A period is the set of all elements in the same horizontal row of the periodic table. Across a period, the atomic number increases by one from element to element, and properties change gradually from metallic on the left to non-metallic on the right. Sodium and chlorine, for instance, lie in the same period but differ greatly in their properties.

周期是周期表中同一横行的所有元素。沿一个周期,原子序数从一个元素到下一个逐一递增,性质则由左侧的金属性逐渐过渡到右侧的非金属性。例如,钠和氯位于同一周期,但性质相差很大。