How is an element's atomic mass determined?

Expert Answers
justaguide eNotes educator| Certified Educator

Mass is a physical property of matter and is expressed in SI units as kilogram. The atomic mass of elements is not actually the mass of their atoms in kilograms, rather it is a value giving the ratio of the atom's mass to that of the mass of a carbon atom which has been fixed at 12 a.m.u (atomic mass unit).

There are three particles that make up an atom, proton, neutron and electron. The mass of protons and neutrons is considered to be equal though in reality neutrons are heavier than protons; the mass of electrons is very less and ignored in the calculation of atomic mass.

An atom of Carbon has 6 protons and 6 neutrons, it is given an atomic mass of 12 a.m.u which makes each proton and neutron in the nucleus of any element contribute 1 a.m.u to the atom's atomic mass.

The number of protons is unique for each element but they can have different numbers of neutrons which leads to elements having isotopes with different atomic mass. The atomic mass of an element is the weighted average of the atomic mass of its isotopes. This requires a knowledge of the relative abundance of each isotope of the element.

aishukul | Student

The atomic mass of an element is the element's number of protons and neutrons added together. You can add the protons and neutrons together to calculate the atomic mass. You can also look at the periodic table of elements where at the bottom of each element's symbol there is the atomic mass number of the specific element you are looking for. Hope this helps!

atyourservice | Student

The atomic mass is found by adding the amount of neutrons and protons in an element. The amount of protons can be told by looking at the atomic number.

malkaam | Student

Atomic mass is an element's mass that is the sum of masses of protons and neutrons forming an element. The easiest way to determine the atomic mass is to look up in the periodic table. In a periodic table, the atomic masses increase as you go across the table. The atomic mass is the decimal figures given therein. All you need to know is the element symbol. Atomic mass is in terms of grams per mole or g/mol.

Another way to calculate the atomic mass of an element is to add up the masses of protons and neutrons.
Example: Find the atomic mass of an isotope of carbon that has 7 neutrons. You can see from the periodic table that carbon has an atomic number of 6, which is its number of protons. The atomic mass of the atom is the mass of the protons plus the mass of the neutrons, 6 + 7, or 13.

Moreover, the atomic mass of an element is a weighted average of all the element's isotopes based on their natural abundance. It is simple to calculate the atomic mass of an element with these steps:

  1.      The first step is to multiply the mass of each isotope by its abundance. But of the abundance is a percent, divide your answer by 100.
  2.      After values have been derived add them up.

The answer is the atomic mass or the atomic weight of the element.

Example: You are given a sample containing 98% carbon-12 and 2% carbon-13. What is the relative atomic mass of the element?

First convert the percentages to decimal values by dividing each percentage by 100. The sample becomes 0.98 carbon-12 and 0.02 carbon-13. (Tip: You can check your math by making certain the decimals add up to 1. 0.98 + 0.02 = 1.00)

Next, multiply the atomic mass of each isotope by the proportion of the element in the sample:

0.98 x 12 = 11.76
0.02 x 13 = 0.26

For the final answer, add these together:

11.76 + 0.26 = 12.02 g/mol

(Examples taken from

chrisyhsun | Student

The mass of an atom naturally comes from the summation of the masses of the atom's individual parts. Every atom is composed of protons, electrons, and neutrons. However, the mass of each electron is so miniscule, even in comparison to the already tiny protons and electrons, that it is just disregarded as a negligible component. As such, the mass of an atom can be simplified to just being the mass of its component protons and neutrons. The mass of an individual proton is essentially equivalent to the mass of a single neutron. Based on this observation, each proton and each neutron is figured as one atomic mass unit. Therefore, any element's atomic mass will simply be the sum of its protons and neutrons. The number of protons in any given element can be found on the periodic table as its atomic number. In normal, regular cases, the number of neutrons in the atom will be the same as the number of protons, so the atomic mass is simply double the atomic number. However, isotopes of the same element contain different number of neutrons, in which case doubling the atomic number will not give the correct answer. For instance, there are several isotopes of carbon, including carbon-12 (6 protons and 6 neutrons) and carbon-14 (6 protons, 8 neutrons).

sid-sarfraz | Student


A genuine chemical matter consisting of a particular type of atom well-known by its atomic number is known as an element. Elements are divided into metals, metalloids, and nonmetals.


Mass is a fundamental, physical property of any substance.


The atom is the smallest unit that defines the chemical elements and their isotopes.

Atomic Mass

The Mass of an atom is referred to as atomic mass of any particular chemical element.

The average atomic mass can be determined by multiplying the sum of an elements isotope with their natural abundance on earth. It can also be stated as sum of the masses of protons, neutrons and electrons in an atom. Hence the equation is;

Atomic Mass = (mass of isotope x relative abundance) + (mass of isotope x relative abundance)

The atomic mass is used as a relative unit to find out the mass of molecules involved in chemical reactions. It can also show the number of atoms or molecules in a mass of substance.

For Example;

In a sample of 400 lithium atoms, it is found that 30 atoms are lithium-6 (6.015 g/mol) and 370 atoms are lithium-7 (7.016 g/mol). Calculate the average atomic mass of lithium.


1) Calculate the percent abundance for each isotope:

Li-6: 30/400 = 0.075
Li-7: 370/400 = 0.925

2) Calculate the average atomic weight:

x = (6.015) (0.075) + (7.016) (0.925)

x = 6.94 g/mol

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rachellopez | Student

Atomic mass is the mass of an atom, particle, or molecule. The atomic mass is determined by the number of protons and neutrons in the atom. For example, Oxygen has 8 protons (as seen by the atomic number) and 8 neutrons which gives oxygen an atomic mass of 16. However, different isotopes of oxygen (with a different amount of neutrons) can have different atomic masses.