1. The Periodic Law
When elements are arranged according to increasing atomic number, their physical and chemical properties show repeating patterns. This is the basis of the modern periodic table.
The properties of elements are periodic functions of their atomic numbers.
2. Groups and Periods
The periodic table is organised into horizontal rows called periods and vertical columns called groups.
There are seven periods. Elements within a period have the same number of occupied principal electron shells.
Elements in the same group often have similar chemical properties because of related valence-electron arrangements.
3. Blocks of the Periodic Table
Elements can also be classified according to the subshell into which their differentiating electron enters.
Includes groups 1 and 2, along with helium by electron configuration.
Includes groups 13 through 18.
Contains the transition elements.
Contains the lanthanides and actinides.
4. Atomic Radius
Atomic radius is a measure of the size of an atom. Its trend is strongly influenced by nuclear charge, electron shells, and shielding.
Across a period
Atomic radius generally decreases from left to right because effective nuclear attraction increases.
Down a group
Atomic radius generally increases because additional electron shells are added.
5. Ionisation Energy
Ionisation energy is the energy required to remove an electron from an isolated gaseous atom.
Across a period
Increasing effective nuclear charge generally makes electrons harder to remove.
Down a group
Outer electrons become farther from the nucleus and are generally easier to remove.
6. Electronegativity
Electronegativity describes the tendency of an atom in a chemical bond to attract shared electrons toward itself.
Electronegativity generally increases from left to right across a period and decreases down a group.
Fluorine has the highest electronegativity on the commonly used Pauling scale.
7. Electron Affinity
Electron affinity describes the energy change associated with adding an electron to a gaseous atom.
Its exact trend contains several exceptions because electron configurations and electron-electron repulsions also influence the energy change.
8. Metals, Nonmetals and Metalloids
| Property | Metals | Nonmetals |
|---|---|---|
| Electrical conductivity | Generally high | Generally low |
| Appearance | Often lustrous | Often non-lustrous |
| Malleability | Generally malleable | Usually brittle when solid |
| Typical ion formation | Often form cations | Often form anions or share electrons |
Metalloids display a mixture of metallic and nonmetallic properties and occur near the boundary between the two broad regions.
Key Ideas
The modern table is arranged according to increasing atomic number.
The arrangement of electrons helps explain why elements in particular groups behave similarly.
General trends are useful for prediction, but individual electron configurations can produce exceptions.