What you'll learn

• How proton number determines an element’s position in the Periodic Table.
• What s-, p-, d- and f-block mean.
• How to classify an element from its position or electron configuration.
• Common exam traps, especially around helium and “atomic mass” wording.

The big picture

The Periodic Table is not just a list of elements. It is a map built around proton number and electron structure.

In this small but important part of Periodicity, you learn how elements are classified into blocks: s-block, p-block, d-block and f-block. This classification helps explain patterns in chemical properties later on, especially in Group 2, Period 3, Group 7 and transition metals.

Elements and proton number

For example, every carbon atom has proton number 6. If an atom has 6 protons, it is carbon. If it has 7 protons, it is nitrogen.

The key point is that the proton number defines the element. Different isotopes of the same element have different numbers of neutrons, but they still have the same proton number.

How the Periodic Table is arranged

The modern Periodic Table is arranged in order of increasing proton number.

This means hydrogen, with proton number 1, comes first. Helium, with proton number 2, comes next. Lithium, with proton number 3, follows, and so on.

The table has:

• groups: vertical columns
• periods: horizontal rows

Elements in the same group often have similar chemical reactions because they have similar outer-shell electron arrangements.

Why electron arrangement matters

A neutral atom has the same number of electrons as protons. So if you know the proton number, you also know how many electrons a neutral atom has.

Those electrons are arranged in shells and subshells.

The Periodic Table can be split into blocks depending on which type of subshell is being filled as proton number increases.

This is why block classification links Periodicity to electron configuration.

The four blocks of the Periodic Table

The Periodic Table is divided like this:

• s-block: left-hand side, mainly Groups 1 and 2
• p-block: right-hand side, mainly Groups 13–18, or Groups 3–0 in some UK school tables
• d-block: central block, containing the transition metals
• f-block: two rows usually shown separately at the bottom

The block layout is much easier to see visually.

The s-block

The s-block is on the left of the Periodic Table.

It includes:

• Group 1 elements, such as lithium, sodium and potassium
• Group 2 elements, such as magnesium and calcium
• hydrogen, because its electron is in a 1s subshell
• helium by electron configuration, even though it is usually placed with the noble gases

Atoms of s-block elements have their highest-energy electron in an s subshell.

For example:

• sodium: $1s^{2}2s^{2}2p^{6}3s^1$
• magnesium: $1s^{2}2s^{2}2p^{6}3s^2$

Both are s-block because the final electrons are in a 3s subshell.

The p-block

The p-block is on the right-hand side of the Periodic Table.

It includes many important non-metals, metalloids and metals, such as:

• carbon
• nitrogen
• oxygen
• sulfur
• chlorine
• aluminium
• silicon

Atoms of p-block elements have their highest-energy electron in a p subshell.

For example, sulfur has electron configuration:

$$1s^{2}2s^{2}2p^{6}3s^{2}3p^4$$

The final electrons are in the 3p subshell, so sulfur is in the p-block.

The d-block

The d-block is the central section of the Periodic Table. It contains the transition metals, although not every d-block element is always classified as a transition metal under the stricter A-Level definition.

You will meet the d-block in more detail later when studying transition metals.

For now, the essential idea is:

• d-block elements are in the middle of the Periodic Table
• their atoms are associated with filling d subshells
• examples include iron, copper, zinc, chromium and nickel

The f-block

The f-block is usually shown as two detached rows underneath the main Periodic Table.

These elements are associated with filling f subshells. You do not need detailed f-block chemistry at this stage, but you should recognise the block as part of Periodic Table classification.

The f-block contains:

• the lanthanoids
• the actinoids

Using position versus using electron configuration

You can classify an element in two main ways.

Method 1: Use its position

If you are given a Periodic Table, locate the element using its proton number or symbol, then identify which block it sits in.

This is usually the quickest method.

Method 2: Use its electron configuration

If you are given proton number and expected to use electron configuration, write the arrangement and look at the type of subshell being filled.

For main-group elements, this is usually straightforward:

• final electron in s subshell → s-block
• final electron in p subshell → p-block
• d subshell being filled across the central section → d-block
• f subshell being filled → f-block

The helium issue

Helium is the awkward element in block classification.

Its electron configuration is:

$$1s^2$$

So, by electron configuration, helium is an s-block element.

However, helium is normally placed on the far right of the Periodic Table with the noble gases because it has very similar chemical properties to them: it is extremely unreactive and has a full outer shell.

A simple decision process

When classifying an element, ask yourself:

1. What is its proton number?
2. Where does that place it in the Periodic Table?
3. Which block is that region: s, p, d or f?

For most exam questions, you will not need to draw full electron configurations. You just need to understand that proton number determines position, and position determines block.