ChemGemTM

How to Find Protons, Neutrons and Electrons (Step-by-Step, with Practice)

Protons = atomic number. Neutrons = mass number − protons. Electrons = protons − charge.

3 steps, 1 worked example, 4 practice problems. About 10 minutes. Free.

ChemGemUpdated Section 1.3

The route · 3 steps

  1. Protons: atomic number (Z) on the periodic table.
  2. Neutrons: mass number (A) − protons.
  3. Electrons: protons − charge. Neutral: same as protons.
Try one now See a worked example
Start here

Periodic table

Protons come from here. Tap any element.

Tap an element.

123456789101112131415161718Row11 shell22 shells33 shells44 shells55 shells66 shells77 shells57–7189–103

Swipe sideways for the full table.

Spot it

How to recognize a protons, neutrons and electrons question

Given an isotope symbol, a name like oxygen-18, or an ion; asked for protons, neutrons or electrons.

Clue words marked:

How many protons, neutrons and electrons are in ³⁷Cl⁻?

Not this type?

Step-by-step

Worked example: inside a chlorine-37 ion

The idea

Reading an isotope symbol
37 17 Cl − 37 = mass number (protons + neutrons) 17 = atomic number (protons) − = charge (1 extra electron)
³⁷Cl⁻: 17 protons, 37 − 17 = 20 neutrons, 17 + 1 = 18 electrons.
Examples
SymbolProtonsNeutronsElectrons
126C666
146C686
2311Na+111210
3216S2−161618
Neutrons = A − Z. Electrons = protons − charge. Charge changes electrons only.

Positive ion (+)

Lost electrons.

Na⁺: 10 e⁻

Negative ion (−)

Gained electrons.

S²⁻: 18 e⁻

On the periodic table: C 6, Na 11, S 16, Cl 17 protons. Mass numbers come from the symbol or name.

Worked example · 3 steps

How many protons, neutrons and electrons are in ³⁷Cl⁻?

  1. 1
    Protons

    17Cl35.45On the periodic table: chlorine (Cl) is 17: 17 protons.

  2. 2
    Neutrons
    n = 37 − 17 = 20
  3. 3
    Electrons
    e = protons − charge
    e = 17 − (−1) = 18
  4. ✓
    Answer

    17 protons, 20 neutrons, 18 electrons.

    Chlorine-37 ion: 17 protons and 20 neutrons in the nucleus; 18 electrons.
    ³⁷Cl⁻: 17 p, 20 n, 18 e.
Your turn

Protons, neutrons and electrons practice

Try first, then check. Hints if you need them.

Problem 1 of 4 · warm-up

How many neutrons are in nitrogen-15?

Hint 1 · the clue

15 is the mass number.

Hint 2 · the next move

Nitrogen’s protons from the periodic table, subtracted from 15.

Your path:

  1. A = 15
  2. Z = 7
  3. n = A − Z
Answer

Answer: 8

Full worked solution

What you know: nitrogen-15.

  1. Nitrogen has 7 protons.
  2. 15 − 7 = 8 neutrons.
Nitrogen-15: 7 protons, 8 neutrons, 7 electrons.
Nitrogen-15: 7 p, 8 n, 7 e.

7N14.01On the periodic table: nitrogen (N) is 7.

Answer: 8

Problem 2 of 4

How many electrons are in a neutral 4020Ca atom?

Hint 1 · the clue

Neutral: electrons = protons.

Hint 2 · the next move

Read the bottom number (or the table).

Your path:

  1. Neutral
  2. Electrons = protons
  3. Z = 20
Answer

Answer: 20

Full worked solution

What you know: neutral calcium-40.

  1. Calcium has 20 protons.
  2. Neutral, so 20 electrons. (40 is the mass number.)
Calcium-40: 20 protons, 20 neutrons, 20 electrons.
Calcium-40: 20 p, 20 n, 20 e.

20Ca40.08On the periodic table: calcium (Ca) is 20.

Answer: 20

Problem 3 of 4 · watch for the trap

How many electrons are in ³²S²⁻?

Hint 1 · the clue

2− means 2 extra electrons.

Hint 2 · the next move

Sulfur’s protons (periodic table) + 2.

Your path:

  1. Z = 16
  2. Charge 2−
  3. e = 16 + 2
Answer

Answer: 18

Full worked solution

What you know: ³²S²⁻.

  1. Sulfur has 16 protons.
  2. 16 − (−2) = 18 electrons.

Watch out: 14 means you subtracted for a negative ion.

Sulfur-32 ion S2−: 16 protons, 16 neutrons, 18 electrons.
S²⁻: 16 p, 16 n, 18 e.

16S32.06On the periodic table: sulfur (S) is 16.

Answer: 18

Problem 4 of 4 · exam-style

An ion has 26 protons, 30 neutrons and 23 electrons. What is its charge?

Hint 1 · the clue

Charge = protons − electrons.

Hint 2 · the next move

26 − 23. Then name it from the periodic table.

Your path:

  1. Charge = p − e
  2. 26 − 23
  3. Positive: fewer electrons
Answer

Answer: 3+

Full worked solution

What you know: 26 p, 30 n, 23 e.

  1. 26 − 23 = 3+.
  2. Z = 26 is iron, A = 26 + 30 = 56: this is ⁵⁶Fe³⁺.
Iron-56 ion Fe3+: 26 protons, 30 neutrons, 23 electrons.
⁵⁶Fe³⁺: 26 p, 30 n, 23 e.

26Fe55.85On the periodic table: atomic number 26 is iron (Fe).

Answer: 3+

Watch out

Common mistakes