A period is a horizontal row of the periodic table; the period number equals the number of occupied electron shells in an atom of the element.
A period is a horizontal row of the periodic table; the period number equals the number of occupied electron shells in an atom of the element. Malay: kala · Chinese: 周期.
Where a group runs down the table and tells you the valence electrons, a period runs across it and tells you the number of shells. As you move from left to right along a period the proton number rises by one at each element, one more electron is added to the same outermost shell, and the atom is pulled in a little more tightly by the growing nuclear charge, so, unlike down a group, the atoms actually get smaller across a period even though electrons are being added.
Example. Sodium (2.8.1), magnesium (2.8.2) and chlorine (2.8.7) are all in Period 3 because each has three occupied shells. They are not chemically alike, sodium is a very reactive metal and chlorine a reactive non-metal, but they share the same number of shells, and their properties change gradually across the row from metal, through metalloid, to non-metal.
Confusion to avoid. The commonest error is to swap “period” and “group”. Same period means same number of occupied shells, not similar chemistry; same group means same valence electrons and similar chemistry. Also remember that atomic size decreases across a period but increases down a group, and mixing up those two directions is a frequent slip.
In the Periodic Table of Elements chapter the period gives you the second coordinate of an element’s address: the group number places it in its column and the period number places it in its row, and together they pin down the electron arrangement exactly.
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