Atomic number

• The number of protons in one atom of an element is known as its atomic number.
• All the atoms of an element have same atomic number.
• It is generally denoted by the letter Z and written at the bottom of the symbol of that element.

Atomic number of an element = Number of protons in one atom of element

• Nucleus of an atom is composed of protons and neutrons which are collectively called as nucleons.

• Atomic number of sodium is 11 i.e. there are 11 protons in the nucleus of sodium atom.
• The atomic number of carbon is 6, so for carbon, Z = 6 (6 protons in the nucleus)
• The atomic number of hydrogen is 1, therefore, for hydrogen Z = 1 (1 proton in the nucleus)
• All the atoms of the same element have the same number of protons in their nuclei, and hence they have the same atomic number.
• An atom is electrically neutral, the number of protons in an atom is equal to the number of electrons.
• Atomic number (Z) = Number of protons = Number of electrons