Atomic Number and Mass Number
In 1913, Henry Moseley X-rayed 38 elements and discovered each has a unique proton count, the discovery that fixed the order of the periodic table.
Printable Worksheets
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Q1 · How do you think scientists identify one element from another at the atomic level?
Q2 · If you added an extra proton to a carbon atom, do you think it would still be carbon? Why or why not?
● Know
- atomic number counts protons
- mass number counts protons and neutrons
- neutral atoms have equal protons and electrons
● Understand
- proton number identifies the element
- mass number is not the same as atomic number
- electrons do not contribute to mass number in this model
● Can do
- calculate atomic and mass number from simple data
- check whether an atom is neutral
- avoid confusing neutron number with atomic number
Imagine swapping one proton out of a carbon atom, that single change turns it into boron, a completely different element with different properties. This is why the atomic number (Z)the number of protons in an atom's nucleus, matters so much. This number defines the element. Carbon always has 6 protons. Oxygen always has 8. If you change the number of protons, you change the element entirely. That is why scientists treat proton number as the identity of the element.
The atomic number is listed on the periodic table and is the most important number for identifying any element. At this level, you should memorise the connection: atomic number = proton count = element identity. Everything else, mass number, neutron count, electron count, flows from this starting point.
An atom with 6 protons is carbon, whether it is in a diamond, a pencil lead or a living cell. An atom with 8 protons is oxygen, whether it is in water, air or rust. The proton count is the unchanging fingerprint of the element. An iron atom with 26 protons is iron whether it comes from a meteorite, a blood cell or the Super Pit mine in Kalgoorlie.
Australian geoscience: Geoscientists at Geoscience Australia use atomic number to identify minerals in rock samples. By measuring which elements are present, they can map Australia's mineral resources and guide mining exploration across the continent.
'Mass number defines the element.' It does not. Mass number (A) counts protons plus neutrons, and it can vary between isotopes of the same element. Carbon-12 and carbon-14 both have mass numbers 12 and 14, but they are both carbon because both have 6 protons. Only the proton count, the atomic number, fixes the element's identity.
Use the Atomic Calculator interactive below. What is one thing you learned from using it?
Complete this isotope calculation for carbon-12.
Number of neutrons = A − Z = − = .
Mass number (A) is the total count of protons and neutrons in the nucleus. It is different from atomic number (Z), which counts only protons. To find the number of neutrons, subtract: neutrons = A − Z. Electrons are not included in the mass number because they are much lighter than protons and neutrons.
Practise this with common examples: carbon has Z = 6 and A = 12, so it has 6 neutrons. Oxygen has Z = 8 and A = 16, so it has 8 neutrons. The main goal is using the rule accurately, not memorising many elements.
Sodium has atomic number 11 and mass number 23. This means it has 11 protons and 23 − 11 = 12 neutrons. If the atom is neutral, it also has 11 electrons. The mass number tells you about the nucleus; the atomic number tells you the element. Always check your answer: neutrons must be a whole positive number.
Australian nuclear medicine: At ANSTO, technetium-99m (Z = 43, A = 99) is produced for medical imaging. Technicians must know exactly how many protons and neutrons are in the nucleus to ensure the isotope behaves correctly in the body. Mass number calculations are part of daily work in nuclear medicine.
'Mass number equals protons plus electrons.' This is wrong. Mass number counts only protons and neutrons, the particles in the nucleus. Electrons are outside the nucleus and their mass is roughly 1/2000 of a proton's mass, so it is ignored in mass number calculations. Adding electrons would give a completely wrong answer.
Atomic number counts and defines the element. Mass number counts plus .
Use the Electron Shell 3D interactive below. What is one thing you learned from using it?
Here's a student's working. One line has an error, click it.
- Mass number A = 14, atomic number Z = 6.
- Number of neutrons = A + Z.
- Number of neutrons = 14 + 6 = 20.
Neutral does not mean no particles. It means the positive and negative charges balance.
If an atom is neutral, the number of protons equals the number of electrons. This relationship is useful in simple worked examples and prepares you for later chemistry without actually teaching bonding.
At this stage, you only need the basic equality rule.
Explain to a partner why two atoms with different mass numbers can still be the same element if they have the same proton number.
A strong student uses the three counts together: protons, neutrons and electrons.
If you know proton number, you know atomic number. If you know protons and neutrons, you can find mass number. If the atom is neutral, electrons match protons.
This makes atomic-structure questions much easier and reduces random guessing.
Complete a table for three simple atoms showing proton number, neutron number, electron number, atomic number and mass number.
Use the Periodic Trends Explorer interactive below. What is one thing you learned from using it?
At the start of this lesson, you were asked about adding just one proton to carbon giving you nitrogen (a completely different element) and what an atomic number actually means.
Now that you have worked through everything, write your answer below. How has your thinking changed, and what surprised you most?
Q1. Explain the difference between atomic number and mass number.
Q2. A neutral atom has 11 protons and 12 neutrons. State its atomic number, mass number and electron number.
Q3. Explain why changing neutron number does not automatically make a new element.
Model answers (click to reveal)
Model Answers
+Multiple Choice
1: C. Atomic number counts protons.
2: B. Mass number is protons plus neutrons.
3: A. A neutral atom has equal protons and electrons, so it has 8 electrons.
4: D. Mass number is 6 + 7 = 13.
5: B. Changing proton number changes the element identity.
Short Answer 1
Atomic number is the number of protons, while mass number is the number of protons plus neutrons. They are different counts and should not be treated as the same thing.
Short Answer 2
Atomic number = 11 because there are 11 protons. Mass number = 23 because 11 + 12 = 23. Electron number = 11 because the atom is neutral.
Short Answer 3
Changing neutron number does not automatically create a new element because element identity is determined by proton number. If the proton number stays the same, the atom still belongs to the same element.
Model answers (click to reveal)
Model Answers
+Multiple Choice
1: C. Atomic number counts protons.
2: B. Mass number is protons plus neutrons.
3: A. A neutral atom has equal protons and electrons, so it has 8 electrons.
4: D. Mass number is 6 + 7 = 13.
5: B. Changing proton number changes the element identity.
Short Answer 1
Atomic number is the number of protons, while mass number is the number of protons plus neutrons. They are different counts and should not be treated as the same thing.
Short Answer 2
Atomic number = 11 because there are 11 protons. Mass number = 23 because 11 + 12 = 23. Electron number = 11 because the atom is neutral.
Short Answer 3
Changing neutron number does not automatically create a new element because element identity is determined by proton number. If the proton number stays the same, the atom still belongs to the same element.
● Atomic Number
Atomic number tells you the proton number.
● Mass Number
Mass number counts protons and neutrons together.
● Neutral Atoms
Neutral atoms have equal protons and electrons.
● Misconception
Neutron number alone does not define the element.