Question

What determines the number of elements found in each period in the periodic table?

Answer 1

To solve this question we first need to know how elements are arranged in the periodic table. We know that in the modern periodic table, "the element's physical and chemical properties are the periodic function of their atomic numbers."

Complete answer:
The atomic number Z of an element is the unique number of protons in that element.
In a neutral atom,
number of electrons = number of protons = atomic number.
In a period, all the elements have the same number of electron shells.
The number of electron shells in an atom can be explained using the quantum numbers.

1. Principal quantum number (n): This corresponds to the shells of an atom in increasing order of their energy.
   n=1,2,3,4…. or K, L, M, N…
2. Azimuthal quantum number ($\ell$): This gives the number of subshells in a shell.
   $\ell$=0 to (n-1)… or s, p, d, f
   The maximum number of electrons in a subshell is given by$2(2\ell +1)$.
   ($\ell$= 0) s has maximum 2 electrons
   ($\ell$= 1) p has maximum 6 electrons
   ($\ell$= 2) d has maximum 10 electrons
   ($\ell$= 3) f has maximum 14 electrons
   Now, let us look at the number of elements in each period.
   - Period 1 has s orbital filled.
   $\ell$ = 0
   Hence it can have 2 elements.
   - Period 2 and Period 3 can have s and p orbitals filled.
   $\ell$ = 0,1
   Hence they have 8 elements each.
   - Period 4 and Period 5 can have s, p, and d orbitals filled.
   $\ell$ = 0,1,2
   Hence they have 18 elements each.
   - Period 6 and Period 7 can have s, p, d, and f orbitals filled.
   $\ell$ = 0,1,2,3
   Hence they have 32 elements each.

Note:
It should be noted that the filling of electrons in an atom is according to the following rules
- The Aufbau Principle - The electrons are filled in the increasing order of the energy of the orbitals.
- The Hund's Rule - If two or more orbitals of the same energy are present, all of them must be half-filled before pairing up.
- The Pauli's exclusion principle - Two electrons in an atom cannot have the same values of all four quantum numbers. Hence there can only be two electrons in an orbital.