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

Is the element a metal or a non-metal?

• The atoms are written in this manner ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$ where A denotes mass number and Z denotes the atomic number.
• Here, the atomic number of the atom will be 15.
• 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.
• The atomic number of the element is 15.
• 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.
• M shell will be its valence shell having 5 electrons.
• Metals have 1,2, or 3 electrons in their valence shell.
• Non-metals have 4,5,6,7, or 8 electrons in their valence shell.

Hence, the element is Non-metal.

Phosphorous is the element having the atomic number 15, and it is a non-metal.