Question

what is an atom and what is atomic number,valency of hellium

Answer 1

Atom is the smallest unit of element..... Atomic number is the number of protons and electrons present in it and valency is the number of protons at the outermost orbit .
Helium atomic number is 3 so its valency is 1

Answer 2

An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.
The atomic number is the number of protons in the nucleus of an atom, which is characteristic of a chemical element and determines its place in the periodic table.
Valency, the combining capacity of an element, is equal to the number of electrons needed, or required to be removed from the outermost shell of the atom of that element, to have a complete 'octet'. The helium atom has only the 1s orbital in its only shell, and it is completely filled with 2 electrons.