Which of the following elements is isoelectronic with $Na^{+}$?

$A$ monoatomic anion $(A^-)$ has $45$ neutrons and $36$ electrons. The atomic mass, group in the periodic table, and physical state at room temperature of the element $(A)$ are respectively:

Two atoms have the same atomic mass but different atomic numbers. Such atoms are called as:

The $^{18}O$ isotope of oxygen will have:

The atomic number of an element represents

What are atomic number,nucleons,and mass number? Give the relation between them.