An atom is described by two numbers: the proton number (the number of protons) and the nucleon number (protons plus neutrons). From these you can work out every particle in a neutral atom. This skill opens almost every question in atoms, elements and isotopes and supports bonding, equations and calculations later.
What do the two numbers mean?
Nuclide notation puts the nucleon number at the upper left of the symbol and the proton number at the lower left. For sodium it is written as 23 above and 11 below the symbol Na. In plain text on this page we write it as 23/11 Na, with the upper number first.
The proton number decides which element you have. Every atom with 11 protons is sodium, and no other element has 11 protons. The nucleon number tells you how many particles are in the nucleus.
| Particle | Relative charge | Relative mass | Where |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | about 1/1840 | Shells around the nucleus |
How do I count the particles?
- Protons = proton number. Read the lower number.
- Electrons = protons for a neutral atom.
- Neutrons = nucleon number − proton number.
- Check: protons + neutrons must equal the nucleon number.
The fourth step is a cheap check that catches most slips.
Worked example
An atom of phosphorus is written 31/15 P. Find the number of protons, neutrons and electrons.
Step 1, protons: the proton number is 15, so there are 15 protons.
Step 2, electrons: the atom is neutral, so there are 15 electrons.
Step 3, neutrons: 31 − 15 = 16 neutrons.
Step 4, check: 15 + 16 = 31, which matches the nucleon number.
Answer: 15 protons, 16 neutrons, 15 electrons.
The mistake to watch for
A frequent slip is to treat the larger number as the number of protons, or to give the neutrons as the nucleon number.
Mistaken answer: 31 protons, 31 neutrons, 15 electrons.
The student read the upper number as “the protons” and never subtracted.
The correction is to remember what each number counts. The nucleon number counts both protons and neutrons, so it cannot be the neutron count on its own. The proton number names the element and sets the electron count in a neutral atom.
Check yourself
Try these first, then open each answer. Use a periodic table if a question asks for an element name.
1. An atom is written 35/17 Cl. How many protons, neutrons and electrons does it have?
Show answer
Protons = 17. Electrons = 17 (neutral). Neutrons = 35 − 17 = 18. Check: 17 + 18 = 35.
17 protons, 18 neutrons, 17 electrons.
2. A neutral atom has 26 protons and 30 neutrons. What is its nucleon number, and which element is it?
Show answer
Nucleon number = 26 + 30 = 56. The element with 26 protons is iron.
56/26 Fe
3. An atom has 13 electrons and a nucleon number of 27. It is neutral. How many neutrons does it have?
Show answer
Neutral, so protons = electrons = 13. Neutrons = 27 − 13 = 14.
14 neutrons (this is aluminium).
Where this leads next
Next, see what changes when an atom gains or loses electrons in determining particles in a simple ion. The same atom-counting habit is what the equation balance reasoning trainer asks you to use on both sides of an equation, and the mixed practice set tests the whole module.
Some students understand each step here but grab the wrong number under pressure. Spotting that kind of habit is what our teachers do in online one-to-one Chemistry tuition.