Atomic structure: protons, neutrons and electrons

• Atomic number = number of protons (which also equals electrons in an atom).
• Mass number = protons + neutrons.
• Neutrons = mass number − atomic number.

Example: sodium has atomic number 11 and mass number 23, so it has 11 protons, 11 electrons and $23 - 11 = 12$ neutrons.

Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 has 6 neutrons and carbon-14 has 8; both have 6 protons. Isotopes have the same chemical properties but different masses, and some (like carbon-14) are radioactive. Radioactive decay is measured by half-life.

Electrons fill shells from the inside: up to 2 in the first shell, then 8 in the second and 8 in the third (for the first 20 elements). Sodium (11 electrons) is 2, 8, 1. The number of outer electrons gives the element's group in the periodic table, and the number of shells gives its period. Explore it with our interactive periodic table.

1. Dalton (early 1800s): atoms as solid spheres.
2. Thomson (1897): discovered the electron; the "plum pudding" model of a ball of positive charge with electrons in it.
3. Rutherford (1911): the alpha-scattering experiment showed most of the atom is empty space with a small, dense, positive nucleus.
4. Bohr (1913): electrons orbit at fixed distances (shells).
5. Chadwick (1932): discovered the neutron.

Each model changed because new evidence didn't fit the old one.

• Using the mass number as the number of neutrons.
• Saying ions change the number of protons (ions gain or lose electrons).
• Mixing up groups (columns) and periods (rows).

Calculate relative formula masses with our Mr calculator.

Atoms become ions by gaining or losing electrons, never protons. A sodium atom (2, 8, 1) loses one electron to become Na⁺ (2, 8), with a full outer shell. A chlorine atom (2, 8, 7) gains one electron to become Cl⁻ (2, 8, 8). Metals tend to form positive ions; non-metals form negative ions.

1. Chlorine-35 has atomic number 17. How many protons, neutrons and electrons does it have?
2. Write the electron arrangement of magnesium (atomic number 12).
3. How do chlorine-35 and chlorine-37 differ?
4. How many electrons does a magnesium ion, Mg²⁺, have?

Answers: 1. 17 protons, 18 neutrons, 17 electrons. 2. 2, 8, 2. 3. Chlorine-37 has two more neutrons. 4. 10.

Most elements are a mixture of isotopes, so the relative atomic mass on the periodic table is a weighted average. Chlorine is about 75% chlorine-35 and 25% chlorine-37:

That's why chlorine's relative atomic mass isn't a whole number. Use $A_r$ values to work out formula masses with our Mr calculator.