The atomic radius is a measure of the size of an atom; it decreases across a period and increases down a group of the periodic table.
The atomic radius is a measure of the size of an atom; it decreases across a period and increases down a group of the periodic table. Malay: jejari atom · Chinese: 原子半径.
The value of the atomic radius comes from a tug-of-war between two things: how many shells the electrons occupy, which pushes the outer electrons further out, and how strong the nuclear charge is, which pulls them in. Down a group each element has one more occupied shell than the one above, so the atoms get larger even though the nuclear charge also grows. Across a period the number of shells stays the same but the nuclear charge rises element by element, so the same shells are pulled in more tightly and the atoms get smaller.
Example. In Group 1, lithium is smaller than sodium, which is smaller than potassium, because each added element gains a whole extra shell. In Period 3, sodium is the largest atom and the atoms shrink steadily towards chlorine and argon on the right, because the nuclear charge keeps rising while the number of shells is fixed at three.
Confusion to avoid. Keep the two directions straight: size decreases across a period (left to right) but increases down a group (top to bottom). Students often reverse one of them. It also helps to remember that a larger atom holds its outer electron more loosely, which is why atomic size is the key to explaining reactivity trends.
In the Periodic Table of Elements chapter the atomic radius is the bridge between position and reactivity: it explains why alkali metals get more reactive down the group and halogens get less reactive, since a bigger atom loses its outer electron more easily and gains one less easily.
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