Definition · Plain-language
The periodic table
The periodic table is an arrangement of the chemical elements, ordered by atomic number, that groups them so recurring patterns in their properties line up.
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Ordered by atomic number
The periodic table lists the elements in order of increasing atomic number — the number of protons in each atom’s nucleus. This single ordering principle places every element in a unique position. The arrangement is not merely a list, though: the elements are laid out in rows so that those with similar chemical properties fall into the same columns. This reflects the periodic law, the observation that the properties of the elements recur at regular intervals when they are ordered by atomic number, an insight central to Mendeleev’s original table.
Periods, groups and blocks
The horizontal rows are called periods, numbered from the top; moving across a period, properties change gradually as electrons fill the same outer shell. The vertical columns are called groups; elements in the same group have the same number of valence electrons and so behave similarly, which is why group 1 (the alkali metals) and group 0/18 (the noble gases) each share a family resemblance. The table is also divided into blocks (s, p, d and f) according to which type of orbital the outermost electrons occupy, linking the table’s shape directly to electron configuration.
Reading trends across the table
The table’s real power is predictive. Because position reflects electron arrangement, properties vary in regular trends: electronegativity and ionisation energy generally rise across a period and fall down a group, while atomic radius does the reverse. Metals cluster on the left and centre, non-metals toward the top right, with metalloids along the dividing line. Knowing only an element’s place lets a chemist anticipate how it will react, what ions it forms and which elements it resembles — which is why Mendeleev could even predict undiscovered elements from gaps in his table.
Key facts
At a glance
- Definition: elements arranged by atomic number to reveal recurring patterns
- Ordering: increasing atomic number (number of protons)
- Rows: periods (properties change gradually across)
- Columns: groups (similar properties, same valence electrons)
- Blocks: s, p, d and f, by outer-electron orbital type
- Devised by: Dmitri Mendeleev (periodic law)
Common misconceptions
What people often get wrong
Often heard: The periodic table is arranged by atomic mass.
Actually: It is arranged by atomic number — the number of protons. Mendeleev originally used atomic mass, but ordering by atomic number resolved the anomalies and is the modern basis.
Often heard: Elements in the same row (period) have similar properties.
Actually: Elements with similar properties share a column (group), not a row. Across a period the properties change gradually from metal to non-metal.
Often heard: The periodic table is just a reference list with no predictive value.
Actually: Its layout encodes regular trends in properties, so an element’s position predicts its reactivity, the ions it forms and which elements it resembles — Mendeleev even predicted undiscovered elements.
