There are two ways to calculate atomic mass:
- By adding the number of protons and neutrons in an atom. The number of protons in an atom is its atomic number, and the number of neutrons is the difference between the mass number and the atomic number. For example, the atomic mass of carbon-12 is 12 because it has 6 protons and 6 neutrons.
- By averaging the masses of the naturally occurring isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons. For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. The atomic mass of carbon is calculated by averaging the masses of these three isotopes, weighted by their natural abundance.
The formula for calculating the atomic mass of an element by averaging the masses of its isotopes is:
atomic mass = Σ(isotope abundance * isotope mass)
where:
- Σ means “sum of”
- isotope abundance is the percentage of the element that is made up of that isotope
- isotope mass is the mass of the isotope
For example, the atomic mass of carbon is calculated as follows:
atomic mass = (0.9893 * 12 amu) + (0.0107 * 13 amu) + (0.0000 * 14 amu)
= 12.011 amu
The atomic mass of an element is usually listed on the periodic table.
How to find an atomic number?
The atomic number of an element is the number of protons in the nucleus of an atom of that element. It is unique to each element and is always listed on the periodic table. The atomic number is also the same as the number of electrons in a neutral atom of that element.
There are a few ways to find the atomic number of an element:
- Look up the element on the periodic table. The atomic number is always listed at the top of the element’s box on the periodic table.
- Use a periodic table app or website. Many periodic table apps and websites allow you to search for elements by name or symbol. Once you have found the element, you can see its atomic number.
- Calculate the atomic number from the mass number and number of neutrons. The mass number of an element is the total number of protons and neutrons in the nucleus of an atom of that element. The number of neutrons is the difference between the mass number and the atomic number. For example, the atomic number of carbon is 6 because it has 6 protons and 6 neutrons. The mass number of carbon is 12 because 6 + 6 = 12.
Here are some examples of how to find the atomic number of an element:
- The atomic number of oxygen is 8.
- The atomic number of hydrogen is 1.
- The atomic number of carbon is 6.
- The atomic number of gold is 79.
