How to Read the Periodic Table: Groups, Periods, Blocks and Trends

The genius of the periodic table is that an element’s position tells you how it behaves. Read the columns and rows correctly and you can predict reactivity, size and bonding without memorising every element. Here’s how the layout works — and the handful of trends that fall out of it.

The two directions

Groups run down (columns). Every element in a group has the same number of valence electrons, which is why they react alike:

• Group 1 (alkali metals) — one outer electron, extremely reactive.
• Group 17 (halogens) — seven outer electrons, reactive nonmetals.
• Group 18 (noble gases) — full outer shell, almost completely inert.

Periods run across (rows). Moving left to right, the atomic number increases by one each step and electrons fill the same principal shell until it’s full — then a new period (a new shell) begins.

Click any element in the interactive periodic table to see its group, period, block and electron configuration for yourself.

The four blocks

The table is split into blocks by which orbital is filling:

Block	Where	What’s filling
s	groups 1–2 (+ He)	s-orbital
p	groups 13–18	p-orbital
d	the transition metals	d-orbital
f	lanthanides & actinides (bottom rows)	f-orbital

The block matches the last electron added — which is exactly what the electron-configuration calculator shows.

The trends that matter

Four trends do most of the heavy lifting, and they all point the same way relative to fluorine (top-right):

• Electronegativity (pull on shared electrons) — increases up and to the right. Fluorine is the highest.
• Ionisation energy (energy to remove an electron) — also increases up and to the right.
• Atomic radius (size) — increases down and to the left, as new shells are added.
• Metallic character / reactivity of metals — increases down and to the left; caesium is about as reactive a metal as they come.

So the most reactive nonmetals huddle top-right, the most reactive metals bottom-left, and the calm noble gases sit in the far-right column. Recolour the periodic table by electronegativity to see the trend as a heat-map.

Metals, nonmetals and the staircase

Draw the diagonal “staircase” from boron down to astatine: metals are to its left (most of the table), nonmetals to its upper right, and the metalloids — boron, silicon, germanium, arsenic, antimony, tellurium — sit right along the line, sharing properties of both.

Why the bottom two rows are separated

The lanthanides (57–71) and actinides (89–103) are the f-block. Slotted into their true position the table would be 32 columns wide, so by convention they’re lifted out and shown below to keep it readable. They still belong between groups 2 and 3 of periods 6 and 7.

Quick summary

Read the periodic table by position: groups (columns) share valence electrons and therefore chemistry; periods (rows) fill electron shells; blocks (s/p/d/f) show which orbital is filling. Electronegativity and ionisation energy rise up and to the right; atomic radius rises down and left; metals sit left, nonmetals upper-right, metalloids on the staircase. Explore it all in the interactive periodic table.

Sources: standard chemistry (IUPAC periodic table organisation and periodic trends). Educational information.