ChemGem

How to Identify an Element from Its Protons (Step-by-Step, with Practice)

Atoms are built from protons, neutrons and electrons, each with its own job. Protons decide the element; their count is the atomic number.

Spot it, solve it, practice it: 3 steps, 1 worked example, 7 practice problems. Free, no account.

ChemGemUpdated Section 1.1

The route · 3 steps

  1. Find the proton count.
  2. That count is the atomic number.
  3. Find that atomic number on the periodic table to name the element.
Try one now See a worked example
Spot it

How to recognize proton count questions

Look for a count of protons, electrons, or neutrons and "which element": protons identify the element, and a neutral atom has as many electrons as protons.

Example wording, with the clue words marked:

An atom has atomic number 13 and mass number 29. How many protons does it have?

Not sure it’s this type? Compare the neighbors:

Step-by-step

Worked example: proton count

The idea

How we learned what atoms are made of
Who and whenFinding
Dalton (early 1800s)Matter is tiny atoms. One element’s atoms are alike. Compounds use whole-number ratios.
Thomson (1897, 1904)Found the electron. “Plum pudding” model: negative electrons in a ball of positive charge.
Rutherford (1909–1911)Gold foil: an atom is mostly empty space around a tiny, dense, positive nucleus.
Later (1919, 1932)Proton found (1919), then neutron (1932).
Bohr and the quantum modelDescribe where the electrons are (Chapter 3).
Rutherford’s gold-foil experiment
alphasourcegold foilmost pass straight througha few are deflecteda very few bounce back= nucleus
Alpha particles: small, fast, positive particles given off by radioactive elements such as radium. Not to scale: the real nucleus is far smaller.

Quick check: why did most alpha particles pass straight through, while a few were deflected or bounced back? Answer: an atom is mostly empty space, so most met nothing. Nearly all the mass and all the positive charge sit in a tiny nucleus. The few that came close were repelled (like charges repel).

A lithium-7 atom with 3 red protons (+) and 4 gray neutrons (0) in the nucleus and 3 blue electrons (−) outside it. Labels name each particle and say where it sits.
Lithium-7: protons and neutrons in the nucleus, electrons outside. Electron positions are schematic; the nucleus is enlarged.
Three particles, three jobs
ParticleChargeWhere it isIts job
Proton+1NucleusIdentifies the element
Neutron0NucleusSets the isotope (1.2)
Electron−1Outside the nucleusSets the charge (1.3)
An atom is the smallest unit of an element. Changing electrons or neutrons does not change the element.
Protons = atomic number
p = Z
Neutral atom: electrons = protons
p is the proton count; Z is the atomic number. Every oxygen atom has 8 protons.

On the periodic table: Oxygen (O) has atomic number 8, the whole number in its square, so every oxygen atom has 8 protons. Its square also shows 16.00, Group 16, Period 2.

On the periodic table: The decimal mass (16.00) is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Worked example · 3 steps

Which element is this? An atom has 11 protons, 12 neutrons, and 10 electrons.

  1. 1
    Find the proton count

    Protons identify the element. Start here, even though other numbers are given.

    ProtonsNeutronsElectrons
    111210
  2. 2
    Read the atomic number

    Atomic number is another name for the proton count.

    Z = p = 11
  3. 3
    Match it to the table

    Neutrons and electrons do not change identity. On the periodic table: sodium (Na) has atomic number 11, the whole number in its square.

    Z = 11 → sodium (Na)
    Sodium’s square: 11, Na, 22.99, Group 1, Period 3.
  4. ✓
    Answer

    The element is sodium (Na).

Your turn

Proton count practice problems

Answer first, then check. Hints open one at a time; the answer and full solution open after you try.

Problem 1 of 7 · warm-up

An atom has atomic number 13 and mass number 29. How many protons does it have?

Hint 1 · the clue

Atomic number is provided.

Hint 2 · the next move

Atomic number is given and protons are asked for: the atomic number is the proton count, so read it straight off.

Your path:

  1. Atomic number = number of protons
  2. Ignore the mass number here
Answer

Answer: 13

Full worked solution

What you know: atomic number 13, mass number 29. What you want: the number of protons.

