Question

Three elements P,Q and R have atomic numbers 4,12 and 20 respectively.

State the number of valence electrons in each of the elements.

Answer 1

• Valence electrons- The electrons present in the valence shell i.e., the outermost shell of an atom are known as valence electrons.
• Valence electrons take part in the chemical reaction.
• These electrons decide the chemical properties of an atom.
• To know the number of valence electrons in P, Q, and R we first need to write the electronic configuration of these elements.
• The electronic configuration of an element describes how electrons are distributed in its atomic orbitals.
• The electrons are filled in the increasing number of shells viz. K,L,M,N,etc.
• Number of electrons in a shell is given by 2n² where n is the number of shells.
• Putting n=1,2,3 and 4, we get the number of electrons 2,8,18,32 that can be filled in each shell respectively.

Element P-

• The atomic number of element P is 4.
• 2 electrons will fill the first shell i.e., the K shell.
• 2 electrons will fill the second shell i.e., the L shell.
• Hence, the electronic configuration of element P can be written as 2,2.

Element Q-

• The atomic number of element Q is 12.
• 2 electrons will fill the first shell i.e., the K shell.
• 8 electrons will fill the second shell i.e., the L shell.
• 2 electrons will enter the third shell i.e., the M shell.
• Hence, the electronic configuration of element Q can be written as 2,8,2.

Element R-

• The atomic number of element R is 20.
• 2 electrons will fill the first shell i.e., the K shell.
• 8 electrons will fill the second shell i.e., the L shell.
• 8 electrons will enter the third shell i.e., the M shell.
• 2 electrons will enter the fourth shell i.e., the N shell.
• Hence, the electronic configuration of element R can be written as 2,8,8,2.

Hence, elements P, Q, and R have 2 valence electrons which are present in the valence shell.