Atomic Number and Mass Number
In 1869, Dmitri Mendeleev arranged all 63 known elements by atomic number and left 3 gaps — within 15 years, scientists discovered exactly those 3 missing elements with the properties he had predicted.
Printable Worksheets
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Q1 · A periodic table box for sodium shows the symbol Na and the numbers 11 and 23. Guess what those two numbers are telling you.
Q2 · Two atoms have the same number of protons. Must they be the same element? Why?
● Know
- Atomic number (Z) = number of protons
- Mass number (A) = protons + neutrons
- In a neutral atom, electrons = atomic number
● Understand
- Why the atomic number defines the element
- Why mass number and atomic number are not the same thing
- How to read a periodic table entry
● Can do
- Identify Z and A from a periodic table box
- Count protons, neutrons and electrons in any neutral atom
- Work out the number of neutrons using A − Z
- Atomic number (Z)
- Mass number (A)
- Element
- Symbol
- Neutral atom
- A letter code like Na or C
- Number of protons
- An atom with equal protons and electrons
- Protons + neutrons
- A substance made of only one kind of atom
Find the box for sodium (Na) on a periodic table and you'll see two numbers: 11 on top and 23 below (or nearby). Without those 2 numbers you cannot tell how many protons, neutrons or electrons a sodium atom holds. Every box on the periodic table shows two numbers along with the element's symbol — and they each tell you something different about the atom:
| Number | What it counts | Why it matters |
|---|---|---|
| Atomic number (Z) | Protons | Defines the element. 6 protons = carbon, always. |
| Mass number (A) | Protons + neutrons | Tells you how heavy the atom is. |
From these two numbers you can work out every particle in a neutral atom:
- Protons = Z
- Electrons = Z (because the atom is neutral)
- Neutrons = A − Z
The atomic number is the smaller of the two on a periodic table entry. The mass number is the bigger one.
The number tells you how many are in an atom. The number is protons plus . In a neutral atom, the number of equals the atomic number.
Wrong: "The mass number tells you how many neutrons are in the atom." Mass number = protons + neutrons. To get just the neutrons you have to subtract Z.
Right: Neutrons = A − Z. Mass number includes the protons too.
Wrong: "Two atoms with the same mass number must be the same element." Two atoms can have the same A but different Z — those are completely different elements.
Right: Same element = same atomic number (Z). The mass number can be anything.
Wrong: "Electrons add a lot of mass, so I should add them when working out the mass number." Electrons have almost no mass. Only protons and neutrons count.
Right: Mass number = protons + neutrons only. Electrons are ignored because their mass is ≈ 0.
A periodic table entry for sodium shows:
- Symbol: Na
- Atomic number (Z) = 11
- Mass number (A) = 23
From this we can work out everything about a neutral sodium atom:
- Protons = Z = 11
- Electrons = Z = 11 (the atom is neutral)
- Neutrons = A − Z = 23 − 11 = 12
So sodium = 11 p, 12 n, 11 e. Apply the 2–8–8 rule from Lesson 12 and the electrons fall into 2, 8, 1.
Use Z and A to fill in the counts:
| Element | Z | A | Protons | Neutrons | Electrons |
|---|---|---|---|---|---|
| Carbon (C) | 6 | 12 | 6 | 6 | 6 |
| Oxygen (O) | 8 | 16 | 8 | 8 | 8 |
| Aluminium (Al) | 13 | 27 | 13 | 14 | 13 |
The pattern always works: protons = electrons = Z, and neutrons = A − Z. Once you can read the two numbers off the periodic table, you can describe any atom of the first 20 elements.
The atomic number is the most important number on the periodic table. Change Z, and the element itself changes.
- 1 proton → hydrogen.
- 2 protons → helium.
- 6 protons → carbon.
- 79 protons → gold.
If you somehow added a proton to a carbon atom (which is incredibly hard), it would stop being carbon and become nitrogen. This is why scientists arrange the periodic table in order of increasing atomic number — each step adds one more proton and gives a brand-new element.
A potassium atom on the periodic table shows Z = 19 and A = 39. Predict (a) how many protons, neutrons and electrons it has, and (b) its electron arrangement using the 2–8–8 rule.
How close was your prediction?
Excellent — you used both Z and A correctly.
That's okay — practise: neutrons = A − Z, electrons = Z for a neutral atom.
At the start of this lesson you were told: Change the number of protons in an atom by even one, and it becomes a totally different element — the atomic number is an element's fingerprint. Does that make more sense now?
Use what you've learned to explain why the atomic number (Z) is like a fingerprint — and state Z, A, the number of protons, neutrons and electrons for sodium.
Q1. Define the atomic number (Z) and the mass number (A). State the formula you use to find the number of neutrons. (3 marks)
Q2. A periodic table entry for fluorine shows Z = 9 and A = 19. Work out the number of protons, neutrons and electrons in a neutral fluorine atom. Show your reasoning. (4 marks)
Q3. Why does the atomic number, not the mass number, decide which element an atom is? Refer to what changes about the atom and what stays the same. Use one specific example. (4 marks)
Answers
▾MCQ 1
A — Atomic number (Z) is the number of protons. This is also the element's "fingerprint" because changing it changes the element.
MCQ 2
C — Neutrons = A − Z = 16 − 8 = 8. (This is oxygen-16.)
MCQ 3
B — In a neutral atom, electrons = protons = Z = 12.
MCQ 4
D — The atomic number identifies the element. Mass number tells you how heavy the atom is but does not identify it.
MCQ 5
C — Mass number = protons + neutrons = 7 + 7 = 14. (This is nitrogen-14.)
Short Answer 1
Model answer: The atomic number (Z) is the number of protons in an atom. The mass number (A) is the total number of protons and neutrons. The number of neutrons is found from neutrons = A − Z.
Short Answer 2
Model answer: Protons = Z = 9. Electrons = Z = 9 (because a neutral atom has equal protons and electrons). Neutrons = A − Z = 19 − 9 = 10. So a neutral fluorine atom has 9 protons, 10 neutrons and 9 electrons.
Short Answer 3
Model answer: The atomic number decides the element because it counts the number of protons, and the number of protons is what gives an atom its identity. If the number of neutrons changes, the mass number changes but it is still the same element (these are called isotopes — Lesson 14). If the number of electrons changes, you get an ion of the same element. But if the number of protons changes, the element itself changes — for example, 6 protons is carbon, but 7 protons is nitrogen. So Z is the only number that locks in which element the atom is.