Mass number facts for kids

The mass number (symbol: A) tells us how many protons and neutrons are inside the center (called the nucleus) of an atom. It's like counting the total number of big particles in the atom's core.

Every different isotope of a chemical element has its own mass number. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

We usually write the mass number after an element's name. For example, the most common type of carbon atom has 6 protons and 6 neutrons. Its mass number is 12 (6 protons + 6 neutrons). We write this as carbon-12. You might also see it written as a small number at the top left of the element's symbol, like ¹²C.

What is Mass Number?

The mass number is a simple count of the two main particles in an atom's nucleus: protons and neutrons. It helps us understand the different types of atoms for each element.

For example:

• A hydrogen atom with 1 proton and 0 neutrons has a mass number of 1 (¹H).
• A different type of hydrogen (called deuterium) has 1 proton and 1 neutron. Its mass number is 2 (²H).
• Another type of hydrogen (called tritium) has 1 proton and 2 neutrons. Its mass number is 3 (³H).

How is Mass Number Different from Other Terms?

It's easy to confuse the mass number with other terms used in chemistry. Here's how it's different:

• atomic number (symbol: Z): This is just the number of protons in an atom's nucleus. It's what makes an element unique. For example, all carbon atoms have 6 protons, so carbon's atomic number is 6.
• atomic mass (symbol: m_a): This is the actual mass of a single atom. It's usually measured in tiny units called unified atomic mass units. The atomic mass is very close to the mass number, but it's not exactly the same because it includes the tiny mass of electrons and the exact mass of protons and neutrons.
• relative atomic mass (symbol: A_r, also called atomic weight): This is the average mass of all the atoms of an element found in nature. It's compared to the mass of a carbon-12 atom. This number is what you usually see on the periodic table.

Finding the Number of Neutrons

You can easily find the number of neutrons (symbol: N) in an atom if you know its mass number (A) and its atomic number (Z).

The formula is: N = A − Z

So, if you have an atom with a mass number of 12 and an atomic number of 6 (like carbon-12), it has 12 - 6 = 6 neutrons.

Mass Number and the Periodic Table

The mass number is not usually shown on the periodic table. Instead, for each element, you will typically see its standard atomic weight (which is the relative atomic mass) and its atomic number. The atomic number is always a whole number, but the atomic weight is often a decimal because it's an average of all the different isotopes of that element.

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In Spanish: Número másico para niños