Atomic Mass Calculator - Find Atomic Mass of Elements

Atomic Mass Definition and Formula

An atom's mass comes from the tiny particles called protons, neutrons, and electrons. However, electrons don't add much to the mass because they are 1,836 times lighter than protons.

Scientists usually measure an atom's mass in atomic mass units (amu) or daltons (Da). One amu equals 1/12 of the mass of a carbon-12 atom. This makes 1 amu equal to 1.66 x 10^-27 kilograms.

To calculate the approximate atomic mass in amu, you add up the number of protons and neutrons. So the atomic mass formula is:

Atomic mass (amu) ≈ Number of protons + Number of neutrons

Or written with symbols:

A = Z + N

Where:
A = Mass number (total protons and neutrons)
Z = Number of protons
N = Number of neutrons

The actual mass is slightly less because of a tiny mass loss from binding energy according to E=mc^2. But this calculator doesn't include that very small effect.

How To Find Atomic Mass with this Calculator

Follow these steps to find an atom's mass:

1. Enter the number of protons. The calculator warns if over 118 (the current highest).
2. Enter the number of neutrons. It warns if over 177 (current max).
3. It shows the atomic mass in amu (default units) and also in kilograms (kg).
4. It also displays the mass number.

Periodic Table with Atomic Mass

Below is a table of elements with atomic mass.

Atomic Number	Symbol	Name	Atomic Mass (amu)
1	H	Hydrogen	1.008
2	He	Helium	4.003
3	Li	Lithium	6.941
4	Be	Beryllium	9.012
5	B	Boron	10.811
6	C	Carbon	12.011
7	N	Nitrogen	14.007
8	O	Oxygen	15.999
9	F	Fluorine	18.998
10	Ne	Neon	20.180
11	Na	Sodium	22.990
12	Mg	Magnesium	24.305
13	Al	Aluminum	26.982
14	Si	Silicon	28.086
15	P	Phosphorus	30.974
16	S	Sulfur	32.065
17	Cl	Chlorine	35.453
18	Ar	Argon	39.948
19	K	Potassium	39.098
20	Ca	Calcium	40.078

FAQ

How do I find atomic mass?

1. Use A = Z + N formula
2. Put in number of protons for Z and neutrons for N
3. Add to get the atomic mass A

What is atomic mass?

The total mass of protons, neutrons and electrons in an atom. Electrons have very little mass compared to protons, therefore they are excluded.

What is average atomic mass?

The average atomic mass is the mean the mean mass of all isotopes of an element, calculated using their percentages and individual atomic masses.

What is the difference between atomic mass and mass number?

Mass number only includes protons and neutrons. Atomic mass also includes tiny electron masses.

What is Carbon-12 atomic mass?

12 amu, from 6 protons + 6 neutrons + 6 tiny electron masses.

What is Helium-4 atomic mass?

2 protons, 2 neutrons
Atomic mass = 2 + 2 = 4 amu

what is Uranium-238 atomic mass?

92 protons, 146 neutrons
Atomic mass = 92 + 146 = 238 amu

What is the atomic weight of chlorine?

The atomic weight (or atomic mass) of chlorine is 35.453 atomic mass units (amu).

Chlorine is a chemical element with the symbol Cl and atomic number 17. It is a halogen and exists as a greenish-yellow gas at room temperature and atmospheric pressure.

The atomic weight of an element is a measure of its mass relative to the mass of a carbon-12 atom, which is assigned a value of exactly 12 amu. The atomic weight is calculated by taking the weighted average of the masses of the naturally occurring isotopes of the element.

Chlorine has two stable isotopes:

• Chlorine-35 (35Cl) with a relative atomic mass of 34.968852721 amu and a natural abundance of 75.78%
• Chlorine-37 (37Cl) with a relative atomic mass of 36.965902594 amu and a natural abundance of 24.22%

To calculate the atomic weight of chlorine, we take the weighted average of the masses of these two isotopes based on their natural abundances:

Atomic weight of chlorine = (34.968852721 × 0.7578) + (36.965902594 × 0.2422) = 35.453 amu

What is the atomic weight of sodium (Na)?

The atomic weight (or atomic mass) of sodium is 22.989769 atomic mass units (amu).

Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive alkali metal.

The atomic weight of an element is a measure of its mass relative to the mass of a carbon-12 atom, which is assigned a value of exactly 12 amu. The atomic weight is calculated by taking the weighted average of the masses of the naturally occurring isotopes of the element.

Sodium has one stable isotope:

• Sodium-23 (23Na) with a relative atomic mass of 22.98976928 amu and a natural abundance of 100%

Since sodium has only one stable isotope, its atomic weight is simply the mass of this isotope.

Atomic weight of sodium = 22.98976928 amu

However, this value is usually rounded to 22.989769 amu for most practical purposes.

It's important to note that the atomic weight is different from the mass number of an isotope. The mass number of an isotope represents the total number of protons and neutrons in the nucleus of that particular isotope. In the case of sodium-23, the mass number is 23 (11 protons + 12 neutrons).