Elements with valency $1$ are

The presence of protons and neutrons at the center of the atom was suggested by:

$(a)$ How many neutrons are present in $C-14$ isotope of carbon?
$(b)$ How many protons does $He^{2+}$ ion possess?
$(c)$ How many maximum electrons can be filled in the $3^{rd}$ orbit of an atom?
$(d)$ How many electrons does $He^{2+}$ ion possess?

$(a)$ What are isobars?
$(b)$ Atomic number of an element $Y$ is $17$.
$(i)$ Write its electronic configuration.
$(ii)$ What is the number of valence electrons in $Y$?
$(iii)$ How many electrons are needed to complete the octet of $Y$?
$(iv)$ Is it a metal or non-metal?
$(c)$ The valency of $Na$ is $1$ and not $7$. Give reason.

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

Write the atomic number of an element which has mass number $23$ and number of neutrons $12$ in the nucleus.