Question

explain why the number of elements in the third period table is 8​

Answer 1

Answer:

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Answer 2

Answer:

The Period Three of the periodic table consists of the elements whose atoms has three shells occupied by electrons in its Ground State (when it is not excited or in a reaction)

According to the Aufbau's Principle (the building up principle) there can be only EIGHT elements that satisfy this. This is because the S orbital of the fourth shell sits at a lower energy level (closer to the nucleus) than the d orbital of the third shell. Therefore when the s and p orbitals of the third shell has been filled the next element (K) has its outer most electron in the S orbital of the fourth shell. Hence it is in period 4.

According to the Aufbau's principle as you list the elements in increasing order of atomic number a single electron gets added to their electronic configuration one by one to the orbital with the lowest energy level possible.

However, in the third energy level the first few sub shells are filled first (8 electrons), then the fourth energy level starts filling up, then it returns to filling up the last subshell in the third energy level (first row of transition metal elements). This happens because there is very little energy difference between the 4s and 3d subshells.

The subshells are:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 etc. (shown in the order they are generally filled).

This shows that the 3s and 3p subshells are filled, then the 4s, then the 3d. This is because of the small energy difference between the energy levels, and electron repulsion in the 3rd energy level making it easier for additional electrons to enter the 4s subshell than to force their way into the 3d. Therefore, the 3d subshell is filled after the 4s. however, there are some exceptions to this, in the transition metals, where the explanation is usually given as the fact that atoms are more stable with fully filled or half filled subshells. However, electrons are removed from the 4s subshell first, rather than the 3d.