The number of positively charged particles or protons present in the nucleus of an atom of each element is called atomic number. It is denoted by z.
The center of the atom is called nucleus.

There is the positive charge on the nucleus due to the presence of protons in it.

The number of protons in the nucleus determines the number of positive charge in the nucleus.

The number of protons in one atom of an element is known as an atomic number.

The total number of protons and neutrons present in one atom of an element is called its atomic mass.

The weight of a molecule is called its molecular weight.

The molecular weight of hydrogen is 2.
The atomic number of helium is 2 and its symbol is He.