The modern periodic table

The periodic table is called periodic because similar properties occur at regular intervals (or periods).

The rows in the periodic table are called periods. All elements in a period have the same number of electron shells. The period number represents the number of electron shells. As you read across a period from left to right, the number of protons and electrons increases by one for each new element.

Columns in the periodic table are called groups. Elements in the same group have similar properties. This similarity is due to them having the same number of outer electrons (electrons in the outer shell), which gives them similar chemical properties.

You can predict reactions and reactivity based on an element’s position. Elements in the same group tend to react in similar ways. For example:

• Group 1 elements, such as lithium, sodium, and potassium, all have one electron in their outer shell, making them very reactive. Reactivity increases as you go down Group 1 (e.g., potassium is more reactive than lithium).

• Group 7 elements, like chlorine and iodine, have seven electrons in their outer shell and similar reactivity patterns. Reactivity decreases as you go down Group 7 (e.g., chlorine is more reactive than iodine).