Atomic Mass Calculator
Atomic Mass Calculator
What Does This Calculator Calculate?
This calculator uses the number of protons and neutrons to determine the atomic number, element symbol, and mass number of an atom.
It also calculates an approximate atomic mass using the masses of protons, neutrons and electrons.
What Is an Atomic Mass Calculator?
An Atomic Mass Calculator is a chemistry tool used to calculate and understand the mass of an atom from its subatomic particles. By entering the number of protons and neutrons, you can determine the mass number and identify the corresponding element.
The number of protons is particularly important because it determines the identity of an element. For example, an atom with 6 protons is carbon, while an atom with 8 protons is oxygen.
The number of neutrons can vary while the number of protons remains the same. Atoms of the same element with different numbers of neutrons are called isotopes.
Atomic Mass Formula
The most important formula used to find the mass number of an atom is:
Using symbols:
where:
- A = mass number
- Z = atomic number or number of protons
- N = number of neutrons
How to Calculate Mass Number
To calculate the mass number, simply add the number of protons and neutrons in the nucleus.
For example, suppose an atom contains 6 protons and 6 neutrons.
Therefore, the mass number is 12. Since the atom contains 6 protons, it is carbon. This isotope is known as carbon-12.
Atomic Number vs Mass Number
Atomic number and mass number provide different information about an atom.
| Quantity | Meaning | Formula |
|---|---|---|
| Atomic Number | Number of protons | Z = Protons |
| Mass Number | Protons + neutrons | A = Z + N |
| Number of Neutrons | Mass number − atomic number | N = A − Z |
How Does the Number of Protons Identify an Element?
Every chemical element has a unique atomic number. The atomic number is equal to the number of protons in the nucleus.
| Protons | Element | Symbol |
|---|---|---|
| 1 | Hydrogen | H |
| 2 | Helium | He |
| 6 | Carbon | C |
| 7 | Nitrogen | N |
| 8 | Oxygen | O |
| 11 | Sodium | Na |
| 17 | Chlorine | Cl |
| 26 | Iron | Fe |
| 29 | Copper | Cu |
| 79 | Gold | Au |
What Are Isotopes?
Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons.
For example, carbon-12 and carbon-13 are both carbon because each contains 6 protons.
Carbon-13: 6 protons + 7 neutrons = 13
The difference in neutron number changes the mass number but does not change the identity of the element.
Examples of Atomic Mass and Mass Number
Example 1: Hydrogen-1
Protons = 1
Neutrons = 0
Example 2: Helium-4
Protons = 2
Neutrons = 2
Example 3: Oxygen-16
Protons = 8
Neutrons = 8
Example 4: Sodium-23
Protons = 11
Neutrons = 12
Example 5: Chlorine-35
Protons = 17
Neutrons = 18
Atomic Mass Unit
The mass of an individual atom is extremely small, so atomic masses are usually expressed in atomic mass units (u) rather than kilograms.
One atomic mass unit is approximately:
To convert atomic mass from atomic mass units to kilograms:
Is Atomic Mass the Same as Mass Number?
No. Atomic mass and mass number are different quantities.
Mass number is a whole number obtained by adding the number of protons and neutrons.
Atomic mass is the measured mass of an atom or isotope and is generally expressed in atomic mass units. It can contain decimal places.
The measured mass of an isotope is not exactly equal to its mass number because of nuclear binding energy and the small mass of electrons.
How to Use This Atomic Mass Calculator
- Enter the number of protons.
- Enter the number of neutrons.
- Click the Calculate button.
- The calculator displays the element symbol and mass number.
- The approximate atomic mass is also displayed.
- Click Show Step-by-Step Solution to see the calculation process.
Important Formulas for Atomic Structure
Why Do Neutrons Affect Atomic Mass?
Neutrons are located in the nucleus and have a mass similar to that of protons. Consequently, changing the number of neutrons changes the mass of the atom.
However, adding a neutron does not change the element because the element is defined by its number of protons.
Why Are Electrons Usually Ignored in Simple Mass Calculations?
Electrons have a much smaller mass than protons and neutrons. Therefore, when learning basic atomic structure, the mass of an atom is often approximated using the mass of its protons and neutrons.
For precise isotope masses, experimentally measured isotope mass data should be used because nuclear binding energy also affects the measured mass.
Frequently Asked Questions
What is the formula for mass number?
The formula is: Mass Number = Protons + Neutrons.
Does the number of neutrons determine the element?
No. The number of protons determines the element. Different numbers of neutrons produce different isotopes of the same element.
What is the difference between atomic number and mass number?
Atomic number is the number of protons. Mass number is the total number of protons and neutrons.
Can two atoms have the same atomic number but different mass numbers?
Yes. They are isotopes of the same element.
How do I calculate the number of neutrons?
Subtract the atomic number from the mass number: Neutrons = Mass Number − Atomic Number.
What unit is used for atomic mass?
Atomic mass is commonly expressed in atomic mass units (u), also called daltons (Da).
Does an electron have mass?
Yes. Electrons have mass, but their mass is much smaller than the mass of a proton or neutron.
Summary
The number of protons and neutrons provides important information about an atom. The number of protons identifies the element, while the total number of protons and neutrons gives the mass number.
Use the Atomic Mass Calculator above to identify an element, calculate its mass number, estimate its atomic mass, and view the complete calculation step by step.