Question

Atomic numbers of elements P, Q, R and S are 1, 2, 7 and 11 respectively. Classify them as metals, non-metals and noble gases.

Answer 1

Metals: Metals are the electropositive elements that have less than 4 valence electrons.

Non-metals: Non-metals are the electronegative elements that have valence electrons equal to or more than four.

Noble gases: Noble gases are those elements that have a complete duplet or octet of electrons in their valence shell.

Step-1: Write the number of electrons

• Atomic number = Number of protons
• But, Number of protons = Number of electrons for an atom
• Therefore, Atomic number = Number of electrons

Step-2: Write electronic configuration

• Begin to fill electrons from the innermost 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: Find out the number of valence electrons and the nature

• If the number of valence electrons is less than 4, then the element is a metal.
• If the number of valence electrons is 4 or more than 4 but less than 8, the element is a non-metal.
• If the number of valence electrons is 8, the element is a noble-gas

Exceptions: For atomic number 1, valence electron number is 1 but it is a non-metal.

For atomic number 2, valence electron number is 2 but it is a noble-gas.

Element	Atomic number	Electronic configuration	Valence electrons	Metal/ Non-metal/ Noble gas
P	1	1	1	Non-metal
Q	2	2	2	Noble gas
R	7	2, 5	5	Non-metal
S	11	2, 8, 1	1	Metal