Chemistry · Lesson

Atomic size, ionisation energy and electron affinity

Compare atomic sizes and energy changes using matched definitions. Use nitrogen/oxygen and fluorine/chlorine data to see where a simple trend needs qualification.

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By the end, you can

State the species and process in an ionization-energy comparison.
Explain why atomic-radius definitions must match.
Distinguish positive electron-affinity convention from attachment energy change.

An atomic radius needs a definition

Atoms do not have one hard visible boundary. Common radius definitions use distances in particular bonding or nonbonding contexts, such as covalent, metallic or van der Waals radii. Compare values from the same definition and compatible context before interpreting a trend. A covalent radius and a nonbonded-contact radius can legitimately differ for the same element.

Many main-group covalent-radius trends show decreasing size across a period and increasing size down a group. Effective attraction and occupied-shell structure help explain them. These patterns are starting points, not exact numerical formulas or a promise of identical behaviour in every material and bonding state.

Ionization energy belongs to a specified process

First ionization energy describes removing an electron from a neutral gaseous atom in its stated initial state, producing a singly positive gaseous ion and an electron. The required energy is positive. A second ionization starts from the singly positive ion, so it is a different process and should not be substituted into a first-ionization comparison.

First ionization often increases across a main-group period, but measured exceptions matter. NIST gives nitrogen about 14.5341 eV and oxygen about 13.61805 eV. Oxygen’s value is lower despite its larger Z. Occupation and electron interactions, including pairing in oxygen’s 2p population, contribute to the explanation; one periodic arrow does not calculate the numerical difference.

A measured exception to a simple across-period arrow

First ionization energy / eV

Atomic number Z

Nitrogen, N
Oxygen, O

Underlying data

Atomic number Z	First ionization energy / eV
7	14.53 · Nitrogen, N
8	13.62 · Oxygen, O

Rounded NIST first-ionization energies for neutral gaseous nitrogen and oxygen atoms. The energy axis begins at zero; only these two source values are plotted, without fitting a general trend.

Define the electron-affinity sign

Consider X(g) + e⁻ → X⁻(g) for a specified atom and states. One electron-affinity convention reports a positive energy released for favourable attachment, equivalently the positive energy required for the reverse detachment. The system’s energy change for that attachment is then negative. Some textbooks instead tabulate the attachment energy change itself with a negative sign for exothermic attachment.

State the reaction and convention before comparing numbers. Using a positive released-energy convention, inspected NIST reference values are about 3.40 eV for atomic fluorine and 3.61 eV for atomic chlorine. The chlorine value is larger. Small-atom electron interactions help explain why the simplest size-based expectation fails; the values are species-specific data, not a monotonic group rule.

Match energy, enthalpy and state

Electron affinity values for specified ground-state processes and electron-gain enthalpies at finite temperature are related but not automatically identical thermodynamic quantities with only an exact sign flip. Thermal contributions and conventions need attention. Do not combine two tables merely because both contain an “electron affinity” heading.

Keep the species, charge, initial state, temperature convention, process and units visible. A useful comparison is narrower than “this element likes electrons”. It says what energy change was measured or evaluated for which process, then explains the trend within that scope.

Worked examples

Compare the specified energies

Using 14.5341 eV for N and 13.61805 eV for O, which first ionization requires more energy, and by how much?

Both values refer to first ionization of neutral gas-phase atoms.
Subtract oxygen’s value from nitrogen’s: 14.5341 − 13.61805 = 0.91605 eV.
Check the sign: nitrogen’s value is larger.

Nitrogen requires about 0.916 eV more in this comparison. The observed exception limits a simple monotonically increasing trend.

Try it yourself

Should a covalent radius and a van der Waals radius be treated as the same measured quantity?
Show the answer and reasoning
No.
Their definitions and physical contexts differ.
Does a second ionization energy describe removal from the same initial neutral atom as the first?
Show the answer and reasoning
No.
Second ionization starts from the singly positive ion.
An attachment process releases 3 energy units under the positive-affinity convention. What is the corresponding system energy change?
Show the answer and reasoning
−3 units for the stated attachment.
Positive released energy and the system’s negative energy change describe opposite sign conventions for the same specified accounting.
Using positive released-energy values F ≈ 3.40 eV and Cl ≈ 3.61 eV, which is larger?
Show the answer and reasoning
Chlorine’s value.
3.61 exceeds 3.40; the difference is about 0.21 eV at these rounded values.

What to carry forward

Compare atomic radii with matched definitions.
First and later ionizations have different initial species.
Write the attachment reaction and energy-sign convention.
Use measured exceptions to refine a periodic pattern.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.