the electrons in the same main energy level.

Group Trends- If there is a gradual increase of atomic radii down a group, there is also a gradual increase of ionic radii.

Valence Electrons

The electrons available to be lost, gained, or shared in the formation of chemical compounds are valence electrons.


Period Trends- Electronegativities tend to increase across each period, although there are exceptions.

Periodic Properties of the d- and f-block elements

Atomic Radii- Gradually decrease across the periods. However, this decrease is less than that for the main-group elements because the electrons added to the d sublevel shield the outer electrons from the nucleus.

Ionization Energy- Ionization energies of the d-block and f-block elements generally increase across the periods. In contrast to the decrease down the main groups, however, the first ionization energies of the d-block elements generally increase down each group.

Ion Formation and Ionic Radii- The electrons in the highest occupied sublevel are always removed first. Comparing 2+ ions across the periods shows a decrease in size that parallels the decrease in atomic radii.

Electronegativity- The d-block elements also follow the general trend for electronegativity values to increase as radii decrease, and vise versa.