Atom size trends

The best way to understand atom size trends is by adding electrons, protons, and neutrons to an atom one by one to see how they affect atom size. You will learn why atom size gradually decreases from left to right across any given row in the periodic table, and increases again when you continue on to the next row.

This lesson is a part of MEL VR Science Simulations. Learn more →

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Re­cent­ly we talked about how to de­ter­mine the size of an atom. To­day, we will look at the pat­terns of change in the size of atoms in the pe­ri­od­ic ta­ble.

First, let's look at the hy­dro­gen atom.

It has one pro­ton in the nu­cle­us and one elec­tron in the 1s-or­bital. It is the first atom in the pe­ri­od­ic ta­ble.

To make the next atom we should add an elec­tron, a pro­ton, and two neu­trons to the hy­dro­gen atom.

Add an elec­tron.

It oc­cu­pies the same or­bital as the first elec­tron, but the size of the atom be­comes much larg­er.

In the nu­cle­us, there is only one pro­ton, and both elec­trons feel its at­trac­tion, but two elec­trons in a small 1S or­bital also strong­ly re­pel each oth­er, so they need much more space.

Add a pro­ton. Now the nu­cle­us at­tracts elec­trons more strong­ly and the size of the atom has de­creased. Add two neu­trons and put the he­li­um atom next to the hy­dro­gen atom.

To make the next atom, again we need to add an elec­tron, a pro­ton, and two neu­trons. Add a pro­ton.

The size of the atom de­creas­es. Add an elec­tron. The atom in­creas­es in size! But why?

The third elec­tron oc­cu­pies the 2s-or­bital, which is much big­ger than the 1s-or­bital.

Add the neu­trons and put lithi­um in its place in the ta­ble.

Ev­ery sub­se­quent el­e­ment in the sec­ond pe­ri­od has one more pro­ton than the pre­ced­ing el­e­ment, so the nu­cle­ar charge in­creas­es and the out­er­most elec­trons come clos­er to the nu­cle­us.

In the third pe­ri­od, the first atom will again be very large and the size of the atoms will de­crease along the pe­ri­od.

So atom­ic size gen­er­al­ly de­creas­es as you move from left to right across a pe­ri­od. And atom­ic size in­creas­es as you move from top to bot­tom through a group.

Look at the pe­ri­od­ic ta­ble and de­ter­mine: which atom is the largest in this ta­ble?

In the first 5 pe­ri­ods, Ru­bid­i­um is the largest atom.

Which atom is the small­est in the pe­ri­od­ic ta­ble?

That’s right, but re­mem­ber that he­li­um is a no­ble gas which does not re­act with oth­er atoms, so among atoms that can form sub­stances, flu­o­rine is the small­est.

Teacher's notes


atoms, atom­ic struc­ture, elec­trons, pro­tons, atom size, atom­ic ra­dius, coulom­bic force

Stu­dents will

  • Re­vise the el­e­ments that are ar­ranged in the Pe­ri­od­ic Ta­ble ac­cord­ing to their atom­ic num­ber: num­ber of pro­tons (equal to the num­ber of elec­trons).
  • Learn that pro­tons at­tract elec­trons, but elec­trons them­selves re­pel.
  • Learn that atom­ic size de­creas­es through the pe­ri­ods.
  • Learn that atom­ic size in­creas­es through the group.
  • Find the largest and the small­est atoms in the ta­ble.

His­to­ry and sources of knowl­edge

Top­ics to dis­cuss

  • Atom sizes: def­i­ni­tion, prob­lems of mea­sure­ment. (Re­vise VR les­son ’Atom size’)
  • Coun­ter­in­tu­itive trends in pe­ri­ods: the atoms be­came heav­ier but atom­ic size de­creas­es.
  • Forces de­ter­min­ing the size of atoms.
  • Pe­ri­od­ic­i­ty in atom­ic size.


  • Why the atom size de­creas­es in pe­ri­ods?
  • Why the atom size in­creas­es in groups?