How does first ionization energy change across the periodic table?

It generally increases across a period and decreases down a group. Across a period the nuclear charge grows while the shell stays the same, so Zeff increases, the atom shrinks and the outer electron is held more tightly. Down a group the outer electron is in a higher shell, farther from the nucleus. Helium has the highest first ionization energy (2372 kJ/mol) and the alkali metals such as caesium (376 kJ/mol) the lowest.

Why is the ionization energy of boron lower than beryllium, and oxygen lower than nitrogen?

Boron's outermost electron is in 2p, which is higher in energy than 2s and partly shielded by the 2s pair, so it is easier to remove: B 801 kJ/mol versus Be 899 kJ/mol. Nitrogen has a half-filled 2p³ subshell with one electron per orbital; oxygen's fourth 2p electron must share an orbital, and the extra repulsion makes it easier to remove: O 1314 kJ/mol versus N 1402 kJ/mol. The same pattern appears at Mg→Al and P→S in period 3.

How do you calculate effective nuclear charge with Slater's rules?

Group the orbitals as [1s] [2s,2p] [3s,3p] [3d] [4s,4p] [4d] [4f] [5s,5p] …. For an s or p electron, each other electron in the same group shields 0.35 (0.30 in 1s), each electron in shell n − 1 shields 0.85, and each electron in shell n − 2 or lower shields 1.00. For a d or f electron, same group 0.35 and everything to the left 1.00. Then Zeff = Z − σ. For sodium's 3s electron σ = 8 × 0.85 + 2 × 1.00 = 8.80, so Zeff = 2.20; for chlorine's 3p electron σ = 6 × 0.35 + 8 × 0.85 + 2 = 10.90 and Zeff = 6.10.

Why does chlorine have a higher electron affinity than fluorine?

Following the trend, fluorine should be highest, but the F atom is so small that its 2p electrons are tightly packed and repel an incoming electron strongly, so less energy is released: F 328 kJ/mol versus Cl 349 kJ/mol, making chlorine the element with the largest electron affinity. Likewise O (141 kJ/mol) is below S (200 kJ/mol). Nitrogen's half-filled 2p³ forces the added electron to pair up, so N⁻ is unbound (EA < 0), as are the anions of the noble gases, Be and Mg.

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📊Periodic Trends Chart - Ionization Energy, Electronegativity, Atomic Radius, Electron Affinity & Zeff

Periodic trends are driven by the effective nuclear charge Zeff and the shell number n. Across a period the nuclear charge rises while the shell stays the same; each same-shell electron shields only 0.35, so Zeff grows by about 0.65 per step, atoms shrink, and first ionization energy and electronegativity increase. Down a group a new shell is added, the outer electron sits farther from the nucleus, and ionization energy and electronegativity fall while the radius grows. This tool plots these properties against atomic number for Z = 1–86 as a line or bar chart with shaded periods, highlights any group or period, and shows that subset on its own with Slater Zeff values and an explanation. First ionization energies are NIST Atomic Spectra Database values (H 1312, He 2372, Li 520, Be 899, B 801, C 1086, N 1402, O 1314, F 1681, Ne 2081, Na 496 kJ/mol; switchable to eV), and the chart marks the classic dips Be→B, N→O, Mg→Al and P→S, plus the electron-affinity exceptions — N below zero and Cl (349 kJ/mol) above F (328 kJ/mol). Atomic radius uses one consistent set, the Cordero 2008 covalent radii; electron affinities include only well-established experimental values, with unbound anions shown as “< 0”. The Slater's-rules calculator lists every shielding term: for the Na 3s electron σ = 8×0.85 + 2×1.00 = 8.80, so Zeff = 2.20; for a Cl 3p electron Zeff = 6.10. You also get successive ionization energies for Z ≤ 20 on a log scale (Na 496, 4562, 6910 kJ/mol — the jump shows one valence electron), a two-property scatter plot with Pearson r, and a periodic-table heat map of the chosen property.

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How to use Periodic Trends

1Choose a property (ionization energy, electronegativity, atomic radius, electron affinity, Zeff, melting or boiling point) and a range of Z
2Highlight a group or period to see its trend in isolation with Zeff values and an explanation, or hover over the chart to read values
3Tap an element in the heat map to see its Slater's-rules Zeff working, successive ionization energies and a scatter plot of two properties

FAQ

How does first ionization energy change across the periodic table?: It generally increases across a period and decreases down a group. Across a period the nuclear charge grows while the shell stays the same, so Zeff increases, the atom shrinks and the outer electron is held more tightly. Down a group the outer electron is in a higher shell, farther from the nucleus. Helium has the highest first ionization energy (2372 kJ/mol) and the alkali metals such as caesium (376 kJ/mol) the lowest.
Why is the ionization energy of boron lower than beryllium, and oxygen lower than nitrogen?: Boron's outermost electron is in 2p, which is higher in energy than 2s and partly shielded by the 2s pair, so it is easier to remove: B 801 kJ/mol versus Be 899 kJ/mol. Nitrogen has a half-filled 2p³ subshell with one electron per orbital; oxygen's fourth 2p electron must share an orbital, and the extra repulsion makes it easier to remove: O 1314 kJ/mol versus N 1402 kJ/mol. The same pattern appears at Mg→Al and P→S in period 3.
How do you calculate effective nuclear charge with Slater's rules?: Group the orbitals as [1s] [2s,2p] [3s,3p] [3d] [4s,4p] [4d] [4f] [5s,5p] …. For an s or p electron, each other electron in the same group shields 0.35 (0.30 in 1s), each electron in shell n − 1 shields 0.85, and each electron in shell n − 2 or lower shields 1.00. For a d or f electron, same group 0.35 and everything to the left 1.00. Then Zeff = Z − σ. For sodium's 3s electron σ = 8 × 0.85 + 2 × 1.00 = 8.80, so Zeff = 2.20; for chlorine's 3p electron σ = 6 × 0.35 + 8 × 0.85 + 2 = 10.90 and Zeff = 6.10.
Why does chlorine have a higher electron affinity than fluorine?: Following the trend, fluorine should be highest, but the F atom is so small that its 2p electrons are tightly packed and repel an incoming electron strongly, so less energy is released: F 328 kJ/mol versus Cl 349 kJ/mol, making chlorine the element with the largest electron affinity. Likewise O (141 kJ/mol) is below S (200 kJ/mol). Nitrogen's half-filled 2p³ forces the added electron to pair up, so N⁻ is unbound (EA < 0), as are the anions of the noble gases, Be and Mg.

Features

First ionization energy for Z 1–86 from NIST, in kJ/mol or eV, as a line or bar chart
Pauling electronegativity, Cordero covalent radius, electron affinity, melting and boiling point trends
Shaded periods, group / period highlighting and hover readout
Trend in isolation for any group or period, with a Slater Zeff table and explanation
Marked and explained anomalies: Be→B, N→O, Mg→Al, P→S, and the N and F/Cl electron affinities
Step-by-step Slater's rules Zeff, including (n−1)d electrons for transition metals
Successive ionization energies for Z ≤ 20 on a log scale with the core-shell jump detected
Two-property scatter plot with Pearson correlation and a periodic-table heat map

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