1_2 The Modern Periodic Table: Groups, Periods, Blocks and Families
The modern periodic table arranges all known elements in order of increasing atomic number, the number of protons in the nucleus. Because elements are placed this way, their electron arrangements repeat in a regular cycle, and so do their properties. Once you know how the table is laid out, an element's position tells you a lot about how it behaves before you learn a single fact about it.
Groups and Periods
The vertical columns are groups. Elements in a group have the same number of electrons in their outermost shell (their valence electrons), so they react in similar ways. Two numbering systems are in use. The older one has eight groups (I to VIII), each split into an A subgroup of representative (main-group) elements and a B subgroup of transition elements. The IUPAC system simply numbers the 18 columns from 1 to 18. Groups IA and IIA are groups 1 and 2, the B subgroups are groups 3 to 12, and IIIA to VIIIA are groups 13 to 18.
The horizontal rows are periods, numbered 1 to 7. The period number is the number of the outermost electron shell that is being filled. Moving along a period, each element has one more proton and one more electron than the last, so properties change step by step until the shell is full and the pattern starts again in the next row.
The seven periods
Period | Elements | Type | What is being filled |
1 | 2 (H, He) | Very short | 1s |
2 and 3 | 8 each | Short | s and p of shells 2 and 3 |
4 and 5 | 18 each | Long | s, p and the d subshell of the shell below |
6 and 7 | 32 each | Very long | s, p, d and an f subshell (lanthanides, actinides) |
In the short periods, every element is a representative element, and the eighth element along resembles the first: lithium and sodium, or fluorine and chlorine. In the long periods, ten transition elements are inserted in the middle, so similar elements are 18 places apart, as potassium (
Z = 19
) and rubidium (Z = 37
) are. Periods 6 and 7 add fourteen inner transition elements each: the lanthanides after lanthanum and the actinides after actinium. These are printed in two rows below the main table to keep it a sensible width. Period 7 has been complete since elements 113, 115, 117 and 118 were named in 2016.Blocks: Where the Last Electron Goes
The table can also be split into four blocks, named after the subshell that receives the last electron when the element's configuration is built up. The block tells you which electrons take part in bonding, and so it predicts the typical oxidation states.
- s-block (groups 1 and 2): valence electrons in an s orbital, e.g. sodium
\mathrm{[Ne]\,3s^1}
. Reactive metals with oxidation states +1 or +2. - p-block (groups 13 to 18): the last electron enters a p orbital, e.g. chlorine
\mathrm{[Ne]\,3s^2 3p^5}
. Metals, metalloids and all non-metals except hydrogen and helium. - d-block (groups 3 to 12): the transition elements, filling the d subshell of the shell below the outermost one. Many show several oxidation states and coloured compounds.
- f-block: the lanthanides and actinides, filling a 4f or 5f subshell. Elements within each series are chemically very alike.
Helium is the one awkward case. Its configuration is
\mathrm{1s^2}
, so by electrons it belongs to the s-block, but it is placed in group 18 because it is an unreactive noble gas like neon and argon.Element Families
Some groups behave so consistently that they have family names, which are worth knowing because chemists use them constantly:
Common family names
Family | Group | Why the name |
Alkali metals | 1 (IA) | React with water to form strongly alkaline hydroxides, e.g. NaOH |
Alkaline earth metals | 2 (IIA) | Their oxides are alkaline and were once called 'earths' |
Halogens | 17 (VIIA) | From Greek for 'salt former': they combine with metals to make salts such as NaCl |
Noble gases | 18 (VIIIA) | Full outer shells make them very unreactive |
Transition elements | 3–12 (B) | Sit between the s- and p-blocks, bridging their properties |
You may also meet the term rare earth elements. It means scandium, yttrium and the lanthanides. The name is historical: they are not especially rare, but they are hard to separate from one another.
Metals, Non-metals and Metalloids
A stepped line runs diagonally through the p-block, from boron down to astatine. Elements to the left of it, plus the whole s-, d- and f-blocks, are metals: they are shiny, conduct electricity, and tend to lose electrons to form positive ions. Elements to the upper right are non-metals, which tend to gain or share electrons. Hydrogen is a non-metal despite sitting on the far left.
Elements along the line, such as boron, silicon, germanium, arsenic, antimony and tellurium, are metalloids (semi-metals). They show a mix of properties. Silicon, for example, looks metallic but is brittle and only a modest conductor, which is exactly why it is useful in semiconductors. Metallic character increases down a group and decreases across a period, which is why the line slants the way it does.
Tips for remembering
- Columns share chemistry, rows share shells: elements in a group have the same outer-electron arrangement; elements in a period fill the same outermost shell.
- Period lengths 2, 8, 8, 18, 18, 32, 32: each length is the number of electrons the subshells being filled in that row can hold.
- Block = last subshell filled: s on the left, p on the right, d in the middle, f pulled out underneath.
- Metalloids hug the staircase: boron, silicon, germanium, arsenic, antimony and tellurium sit along the stepped line between metals and non-metals.