Periodic Table Trends

Q: What causes all these trends?

Two competing effects: effective nuclear charge (Zeff) rises across a period because shielding by same-shell electrons is weak, and the number of shells rises down a group. Use the explainer above to see both.

Atomic radius, ionization energy, electronegativity and electron affinity all follow from one idea: how hard the nucleus pulls on the outermost electrons.

Property	Across a period →	Down a group ↓
Atomic radius	decreases	increases
Ionization energy	increases	decreases
Electronegativity	increases	decreases
Electron affinity (energy released)	increases	decreases (roughly)
Metallic character	decreases	increases

The cause: effective nuclear charge

An outer electron does not feel the full charge of the nucleus. Electrons in shells below it cancel, or shield, most of that charge. What is left is the effective nuclear charge, Z_eff. John Slater’s 1930 rules estimate it by counting how much each other electron screens.

Step through the model below. Watch Z_eff, the size of the atom and its ionization energy move together, then switch direction.

Explainer: why atoms shrink across a row and grow down a column

Sodium (Na) · shells 2, 8, 1

Z_eff = 11 protons − 8.8 shielding = 2.2

Inner (shielding) electrons: 10
Outer electrons: 1
Atomic radius: 154 pm
1st ionization energy: 496 kJ/mol

Each step adds one proton and one electron, but the new electron joins the same shell. Electrons in the same shell screen each other poorly (0.35 each in Slater’s rules), so Z_eff rises by about 0.65 per step. The outer shell is pulled in and gets harder to ionize.

If you like learning by stepping through a model like this, ahaboo has narrated interactive explainers of how everyday things work, from why the seasons happen to how photosynthesis works.

See each trend on the table

Swipe sideways to see the whole table →

1H37 2He32 3Li134 4Be90 5B82 6C77 7N75 8O73 9F71 10Ne69 11Na154 12Mg130 13Al118 14Si111 15P106 16S102 17Cl99 18Ar97 19K196 20Ca174 21Sc144 22Ti136 23V125 24Cr127 25Mn139 26Fe125 27Co126 28Ni121 29Cu138 30Zn131 31Ga126 32Ge122 33As119 34Se116 35Br114 36Kr110 37Rb211 38Sr192 39Y162 40Zr148 41Nb137 42Mo145 43Tc156 44Ru126 45Rh135 46Pd131 47Ag153 48Cd148 49In144 50Sn141 51Sb138 52Te135 53I133 54Xe130 55Cs225 56Ba198 57La169 58Ce—59Pr—60Nd—61Pm—62Sm—63Eu—64Gd—65Tb—66Dy—67Ho—68Er—69Tm—70Yb—71Lu160 72Hf150 73Ta138 74W146 75Re159 76Os128 77Ir137 78Pt128 79Au144 80Hg149 81Tl148 82Pb147 83Bi146 84Po—85At—86Rn145 87Fr—88Ra—89Ac—90Th—91Pa—92U—93Np—94Pu—95Am—96Cm—97Bk—98Cf—99Es—100Fm—101Md—102No—103Lr—104Rf—105Db—106Sg—107Bh—108Hs—109Mt—110Ds—111Rg—112Cn—113Nh—114Fl—115Mc—116Lv—117Ts—118Og—

Atomic radius (pm)

32225

Atomic radius trend

Across period 3, radius falls from sodium (186 pm metallic radius, 154 pm covalent) to chlorine (99 pm) as Z_eff rises. Down group 1 it climbs from lithium to caesium as shells are added. The values here are covalent radii from the CRC Handbook; tables that use metallic or van der Waals radii give larger numbers but the same trend.

Ionization energy trend

The first ionization energy is the energy to remove one electron from a gaseous atom. A higher Z_eff and a smaller atom both hold the electron tighter, so the trend mirrors radius. Look for the dips at boron and oxygen in period 2: boron’s electron is in a higher-energy 2p orbital, and oxygen’s fourth 2p electron has to pair up.

Electronegativity trend

Electronegativity is the pull on shared electrons in a bond, so it follows Z_eff and size the same way. See the full electronegativity chart with a bond-polarity checker.

Electron affinity trend

Electron affinity is the energy change when a neutral atom gains an electron. The halogens release the most, because one extra electron completes their shell. Values in this table are signed: negative means energy is released.

Questions students ask

What is the atomic radius trend?

Atomic radius decreases from left to right across a period and increases from top to bottom down a group. Caesium and francium are the largest atoms; helium is the smallest.

What is the ionization energy trend?

First ionization energy increases across a period and decreases down a group: the opposite of atomic radius. Helium has the highest (2372 kJ/mol) and caesium one of the lowest (376 kJ/mol). There are small dips at group 13 (a p electron is easier to remove than an s electron) and group 16 (the first paired p electron repels its partner).

What is the electron affinity trend?

Electron affinity generally becomes more negative (more energy released) across a period, peaking at the halogens. Chlorine, not fluorine, has the most negative value, because fluorine’s small 2p shell is crowded. Noble gases and group 2 have values near zero or positive.

What is metallic character and how does it trend?

How easily an atom loses electrons. It is the reverse of ionization energy: it increases down a group and decreases across a period, so the most metallic elements sit bottom-left and the least metallic top-right.

What causes all these trends?

Two competing effects: effective nuclear charge (Z_eff) rises across a period because shielding by same-shell electrons is weak, and the number of shells rises down a group. Use the explainer above to see both.