What is the difference between mass number and atomic mass? Mass number, usually referred to simply as “mass,” is the total number of protons and neutrons in an atom. Atomic mass, sometimes called “natural abundance” or “atomic weight,” is the average mass of an atom of a certain element relative to carbon-12. Though they are related, these two terms are not synonymous. In this post, we’ll explore the difference between mass number and atomic mass, and provide some examples to help illustrate the concepts.
What is Mass Number?
The mass number is a physical property of an atom. It is defined as the number of protons in the nucleus of an atom. Mass number is a whole number, and it is not affected by the presence or absence of electrons. The mass number is used to identify different isotopes of an element.
- The mass number of an atom can be determined by adding the number of protons and neutrons in the nucleus. Mass number is sometimes abbreviated as “A.” The symbol “A” comes from the German word “atomgewicht,” which means “atomic weight.” Mass number is not to be confused with atomic weight, which is a measure of the average mass of an element’s atoms.
- Mass number is also not to be confused with molar mass, which is a measure of the mass of a molecule. Mass number has no units because it is simply a count. Mass number cannot be negative.
- Atoms with different mass numbers are called isotopes. Isotopes of an element have different numbers of neutrons in their nuclei. Most elements have multiple isotopes. For example, hydrogen has three isotopes: protium (1H), deuterium (2H), and tritium (3H). Carbon has two stable isotopes: carbon-12 (12C) and carbon-13 (13C).
There are also many radioactive isotopes, such as carbon-14 (14C), which is used for radiocarbon dating. Mass numbers can range from 1 to 118 for the known elements. The highest mass number belongs to oganesson ( Og), which has 118 protons in its nucleus. Mass numbers can be used to write nuclear symbols, which specify an element and its mass number. For example, the symbol for hydrogen-3 would be written as 3H or 3 1 H.
What is Atomic Mass?
Atomic number is a measure of the number of protons in an atom, and therefore its place in the periodic table. It is usually represented by the symbol Z. The atomic number of an element is important because it determines how the element behaves chemically.
- For example, elements with a low atomic number are more reactive than those with a high atomic number. Atomic number can also be used to predict an element’s properties, such as its boiling point and melting point.
- Atomic number is determined by the number of protons in an atom’s nucleus. The nuclei of atoms are held together by the strong nuclear force, which is unaffected by electric or magnetic fields.
- This means that atoms with different numbers of protons can exist peacefully side-by-side. However, when atoms lose or gain protons, they become ions and their chemical properties change. Atomic number is therefore a key concept in understanding atomic structure and chemical reactions.
Differences between Mass Number and Atomic Mass
Mass number and atomic mass are often used interchangeably, but they are actually two different things. The mass number is the sum of the protons and neutrons in an atom, while atomic mass is the average mass of all atoms of an element.
- Because protons and neutrons have different masses, the atomic mass of an element is usually different from its mass number. For example, the mass number of oxygen is 16 (8 protons + 8 neutrons), but its atomic mass is 15.9994 amu.
- The small discrepancy is due to the fact that there are different isotopes of oxygen, each with a different number of neutrons. A mass number is a whole number, while atomic mass is always reported as a decimal.
- When writing the symbols for elements, the mass number is written as a superscript before the symbol, while the atomic mass is written as a subscript after the symbol. For example, the symbol for carbon-12 would be written as 12C, indicating that it has 6 protons and 6 neutrons. Finally, it’s important to remember that Mass Number = Atomic Mass + Electrons.
Conclusion
So, what is the difference between mass number and atomic mass? The main difference is that the mass number includes the total number of protons and neutrons in an atom, while the atomic mass only considers the protons. This means that the atomic mass is always lower than the mass number. While they may be used interchangeably in some cases, it’s important to understand the distinction between these two terms so you can use them correctly in your own scientific work.
