Name the particles which determine the mass of an atom.

$(a)$ Both helium and beryllium have two electrons in their valence shells. Helium is a noble gas whereas beryllium is a metal. Justify.
$(b)$ Hydrogen exists in three isotopic forms. Why are the isotopes of hydrogen chemically alike?

Explain with examples $(i)$ Atomic number,$(ii)$ Mass number. Give any two uses of isotopes.

Which will be chemically more reactive: element $A$ with atomic number $17$ or element $B$ with atomic number $16$? Give a reason as well.

Find out the mass number of oxygen and sulphur atoms.

Isotopes of an element have