Periodic Table
The periodic table lists every known element in order of atomic number, in 7 rows (periods) and 18 columns (groups). Elements in the same group have similar outer-electron arrangements and so behave in similar ways, which lets chemists predict properties from position. Mendeleev's 1869 version was so successful that it predicted elements nobody had yet found; the seventh period was completed in 2015.
From atomic weight to atomic number
Mendeleev's ordering by atomic weight forced a few swaps, such as tellurium before iodine, justified by chemical similarity. Moseley's 1913 X-ray work, in which the square root of characteristic X-ray frequency plotted against atomic number gives a straight line, supplied a physical measurement of atomic number and resolved those inversions.
Predictive power
Mendeleev predicted about 68 for eka-aluminium's atomic weight; gallium, discovered in 1875, has 69.72. Scandium and germanium, also predicted, were found by 1886. The noble gases, discovered in the 1890s, slotted in as a new final group.
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How the table is organised
Reading left to right along a row, the atomic number goes up by one each step. Rows are called periods and columns are called groups; IUPAC has recommended since 1988 that the groups simply be numbered 1 to 18. Some groups have family names: group 1 the alkali metals, group 2 the alkaline earth metals, group 17 the halogens and group 18 the noble gases.
- Group (column): members typically have similar properties and similar outer-shell electron configurations.
- Period (row): the atomic number rises by one from each element to the next.
- Metals sit to the left and centre, non-metals to the upper right, with metalloids between them.
Worked example: reading a position
Chlorine has atomic number 17 and sits in group 17, period 3. Being in group 17 makes it a halogen, like fluorine above it in period 2, and halogens typically gain one electron to form 1− ions. Sodium, in group 1 of the same period, typically loses one electron to form a 1+ ion — which is why the two combine as Na⁺Cl⁻.
Trends you can read off the table
Electronegativity — how strongly an atom attracts the electrons in a bond — generally increases from left to right across a period and decreases down a group, so the most electronegative elements, led by fluorine, are at the upper right.
How we got here
In 1869, with about 60 elements known, Dmitri Mendeleev arranged them by increasing atomic weight and noticed that similar elements recurred at regular intervals. He left gaps and predicted the properties of missing elements; 'eka-aluminium' turned up as gallium in 1875. In 1913 Henry Moseley found a way to measure atomic number, which is why today's table is ordered by atomic number rather than atomic weight. In 2015 IUPAC verified elements 113, 115, 117 and 118, completing period 7, and named them in 2016.
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Sources and methodology
- The modern periodic table arranges the elements in order of increasing atomic number, with vertical columns called groups and horizontal rows called periods: 18 groups and 7 periods. (awaiting scientific review)
- Since 1988 IUPAC has recommended that the groups (columns) of the periodic table simply be numbered from 1 to 18. (awaiting scientific review)
- IUPAC Periodic Table of Elements (release dated 4 May 2022) — Government or standards body
- Members of a group in the periodic table typically have similar properties and similar electron configurations in their outer shell. (awaiting scientific review)
- Chlorine — Element information, properties and uses (Periodic Table) — Primary dataset
- Dmitri Mendeleev's 1869 periodic table ordered the elements by increasing atomic weight and left gaps for undiscovered elements; gallium, which he had predicted as 'eka-aluminium', was discovered in 1875. (awaiting scientific review)
- Development of the periodic table — Science media
- About 60 elements were known when Mendeleev completed his periodic system in 1869. (awaiting scientific review)
- The Periodic Table of Chemical Elements: a window on the universe — Government or standards body
- In 1913 Henry Moseley's X-ray measurements gave a way to measure atomic number, explaining why pairs such as tellurium and iodine sit out of atomic-weight order in the table. (awaiting scientific review)
- Development of the periodic table — Science media
- In December 2015 IUPAC verified the discoveries of elements 113, 115, 117 and 118, completing the seventh period (row) of the periodic table. (awaiting scientific review)
- Discovery and Assignment of Elements with Atomic Numbers 113, 115, 117 and 118 — Government or standards body
- On 28 November 2016 IUPAC approved the names nihonium (Nh), moscovium (Mc), tennessine (Ts) and oganesson (Og) for elements 113, 115, 117 and 118. (awaiting scientific review)
- IUPAC Announces the Names of the Elements 113, 115, 117, and 118 — Government or standards body
- Electronegativity generally increases from left to right across a period and decreases down a group, so the most electronegative elements lie towards the upper right of the periodic table. (awaiting scientific review)
- IUPAC's periodic-table page describes which elements belong in group 3 — scandium, yttrium, lutetium and lawrencium, or scandium, yttrium, lanthanum and actinium — as a debated question that it set up a project to resolve. (awaiting scientific review)
- IUPAC Periodic Table of Elements (release dated 4 May 2022) — Government or standards body
- Mendeleev predicted an atomic weight of about 68 for eka-aluminium, and gallium has 69.72; scandium and germanium, two more of his predicted elements, had been discovered by 1886, and the noble gases discovered in the 1890s fitted in as a new final group. (awaiting scientific review)
- Development of the periodic table — Science media
- Moseley found that plotting the square root of the frequency of an element's characteristic X-rays against atomic number gives a straight line, which gave a way to measure atomic number. (awaiting scientific review)
- Development of the periodic table — Science media
- For groups 1 and 2 the group number equals the number of valence electrons, and so the charge of the cation formed when they are all lost, while most monatomic anions form when a non-metal atom gains enough electrons to fill its outer s and p orbitals. (awaiting scientific review)
- IUPAC has no recommendation for a specific form of the periodic table, such as the 18-column or 32-column format. (awaiting scientific review)
- IUPAC Periodic Table of Elements (release dated 4 May 2022) — Government or standards body
Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.
Content status: published 1 October 2026.
- Scientific review: this version has not yet been signed off by a scientific reviewer.
- The Advanced explanation has not yet been reviewed for age suitability.