Question

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

1. State the number of valence electrons in each of these elements
2. Are these elements going to have similar or different chemical properties?
3. Are these elements metals or non-metals?

Answer 1

(i) Number of valence electrons

Step-1: Find the number of electrons in the elements

• Atomic number = Number of protons
• Number of protons = Number of electrons for an atom

Step-2: Write the electronic configuration

• Begin to fill electrons from the innermost shell (or K shell) to outermost shell
• The maximum number of electrons that can be accommodated in first, second and third shells are 2, 8, 18 respectively.
• The electrons are filled in shells in a sequential manner.
• The maximum number of electrons in a valence shell can be 8

Step-3: Identify the number of electrons in the outermost or valence shell.

Element	Atomic number	Electronic configuration	Valence electrons	Metal/ Non-metal
P	4	2, 2	2	Metal
Q	12	2, 8, 2	2	Metal
R	20	2, 8, 8, 2	2	Metal

(ii) These elements will have similar chemical properties because chemical properties depend on the valence electrons.

• As all the elements have same number of valence electrons, they have similar chemical properties like nature of chemical bonds, the compounds formed by them and types of chemical reactions etc.

(iii) Metal/ Non-metal

• If the number of valence electrons is less than 4, it is a metal.
• If the number of valence electrons is 4 or more than 4, it is a non-metal.
• As the number of valence electrons is 2 for all the elements i.e.; less than 4, therefore P, Q and R are metals.