Question

An element is represented by symbol ${}_{15}{}^{31}\mathrm{X}$.

Name its valence shell and the number of electrons in the valence shell.

Answer 1

• Valence shell: The outermost shell of an atom is called its valence shell.
• The electrons of these shells take part in bonding with other atoms.
• The atoms are written in this manner ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$ where A denotes mass number and Z denotes the atomic number.
• 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.
• The 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.
• Here, the atomic number of the element X is 15.
• The electronic configuration of element X is 2,8,5.
• 2 electrons will fill the first shell i.e., the K shell.
• 8 electrons will fill the second shell i.e., the L shell.
• 5 electrons will enter the third shell i.e., the M shell.

Hence, here M shell is the valence shell and there are 5 electrons in its valence shell.

Phosphorous (P) is the element having the atomic number 15.