The modern periodic table

The modern periodic table arranges elements in order of increasing atomic (proton) number. This arrangement reveals patterns in the properties of elements, allowing chemists to make predictions about their behavior.

Periodic table structure

The periodic table is organised into periods (rows) and groups (columns):

• Periods: Elements in the same period have the same number of electron shells. As you move down a period, the number of electron shells increases.
• Groups: Elements in the same group have similar electron configurations in their outer shell. As a result, they exhibit similar physical and chemical properties.

The periodic table can be further divided into three blocks based on the type of subshell being filled in the outermost electron shell:

1. s-block: Elements in the s-block have their outermost electrons in an s orbital. For example, the electronic configuration of lithium (Li) is 1s² 2s¹.

2. p-block: Elements in the p-block have their outermost electrons in an p orbital. For example, the electronic configuration of oxygen (O) is 1s² 2s² 2p⁴.

3. d-block: Elements in the d-block have their outermost electron in a d orbital. For example, the electronic configuration of iron (Fe) is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶.

Determining electron configurations

You can use the table to deduce the electron configuration of elements:

1. Identify the period - This shows how many electron shells.
2. Note which block it falls in - This shows which subshell is being filled in the outer electron shell.
3. Write out the complete configuration by building up from the 1s orbital.

Worked example 1 - Determining the electron configuration of vanadium

Write the electronic configuration of an atom of vanadium, V.

Step 1: Identify which period vanadium belongs to

Vanadium is located in period 4, so it has 4 electron shells.

Step 2: Identify which block vanadium belongs to

Vanadium is part of the d-block, so its outer shell includes a partially filled d subshell.

Step 3: Write the complete electronic configuration

The complete configuration is:

Period 1: 1s²

Period 2: 2s² 2p⁶

Period 3: 3s² 3p⁶

Period 4: 4s² 3d³

Therefore, the electron configuration for vanadium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³.

The abbreviated configuration is [Ar] 4s² 3d³.