  1. Atomic number is defined as the number of protons. Every atom of an element has the same proton count.
  2. Read it straight off: atomic number 13 → 13 protons.

Check: The proton count can never be larger than the mass number, because mass number = protons + neutrons. 13 is less than 29, so it fits.

Watch out: Don't use the mass number (29). It counts protons and neutrons together.

Your route: Read atomic number → Use it as proton count

13Al26.98On the periodic table: Atomic number 13 belongs to aluminum (Al); it is the whole number in aluminum’s box, so an aluminum atom has 13 protons.

Answer: 13

Problem 2 of 7

An atom has atomic number 14 and mass number 31. How many protons does it have?

Hint 1 · the clue

Atomic number is provided.

Hint 2 · the next move

Atomic number is given and protons are asked for: the atomic number is the proton count, so read it straight off.

Your path:

  1. Atomic number = number of protons
  2. Ignore the mass number here
Answer

Answer: 14

Full worked solution

What you know: atomic number 14, mass number 31. What you want: the number of protons.

  1. Atomic number is defined as the number of protons. Every atom of an element has the same proton count.
  2. Read it straight off: atomic number 14 → 14 protons.

Check: The proton count can never be larger than the mass number, because mass number = protons + neutrons. 14 is less than 31, so it fits.

Watch out: Don't use the mass number (31). It counts protons and neutrons together.

Your route: Read atomic number → Use it as proton count

14Si28.09On the periodic table: Atomic number 14 belongs to silicon (Si); it is the whole number in silicon’s box, so a silicon atom has 14 protons.

Answer: 14

Problem 3 of 7

A neutral atom has 15 protons and 18 neutrons. How many electrons does it have?

Hint 1 · the clue

The word neutral fixes the charge balance.

Hint 2 · the next move

"Neutral atom" and electrons asked for: neutral means electrons equal protons; neutrons don't matter.

Your path:

  1. Neutral means total charge is 0
  2. Electrons = protons
  3. Ignore the neutrons
Answer

Answer: 15

Full worked solution

What you know: a neutral atom with 15 protons and 18 neutrons. What you want: the number of electrons.

  1. Neutral means the total charge is 0. Each proton is +1 and each electron is −1.
  2. 15 positive charges need 15 negative charges to cancel: (+15) + (−15) = 0. So there are 15 electrons.

Check: charge = protons − electrons = 15 − 15 = 0, which is neutral.

Watch out: The 18 neutrons have no charge, so they play no part. Don't match electrons to neutrons.

Your route: Neutral → Electrons equal protons

15P30.97On the periodic table: An atom with 15 protons is phosphorus (P), atomic number 15, the whole number in its box. A neutral phosphorus atom has 15 electrons.

Answer: 15

Problem 4 of 7

A neutral atom has 16 protons and 19 neutrons. How many electrons does it have?

Hint 1 · the clue

The word neutral fixes the charge balance.

Hint 2 · the next move

"Neutral atom" and electrons asked for: neutral means electrons equal protons; neutrons don't matter.

Your path:

  1. Neutral means total charge is 0
  2. Electrons = protons
  3. Ignore the neutrons
Answer

Answer: 16

Full worked solution

What you know: a neutral atom with 16 protons and 19 neutrons. What you want: the number of electrons.

  1. Neutral means the total charge is 0. Each proton is +1 and each electron is −1.
  2. 16 positive charges need 16 negative charges to cancel: (+16) + (−16) = 0. So there are 16 electrons.

Check: charge = protons − electrons = 16 − 16 = 0, which is neutral.

Watch out: The 19 neutrons have no charge, so they play no part. Don't match electrons to neutrons.

Your route: Neutral → Electrons equal protons

16S32.06On the periodic table: An atom with 16 protons is sulfur (S), atomic number 16, the whole number in its box. A neutral sulfur atom has 16 electrons.

Answer: 16

Problem 5 of 7

Which change would turn a sodium atom into an atom of another element?

Hint 1 · the clue

The question asks about a change of element.

Hint 2 · the next move

"Another element" asks about identity, not charge or mass: only the proton count sets which element it is.

