Atomic number and atomic mass: what the numbers tell you
The atomic number is the number of protons in an atom, and it tells you which element the atom is. The mass number is the number of protons plus neutrons. With these two numbers you can count every particle: protons and electrons both equal the atomic number, and neutrons equal the mass number minus the atomic number.
Atomic number: an element’s ID number
Every element has its own atomic number, often written with the letter Z. It is the number of protons in the nucleus of each of its atoms. Hydrogen is 1, carbon is 6, iron is 26 and gold is 79.
The atomic number works like an ID number. Any atom with 26 protons is iron, and any atom with 27 protons is cobalt. Change the number of protons, and you have a different element.
On the periodic table, the atomic number is the whole number in each box, usually at the top. The elements are lined up in order of it, from 1 to 118.
Mass number: protons plus neutrons
The mass number, written A, is the total number of protons and neutrons in an atom’s nucleus. Electrons are so light that they are left out of the count.
Because it counts particles, the mass number is always a whole number. It often appears in an atom’s name. An iron atom with 26 protons and 30 neutrons is called iron-56.
Atomic mass: the decimal in the box
The decimal number in each box is the atomic mass, also called the atomic weight. For iron it is 55.845. It is the average mass of the element’s atoms, measured in atomic mass units (u).
It is an average because most elements are a mix of atoms with different numbers of neutrons, called isotopes. That is why the atomic mass is usually not a whole number. The next lesson explains isotopes in more detail.
For elements whose atoms are all radioactive, such as technetium, the table usually shows a whole number instead, like 98. That is the mass number of one of the element’s longest-lasting isotopes.
How to find protons, neutrons and electrons
For a neutral atom, three simple rules give you every particle.
Protons = atomic number. Electrons = atomic number, because a neutral atom has as many electrons as protons. Neutrons = mass number − atomic number.
If you are not given a mass number, round the atomic mass from the table to the nearest whole number and use that. The answer is a good estimate for a typical atom, but not always exact, because the atomic mass is an average.
Worked examples: sodium and iron
Sodium has atomic number 11 and atomic mass 22.99, which rounds to 23. So a sodium atom has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Iron has atomic number 26 and atomic mass 55.845, which rounds to 56. So most iron atoms have 26 protons, 26 electrons and 56 − 26 = 30 neutrons.
What about ions?
An ion is an atom that has gained or lost electrons. Its protons never change, so its atomic number stays the same, but its electron count does not match anymore.
A positive ion has lost electrons and a negative ion has gained them. Na⁺ has lost one electron, so it has 11 − 1 = 10 electrons. Chlorine’s atomic number is 17, so Cl⁻, which has gained one electron, has 17 + 1 = 18 electrons.
Key points
- The atomic number is the number of protons, and it decides which element an atom is.
- The mass number is the number of protons plus neutrons, and it is always a whole number.
- The atomic mass in each box is an average of the element’s isotopes, so it is usually a decimal.
- In a neutral atom, the number of protons and the number of electrons both equal the atomic number.
- Neutrons = mass number − atomic number; if no mass number is given, round the atomic mass for an estimate.
Check yourself
Three quick questions on this lesson.
1. Chlorine has atomic number 17. How many protons does every chlorine atom have?
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17 — The atomic number is the number of protons, so every chlorine atom has 17.2. A sodium-23 atom has atomic number 11. How many neutrons does it have?
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12 — Neutrons equal the mass number minus the atomic number, so 23 − 11 = 12.3. How many electrons does a chloride ion, Cl⁻, have?