An atom of an element has full $K$ and $L$ shells and has $5$ electrons in its $M$ shell. What is its atomic number?

An atom with $3$ protons and $4$ neutrons will have a valency of

The electronic configuration of an element $X$ is $2, 8, 2$.
$(a)$ Find the number of electrons present in the atom of element $X$.
$(b)$ Write the atomic number.
$(c)$ Is this element $X$ a metal or a non-metal?
$(d)$ Find out the valency of the element $X$.

Define the term valency. How is valency different from valence electrons? Find the valency of oxygen and fluorine.

The electron distribution in an aluminium atom is

Which of the $Na^+$ and $He$ has completely filled $K$ and $L$ shells? Give a reason to support your answer.