An s orbital is an orbital with angular momentum quantum number l = 0; it is spherically symmetric, so its electron density depends only on distance from the nucleus and not on direction. There is one s orbital in each shell. Its simple, symmetric shape contrasts with the directional shapes of p, d, and f orbitals.

s 轨道是角动量量子数 l = 0 的轨道;它呈球对称,故其电子密度只与距核距离有关,而与方向无关。每个壳层中有一个 s 轨道。它简单对称的形状与 p、d、f 轨道的方向性形状形成对比。