How does the number of electron shells change across the periods?

The number of electron shells increases by one as you move from one period to the next on the periodic table.

In the periodic table, each period corresponds to a new energy level or electron shell. As you move from left to right across a period, the number of electrons and protons increases, but they are added to the same shell. This is because the electrons are filling up the available spaces in the current energy level before moving on to the next one.

For instance, in the first period, there is only one electron shell, and it can hold up to 2 electrons. Hydrogen and helium, the two elements in this period, fill this shell. As you move to the second period, a second electron shell is added. This shell can hold up to 8 electrons, and it is filled by the elements lithium through neon.

The third period also has a maximum capacity of 8 electrons, filled by the elements sodium through argon. This pattern continues down the periodic table, with each new period representing a new electron shell.

It's important to note that the number of electron shells an atom has determines its period on the periodic table. This is because the electron shells are arranged in energy levels, and the energy level increases as you move down the periods. This is why elements in the same period have similar chemical properties - they have the same number of electron shells, and therefore the same number of valence electrons available for bonding.

In summary, as you move across the periods in the periodic table, the number of electron shells increases by one for each new period. This is a fundamental concept in understanding the structure of the atom and the periodic table.