3.2.1.1 - Periodic Table Blocks

Proton Number Fixes an Element's Position

The modern Periodic Table is arranged in order of increasing proton number. Proton number is unique to each element, so if the proton number changes, the element changes too.

In a neutral atom, the number of electrons equals the number of protons. That means proton number also tells you how many electrons the atom has. As electrons fill orbitals in a regular sequence, the proton number determines the electron arrangement, and that electron arrangement determines where the element sits in the Periodic Table.

This is why position in the table is not arbitrary. For example, magnesium has proton number 12, so a neutral magnesium atom has 12 electrons. Its electron arrangement ends in $3s^{2}$, which places magnesium in the left-hand part of Period 3. Aluminium has proton number 13, so it has one more electron; that extra electron goes into a $3p$ sub-shell, so aluminium moves into the p-block.

What the Four Blocks Mean

Chemists divide the Periodic Table into four broad blocks. Each block is named after the distinguishing type of sub-shell being filled across that region.

The s-block is the left-hand two columns of the table. The distinguishing electrons across these columns enter an s sub-shell. Sodium, for example, has electron arrangement $1s^{2}\,2s^{2}\,2p^{6}\,3s^{1}$, so it is in the s-block.

The p-block is on the right-hand side of the table. The distinguishing electrons across these columns enter a p sub-shell. Sulfur ends $3p^{4}$, so sulfur is a p-block element.

The d-block is the central block between the s- and p-blocks. Across this region of the table, electrons are being added to a d sub-shell. Iron, for example, contains $3d$ electrons and is classified as a d-block element.

The f-block is shown as the two detached rows below the main body of the table. Across this region, electrons are being added to an f sub-shell.

The diagram below shows the four block regions on a simplified Periodic Table and highlights helium's unusual position.