Elements with valency $1$ are

State whether the following statement is true or false:
An atom of each element has a definite combining capacity,called its valency.

Identify the isotopes out of $A, B, C$ and $D$ given the following representations:
${ }_{17}^{35} A, { }_{20}^{40} B, { }_{17}^{37} C, { }_{19}^{38} D$

Write the symbols of two isotopes of uranium.

Two elements are denoted as follows:
$_{20}^{40}A$ and $_{18}^{40}B$
$(a)$ Find the number of electrons present in $A$ and $B$.
$(b)$ Find the number of nucleons present in $A$ and $B$.
$(c)$ Explain the special 'term' used to represent $A$ and $B$ and define the same.

$(a)$ 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?