Difference Between Atomic Number and Mass Number

Atomic Number vs Mass Number

Atoms are characterized by their atomic numbers and mass numbers. In the periodic table, atoms are arranged according to their atomic number. Mass number of an element is more related with the mass of it. However, it is not giving the exact mass of the atom. There are some elements, where atomic number and mass number is similar, and most of the time, mass number is higher than the atomic number.

What is Atomic Number?

Atomic number of an element is the number of protons it has in the nucleus. The symbol for denoting the atomic number is Z. When the atom is neutral, it has the same number of electrons as protons. Thus, atomic number is equal to the number of electrons in this instance. But it is always reliable to get the number of protons as the atomic number. The elements in the periodic table are arranged according to the increasing atomic number. This arrangement has automatically arranged them in increased atomic weight most of the time. Every element has separate atomic number, and no element has the same atomic number. Therefore, atomic number is a convenient way of distinguishing different elements. By looking at the atomic number itself, a lot of information about the element can be withdrawn. For example, it tells the group and the period where the element belongs in the periodic table. Further, it gives information about oxidation states, charge of the ion, bonding behavior, nucleus charge, etc.

What is Mass Number?

Atoms are mainly composed of protons, neutrons and electrons. Atomic mass is simply the mass of an atom. Most of the atoms in the periodic table have two or more isotopes. Isotopes differ from each other by having a different number of neutrons, even though they have the same proton and electron amount. Since their neutron amount is different, each isotope has a different atomic mass.

Mass number is the total number of neutrons and protons in a nucleus of an atom. The collection of neutrons and protons is also known as nucleons. Therefore, mass number can also be defined as the number of nucleons in a nucleus of an atom. Normally, this is denoted in the left, upper corner of the element (as superscript) as an integer value. Different isotopes have different mass numbers, because their numbers of neutrons vary. Therefore, the mass number of an element gives the mass of the element in integers. The difference between the mass number and the atomic number of an element gives the number of neutrons it has.


What is the difference between atomic number and mass number?

• Atomic number is the number of protons in a nucleus of an atom. Mass number is the total number of protons and neutrons in the nucleus.

• Conventionally atomic number is written in the left, bottom corner of an element, whereas the mass number is written in the left, upper corner.

• Atomic number is denoted by Z, and mass number is denoted by the symbol A.