On moving horizontally across a period, the number of electrons in the outermost shell increases from ___________ to __________2,82,181,81,18

The number of electrons in the outermost shell increases from 1 to 8 when moving horizontally across a period in the periodic table.

Here's the step by step explanation:

1. The periodic table is divided into periods (horizontal rows) and groups (vertical columns).

2. When you move horizontally across a period from left to right, you are essentially adding one proton to the nucleus and one electron to the outermost shell with each successive element.

3. The first element in each period (like Hydrogen in period 1, Lithium in period 2) has one electron in its outermost shell.

4. As you move to the right, each element has one more electron (and proton) than the previous one.

5. By the time you reach the end of the period (like Helium in period 1, Neon in period 2), there are 8 electrons in the outermost shell. This is because the outermost shell (also known as the valence shell) can hold a maximum of 8 electrons.

So, the number of electrons in the outermost shell increases from 1 to 8 as you move horizontally across a period.