Every atom of iron has exactly 26 protons. Change that number and it is no longer iron: 25 makes manganese, 27 makes cobalt. Yet iron atoms are not all identical. Some have more neutrons than others, and an iron atom that loses electrons becomes an ion with completely different chemistry.

This lesson shows what atoms are made of and how one short symbol tells you exactly how many protons, neutrons and electrons a particle has (Figure 1).

1. Nuclide symbolA top left, Z bottom left, charge top right
2. readA, Z and the charge
3. Protons and neutronsZ, and A minus Z
4. chargeelectrons lost or gained
5. ElectronsZ minus the charge

Neutral atom: electrons equal protons.Isotopes: same Z, different A.

Learning Objectives

After this lesson, you should be able to:

1. Give the charge, mass and location of protons, neutrons and electrons.
2. Use atomic number and mass number to count particles in atoms, ions and isotopes.

Key Ideas

1. Protons, Neutrons and Electrons

An atom has a tiny, dense nucleus that holds almost all of its mass, surrounded by a much larger region where its electrons move (Figure 2).

• A proton is in the nucleus, has a charge of +1 and a mass of 1.0073 u.
• A neutron is in the nucleus, has no charge and a mass of 1.0087 u.
• An electron is outside the nucleus, has a charge of −1 and a mass of only 0.000549 u, about 1836 times lighter than a proton.

The unit u (the atomic mass unit) is introduced properly in the next lesson; here it is enough that protons and neutrons each have a mass close to 1 u and electrons almost none. The charges are relative: the proton and the electron carry charges of the same size and opposite sign.

Key idea: protons and neutrons make up the nucleus and nearly all the mass; electrons are outside the nucleus, carry the negative charge and add almost no mass.

2. Atomic Number, Mass Number and Nuclide Symbols

The atomic number $Z$ is the number of protons. It defines the element: every atom with 26 protons is iron. The mass number $A$ counts the protons and the neutrons together, so the number of neutrons is $A-Z$.

A nuclide symbol packs this into one line: the mass number at the top left, the atomic number at the bottom left and the charge, if any, at the top right (Figure 3). The atomic number is often left out, because the element symbol already fixes it; the same atom can also be written iron-56 or Fe-56.

In a neutral atom the positive and negative charges balance, so the number of electrons equals the number of protons.

Key idea: Z is protons, A is protons plus neutrons, and a neutral atom has as many electrons as protons.

3. Ions: Gaining or Losing Electrons

An ion is an atom (or group of atoms) with a net charge, because it has gained or lost electrons. The nucleus does not change, so the protons and neutrons stay the same (Figure 4).

• A cation is positive: it has lost electrons. Na⁺ has 11 protons and 10 electrons.
• An anion is negative: it has gained electrons. Cl⁻ has 17 protons and 18 electrons.

Read the sign carefully: a charge of 3+ means three electrons fewer than protons, and 2− means two more.

4. Isotopes

Isotopes are atoms of the same element with different numbers of neutrons, and therefore different mass numbers. Natural chlorine is a mixture of two isotopes: 75.76% chlorine-35 and 24.24% chlorine-37 (Figure 5). Both have 17 protons and 17 electrons, so they react in the same way; they differ only in mass.

Key idea: isotopes share the atomic number and differ in the mass number. Chemistry is decided by the protons and electrons, so isotopes behave alike in reactions.

Worked Example 1: Counting the Particles in an Atom

Problem: How many protons, neutrons and electrons does one neutral atom of ${}^{56}_{26}\mathrm{Fe}$ have?

Step 1. Read the atomic number and the mass number.

• $Z = 26$ (bottom left): protons
• $A = 56$ (top left): protons and neutrons

Step 2. Count the protons and the neutrons.

Step 3. Count the electrons. A neutral atom has as many electrons as protons.

Final Answer: 26 protons, 30 neutrons, 26 electrons

Check: protons plus neutrons give back the mass number, 26 + 30 is 56.

Your turn: counting the particles in an atom

Same three steps as Worked Example 1: read Z and A, subtract for the neutrons, then count the electrons.