Your path:

  1. Identity comes from protons
  2. Changing electrons only makes an ion
  3. Changing neutrons only makes an isotope
Answer

Answer: A change in the number of protons

Full worked solution

What you know: a sodium atom. What you want: the change that makes it a different element.

  1. Notice the clue: "another element" asks about identity, not charge or mass.
  2. Protons identify the element. Atomic number is the proton count, and sodium's is 11.
  3. Changing electrons only makes an ion. Changing neutrons only makes another isotope. So the answer is A change in the number of protons.

Why it matters: An atom with 12 protons is no longer sodium; it is magnesium.

Watch out: Na+ has lost an electron but is still sodium.

Your route: Identify the defining particle → Protons

11Na22.99On the periodic table: Sodium (Na) has atomic number 11, the whole number in its box, so every sodium atom has 11 protons. An atom with a different proton count sits in a different box.

Answer: A change in the number of protons

Problem 6 of 7

An atom has 9 protons. Which element?

Hint 1 · the clue

Find that proton count as an atomic number on the periodic table; read the element symbol.

Hint 2 · the next move

A proton count is given and "Which element?" is asked: find that atomic number on the periodic table.

Your path:

  1. Protons = atomic number
  2. Find that number on the periodic table
  3. Read the element's name
Answer

Fluorine, atomic number 9.

On the periodic table: Fluorine (F) has atomic number 9, the whole number in its box.

Full worked solution

What you know: 9 protons. What you want: the element.

  1. The proton count equals the atomic number, so look for atomic number 9.
  2. On the periodic table, the box with whole number 9 is F. The answer is Fluorine, atomic number 9.

Check: Counting across period 2 (Li 3, Be 4, B 5, C 6, N 7, O 8, F 9) lands on fluorine.

Watch out: Match the whole-number atomic number, not the decimal atomic mass (about 19.00 for fluorine).

Answer: Fluorine, atomic number 9.

YOUR PERIODIC-TABLE REFERENCE

9FFluorine19.00Group 17 · Period 2

Atomic number identifies proton count. The decimal mass is an average, not an isotope’s mass number. Use supplied rounded masses when a problem gives them.

Problem 7 of 7

Put these models in time order: Rutherford’s nuclear atom, Dalton’s tiny atoms, Thomson’s plum pudding. Then explain what gold-foil result made Rutherford replace the plum pudding model.

Hint 1 · the clue

Each new model kept what still fit and changed what new evidence ruled out. Ask which model needed the discovery of the electron and which needed the gold-foil results. Then ask what a spread-out ball of positive charge could not do to a fast alpha particle.

Hint 2 · the next move

Atomic models to order plus the "gold-foil result": recall the model history and what Rutherford's surprise showed.

Your path:

  1. Order: Dalton → Thomson → Rutherford
  2. A few alpha particles deflected or bounced back
  3. So mass and + charge sit in nucleus
Answer

Dalton → Thomson → Rutherford. A few alpha particles were deflected sharply or bounced straight back, so the positive charge and mass must sit in a tiny, dense nucleus.

Full worked solution

What you know: three models and the gold-foil results (most alpha particles passed through, a few deflected sharply, a very few bounced back). What you want: the time order and the reason for the change.

  1. Dalton (early 1800s): matter is made of tiny atoms; atoms of one element are alike.
  2. Thomson (1897 electron; 1904 model): negative electrons scattered through a ball of positive charge, the plum pudding.
  3. Rutherford (1909–1911): alpha particles fired at gold foil. Most passed straight through, so the atom is mostly empty space. A few were deflected sharply and a very few bounced almost straight back, which spread-out positive charge could not cause, so the positive charge and nearly all the mass are packed into a tiny, dense nucleus.

Check: Each model kept what still fit the evidence (atoms exist; atoms contain electrons) and changed only what the new evidence ruled out.

Watch out: Rutherford did not show where the electrons are; Bohr and the current quantum model describe that (Chapter 3).

Answer: Dalton → Thomson → Rutherford. A few alpha particles were deflected sharply or bounced straight back, so the positive charge and mass must sit in a tiny, dense nucleus.

Watch out

Common proton count mistakes