Understanding the Difference between Atomic Mass and Mass Number in Chemistry
Atomic Mass
Atomic mass refers to the mass of an element’s atom. It is a relative quantity and is measured in atomic mass units (amu), which is a unit of measurement used to represent the mass of atoms or molecules. The atomic mass of an element is generally calculated by taking the weighted average of the masses of all the isotopes of that element found in nature, considering the percentage abundance of each isotope in the element.
The atomic mass of an element is a significant physical property because it can be used to determine the number of atoms in a sample of an element, the reaction stoichiometry, and the molar mass of a compound. It is denoted by the symbol “m” or “M” and is typically found on the periodic table of elements.
Mass Number
Mass number refers to the number of protons and neutrons present in an atom’s nucleus. It is an absolute quantity and is measured in atomic mass units (amu). Unlike atomic mass, mass number is a whole number and not a weighted average.
The mass number helps to identify the isotope of an element since the number of neutrons present in the atom’s nucleus varies from isotope to isotope. To calculate the number of neutrons present in an atom, one can subtract the number of protons (which is the atomic number) from the mass number of the atom.
The Key Difference
The significant difference between atomic mass and mass number is that atomic mass is a weighted average of all the isotopes of an element, whereas mass number is an absolute value that represents the total number of protons and neutrons in the atom’s nucleus.
Another key difference between these two quantities is that atomic mass can have decimal values, while mass number is always a whole number.
In conclusion, understanding these two essential concepts in chemistry is vital as it helps to distinguish between different isotopes of the same element, which can have significant implications in various fields of study such as medicine, physics, and environmental science.
Table difference between atomic mass and mass number
| Atomic Mass | Mass Number |
|---|---|
| It is the average mass of all the isotopes of an element. | It is the number of protons and neutrons in the nucleus of an atom. |
| It is measured in atomic mass units (amu). | It is usually represented by the symbol ‘A’. |
| It takes into account the relative abundance of each isotope of an element. | It is a whole number and does not take into account the relative abundances of the isotopes. |
| It is used to calculate the molar mass of an element or compound. | It is used to identify the different isotopes of an element. |