Blocks of the Periodic Table

Introduction

Look at the shape of the periodic table: a tall pair of columns on the left, a wide section on the right, a low central area and two long rows floating underneath. That odd shape is not an accident of printing. It divides naturally into four rectangular regions called blocks : the s-block, p-block, d-block and f-block. Each block is named after the type of electron sub-shell being filled, and the width of each block tells you how many electrons that sub-shell can hold.

Core explanation

Shells contain sub-shells. So far you have treated each electron shell as a single level holding 2, 8, 8 and so on electrons. More detailed models show that shells are divided into sub-shells , labelled s, p, d and f. Each type holds a fixed maximum number of electrons:

Sub-shell Maximum electrons Width of block --- --- --- s 2 2 columns p 6 6 columns d 10 10 columns f 14 14 columns

The second shell, for example, has one s sub-shell (2 electrons) and one p sub-shell (6 electrons), which is why it holds 8 in total.

The four blocks.

- s-block: groups 1 and 2 on the left, plus helium. The last electron added goes into an s sub-shell. These are reactive metals, apart from hydrogen and helium. - p-block: groups 13 to 18 on the right. The last electron goes into a p sub-shell. This block contains all the non-metals except hydrogen and helium, the metalloids and some metals such as aluminium, tin and lead. - d-block: groups 3 to 12 in the centre. The last electron goes into a d sub-shell of an inner shell. These are the transition metals, such as iron, copper and zinc. - f-block: the two rows at the bottom — the lanthanides and actinides. Here electrons enter f sub-shells deeper inside the atom. All are metals.

Why the table has this shape. Going across a period, electrons fill the sub-shells in a set order. Period 2 fills an s sub-shell (2 elements, lithium and beryllium) and then a p sub-shell (6 elements, boron to neon). From period 4, a d sub-shell of the shell below is filled between the s and p, inserting 10 extra elements. From period 6, an f sub-shell adds 14 more. The block widths — 2, 6, 10 and 14 — build the shape you see.

Helium's odd position. Helium's two electrons fill an s sub-shell, so strictly it belongs in the s-block. It is placed above neon in group 18 because it behaves like a noble gas: its shell is full.

Blocks and properties. Blocks give a quick guide to behaviour: s-block elements are soft, reactive metals; d-block elements are hard, dense metals often forming coloured compounds; p-block elements are mostly non-metals; f-block elements are metals, many of them radioactive in the actinide row.

Step-by-step reasoning

To identify an element's block:

1. Find the element's group on the table. 2. Groups 1 and 2: s-block. 3. Groups 3 to 12: d-block. 4. Groups 13 to 18: p-block (but helium is s-block). 5. In the separate rows below the table: f-block.

Visual explanation

Picture the periodic table shaded in four colours: a narrow two-column strip on the left for s, a ten-column trough in the middle for d, a six-column strip on the right for p, and two long fourteen-box rows underneath for f. Pull the f rows up into place and the table becomes 32 columns wide.

Real-world analogy

Think of a car park split into zones for motorbikes, cars, vans and coaches. Each zone has spaces of a different size and number. Blocks are like those zones: each holds a different type of sub-shell, with its own fixed number of "spaces" for electrons.

Real-world example

Many strong permanent magnets in electric car motors and wind turbines contain neodymium, an f-block element, combined with iron from the d-block. Engineers often talk about "rare earth" and "transition metal" supplies, which correspond to these blocks.

Why?

Why is the d-block ten columns wide? Because a d sub-shell can hold a maximum of 10 electrons. As each transition metal adds one more electron to the d sub-shell, it takes 10 elements to fill it — hence 10 columns.

Common misconception

"The blocks are named after types of element, such as d for 'dense'." The letters s, p, d and f come from the names of lines in atomic spectra (sharp, principal, diffuse and fundamental). They refer to sub-shells, not to properties.

Worked example

Question: Name the block for each element and say whether it is a metal: sodium (group 1), bromine (group 17), iron (group 8) and uranium (an actinide).

Reasoning: Use the group ranges: group 1 is s; 13 to 18 is p; 3 to 12 is d; actinides are f.

Answer: Sodium — s-block, metal. Bromine — p-block, non-metal. Iron — d-block, metal. Uranium — f-block, metal.

Quick check

1. How many columns wide is the p-block, and why? Answer: Six, because a p sub-shell can hold up to six electrons.

Exam focus

At this level, you should be able to label the four blocks on an outline of the table and name the type of element found in each. Remember that the transition metals are the d-block, and that helium is the exception placed outside its block.

Advanced insight

The full electron configuration shows the block directly. Sodium is 1s² 2s² 2p⁶ 3s¹, ending in s; chlorine is 1s² 2s² 2p⁶ 3s² 3p⁵, ending in p. In the d-block, the 3d sub-shell fills after 4s, which is why period 4 contains the first row of transition metals.

Summary

The periodic table is divided into four blocks named after the sub-shell being filled: s (groups 1–2 and helium), p (groups 13–18), d (groups 3–12, the transition metals) and f (lanthanides and actinides). The widths 2, 6, 10 and 14 match the maximum electrons in s, p, d and f sub-shells, which explains the table's shape.

Practice questions

1. Which groups make up the s-block? Answer: Groups 1 and 2, together with helium. 2. Explain why the d-block contains ten columns. Answer: A d sub-shell holds up to 10 electrons, so ten elements are needed to fill it, one column each. 3. In which block are most non-metals found? Answer: The p-block. 4. Why is helium placed in group 18 even though its outer electrons are in an s sub-shell? Answer: Its outer shell is full, so it is unreactive like the noble gases in group 18.