Worked Example 2: Counting the Particles in an Ion

Problem: How many protons, neutrons and electrons does the ion ${}^{27}_{13}\mathrm{Al}^{3+}$ have?

Step 1. Read the atomic number, the mass number and the charge.

• $Z = 13$, $A = 27$, charge 3+

Step 2. Count the protons and the neutrons. They are not changed by the charge.

Step 3. Count the electrons from the charge. The charge is 3+, so 3 electrons were lost.

Final Answer: 13 protons, 14 neutrons, 10 electrons

1. Symbol²⁷Al³⁺A 27, Z 13, charge 3+
2. readZ and A
3. Protons and neutrons13 protons, 14 neutrons
4. charge 3+3 electrons lost
5. Electrons10Final answer

A positive ion has fewer electrons than protons. Adding the 3 instead of subtracting it would give 16 electrons, the count for a 3− ion.

Your turn: counting the particles in an ion

Same method as Worked Example 2: protons and neutrons from Z and A, then electrons from the charge.

Worked Example 3: Comparing Isotopes

Problem: The atomic number of chlorine is 17. How many neutrons are in one atom of chlorine-35, and how many in one atom of chlorine-37?

Step 1. Read the mass numbers from the names. The number after the name is the mass number: $A = 35$ and $A = 37$.

Step 2. Subtract the protons from each mass number.

Final Answer: chlorine-35 has 18 neutrons; chlorine-37 has 20 neutrons

Both isotopes have 17 protons and, as neutral atoms, 17 electrons. The two extra neutrons are the only difference.

Your turn: isotopes and mass numbers

Same method as Worked Example 3: the mass number is protons plus neutrons, so subtract (or add) the atomic number.

Worked Example 4: Writing the Symbol from the Particle Counts

Problem: A particle has 26 protons, 30 neutrons and 23 electrons. Write its nuclide symbol.

Step 1. Find the element from the number of protons. $Z = 26$, so the element is iron, Fe.

Step 2. Add the protons and the neutrons to get the mass number.

Step 3. Subtract the electrons from the protons to get the charge.

Final Answer: ${}^{56}_{26}\mathrm{Fe}^{3+}$, the iron(III) ion

Three more protons than electrons means a 3+ charge. Writing the number of neutrons (30) as the mass number is a common slip: the mass number counts protons and neutrons together.

Your turn: writing the symbol

Same three steps as Worked Example 4: protons give the element, protons plus neutrons give A, protons minus electrons give the charge.

Common Mistakes

Common mistake: Using the mass number as the number of neutrons. Iron-56 has 30 neutrons, not 56: the mass number counts protons and neutrons together, so subtract Z.

Common mistake: Getting the electrons of an ion backward. A positive charge means electrons were lost. Al³⁺ has 10 electrons, not 16; Cl⁻ has 18, not 16.

Common mistake: Changing the protons when an atom becomes an ion. Only the electrons change. Fe³⁺ still has 26 protons: a different number of protons would be a different element.

Common mistake: Mixing up Z and A in the symbol. The larger number at the top left is the mass number A; the smaller number at the bottom left is the atomic number Z.

Common mistake: Thinking isotopes are different elements. Chlorine-35 and chlorine-37 are both chlorine: same protons, same electrons, same chemistry. Only the neutrons, and so the mass, differ.

Key Takeaway

The atomic number Z counts protons and fixes the element; the mass number A counts protons and neutrons, so neutrons are A minus Z. A neutral atom has Z electrons; an ion has lost electrons (positive charge) or gained them (negative charge). Isotopes of an element differ only in their neutrons.

Practice: randomized problems

Each problem below uses fresh numbers every time: work it out, check your answer, then open the worked solution to compare your steps.

Your results

This summary collects every practice problem you try on this page and suggests what to review next.

Figures: original diagrams by Dr. Ray’s Learning Center, licensed CC BY 4.0. Isotope data: NIST Standard Reference Database 144, Atomic Weights and Isotopic Compositions. Particle masses: CODATA 2022 recommended values (NIST). The practice problems are generated and checked by an audited engine; see the lesson audit log.