How to Interpret Successive Ionization Energy Graphs

If you remove the electrons from an atom one at a time and measure the energy needed for each, you get a list of successive ionisation energies. Those numbers are a powerful piece of evidence: they show that electrons are arranged in shells, reveal how many are in the outer shell, and let you identify an element’s group from data alone. This guide explains how to read the data and the graphs step by step.

What are successive ionisation energies?

The first ionisation energy is the energy needed to remove one electron from each atom in a mole of gaseous atoms:

X(g) → X⁺(g) + e⁻

The second ionisation energy removes an electron from each 1+ ion:

X⁺(g) → X²⁺(g) + e⁻

And so on: the nth ionisation energy removes the nth electron. They’re measured in kJ/mol.

See ionization energy trend for how first ionisation energies vary across the periodic table.

Rule 1: each ionisation energy is larger than the one before

Successive ionisation energies always increase, because:

• Each electron is removed from an increasingly positive ion, so it’s held more strongly.
• With fewer electrons, there’s less repulsion and less shielding, so the remaining electrons are pulled closer to the nucleus.

Rule 2: big jumps mark a new shell

When an electron has to be removed from a shell closer to the nucleus, there’s a large jump in ionisation energy. That electron:

• is much closer to the nucleus
• is shielded by fewer inner electrons
• feels a much larger effective nuclear charge

So the position of the first big jump tells you how many electrons were in the outer shell, and therefore the group.

Step-by-step method

1. List the ionisation energies in order (or read them from the graph).
2. Look at the ratio or difference between consecutive values.
3. Find the first large jump, much bigger than the steady increases before it.
4. Count the number of electrons removed before the jump. That’s the number of outer (valence) electrons.
5. Identify the group: 1 electron → group 1; 2 → group 2; 3 → group 13; 4 → group 14; and so on.
6. Check for later jumps, which mark further shells.

Worked example 1: sodium

n	Ionisation energy (kJ/mol)
1	496
2	4,562
3	6,910
4	9,543
5	13,354

• 1st to 2nd: 496 → 4,562, a jump of more than nine times.
• Only one electron is removed before the jump.
• One outer electron: group 1. This matches sodium’s configuration, [Ne] 3s¹. The second electron comes from the 2p subshell of the full inner shell.

That enormous second ionisation energy is why sodium forms Na⁺ and never Na²⁺ in compounds.

Worked example 2: aluminium

n	Ionisation energy (kJ/mol)
1	578
2	1,817
3	2,745
4	11,577
5	14,842

• The big jump is between the 3rd and 4th (2,745 → 11,577, over four times).
• Three outer electrons: group 13. Aluminium is [Ne] 3s² 3p¹.

Worked example 3: identify an unknown element

An element in period 3 has these successive ionisation energies (kJ/mol): 786, 1,577, 3,232, 4,356, 16,091, 19,805. Identify it.

• Increases from 1 to 4 are steady (roughly doubling at most).
• 4th → 5th: 4,356 → 16,091, a jump of nearly four times.
• Four outer electrons: group 14.
• Period 3, group 14: silicon ([Ne] 3s² 3p²).

Using graphs

Ionisation energies range from hundreds to hundreds of thousands of kJ/mol, so graphs usually plot log₁₀(ionisation energy) against the number of the electron removed. A log scale makes the pattern much clearer.

The shape for a whole atom

For an atom such as potassium (19 electrons), a log graph of all nineteen ionisation energies shows:

• 1 electron (4s), then a big jump
• 8 electrons (3s² 3p⁶), then a big jump
• 8 electrons (2s² 2p⁶), then a big jump
• 2 electrons (1s²)

Pattern: 1, 8, 8, 2, which matches potassium’s shell arrangement, 2, 8, 8, 1 (read in reverse, because the outermost electrons are removed first). This is some of the most direct evidence that electrons are arranged in shells. See electron shells explained.

Smaller steps within a shell: subshells

Look closely at the eight electrons of one shell and you’ll often see a smaller step between the p electrons and the s electrons. For example, within the second shell, removing the 2s electrons takes noticeably more energy than removing the 2p electrons, because s electrons penetrate closer to the nucleus. This is evidence for subshells. See shells vs subshells vs orbitals.

First ionisation energies vs successive ionisation energies

Don’t confuse these two kinds of data:

	First ionisation energies across a period	Successive ionisation energies of one element
What changes	the element	the number of electrons already removed
General trend	increases across a period (with dips at groups 13 and 16)	always increases
Main use	periodic trends, subshell evidence	finding the group, shell structure

Common exam questions

1. “Explain why the second ionisation energy of an element is greater than the first.” The second electron is removed from a positive ion, with the same number of protons attracting fewer electrons, so there’s less repulsion and a greater effective attraction.
2. “Explain the large increase between the 2nd and 3rd ionisation energies of magnesium.” The third electron is removed from the second shell (2p), which is closer to the nucleus and less shielded, so it’s held much more strongly.
3. “Deduce the group from these data.” Find the first big jump and count the electrons before it.

Tips

• Compare ratios, not just differences. A jump of 2,000 kJ/mol might be small for large values but huge for small ones.
• Use the first big jump for the group; later jumps confirm inner shells.
• Link to configurations in your explanation: name the subshell each electron comes from.
• Mention distance, shielding and nuclear attraction when explaining jumps.

Practice

An element has successive ionisation energies (kJ/mol): 738, 1,451, 7,733, 10,543, 13,630. Which group is it in, and why is there a large jump?

Answer: The big jump is between the 2nd and 3rd (1,451 → 7,733), so it has two outer electrons: group 2 (magnesium). The third electron is removed from an inner, full shell (2p), which is closer to the nucleus and less shielded.

For the concepts behind this, see core vs valence electrons and effective nuclear charge.

Key takeaways

• Successive ionisation energies always increase, because each electron is removed from a more positive ion.
• A large jump shows that an electron is being removed from an inner shell.
• The number of electrons removed before the first large jump equals the number of outer electrons, which gives the group.
• Log graphs of all ionisation energies reveal shells (e.g. 1, 8, 8, 2 for potassium), and smaller steps reveal subshells.
• Explain jumps using distance from the nucleus, shielding and effective nuclear charge.