Period
A horizontal row of the periodic table. All elements in a period have the same number of electron shells.
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Analogy
A period is like a floor in a building. Every room on one floor is at the same height.
In practice
Sodium, magnesium and chlorine are all in period 3, so each has three shells.
Practice
Flashcards
- What does the period number tell you?The number of electron shells the atom has.
Questions people ask
Why does the periodic table have rows of different lengths?
Each period begins when a new shell starts to fill. The first shell holds only 2 electrons, so period 1 has only two elements. The next shells hold more, so periods 2 and 3 have eight each. Later periods include the transition metals and are longer.
What changes as you move across a period?
The number of protons and outer electrons rises by one each step, so atoms go from metals on the left to non-metals on the right. The number of shells stays the same. Reactivity changes, and the last element in each period is a noble gas.