To find the number of elements in a period in the periodic table, you can simply look at the row number of that period. The period number indicates the energy level or principal quantum number of the elements in that row.
In the modern periodic table, there are seven periods labeled from 1 to 7. The first period contains only two elements (hydrogen and helium). The second period contains eight elements (from lithium to neon), the third period contains eight elements (from sodium to argon), and so on.
As you move across a period from left to right, each subsequent element adds one proton to the nucleus and one electron to the outer shell. The number of elements in a period, therefore, corresponds to the number of elements needed to complete the filling of a particular energy level.
Here's a breakdown of the number of elements in each period:
- Period 1: 2 elements
- Period 2: 8 elements
- Period 3: 8 elements
- Period 4: 18 elements
- Period 5: 18 elements
- Period 6: 32 elements
- Period 7: 32 elements
It's important to note that the synthetic or artificially created elements beyond atomic number 118 (oganesson) are not assigned to any specific period.