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Sub-atomic particles is taught in the note for Atomic Structure. Read it below and look for the section on this topic.

Atomic Structure

11 min read

Dalton's atomic theory

In 1803 John Dalton suggested that all matter is made of tiny particles called atoms. His main ideas were:

1. Matter is made of atoms. 2. Atoms of one element are all alike and have the same mass. 3. Atoms of different elements differ in mass and properties. 4. Atoms cannot be created or destroyed in a chemical reaction; they only rearrange. 5. Atoms of different elements combine in simple whole-number ratios to form compounds.

Sub-atomic particles

An atom has a tiny, dense, positively charged nucleus containing protons and neutrons. Electrons move around the nucleus in shells. Most of the atom is empty space.

Structure of an atomA lithium atom with a small dense nucleus of three protons and four neutrons, and three electrons moving in shells around it. A table beside it gives the charge and relative mass of each particle: proton plus one and one, neutron zero and one, electron minus one and about one eighteen hundredth.nucleus: protons + neutronselectronshellParticleChargeMassProton+11Neutron01Electron−1≈ 0Atomic number Z = protons = electronsMass number A = protons + neutrons

Electronic arrangements

Electrons occupy shells, filling the lowest one first. The first shell holds 2 electrons, the second 8 and the third 8 (for the first 20 elements). The electrons in the outer shell are called valence electrons. They decide how an atom reacts.

Electron arrangements of six atomsSix atoms drawn with their electron shells: hydrogen 1, carbon 2 4, oxygen 2 6, sodium 2 8 1, chlorine 2 8 7 and argon 2 8 8. The outer shell electrons decide how an atom reacts.1+H16+C2,48+O2,611+Na2,8,117+Cl2,8,718+Ar2,8,8
Compare the outer shells of three well-known elements.

Atomic number and mass number

QuantitySymbolMeaning
Atomic numberZNumber of protons (= number of electrons in a neutral atom)
Mass numberANumber of protons + number of neutrons
Number of neutronsNA − Z
An atom is written with its mass number above and atomic number below the symbol, for example ³⁵₁₇Cl has 17 protons, 17 electrons and 18 neutrons.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Chlorine has two common isotopes, chlorine-35 and chlorine-37, so the relative atomic mass of chlorine is between 35 and 37.

Worked exampleAverage atomic mass

Chlorine is 75% chlorine-35 and 25% chlorine-37. Find its relative atomic mass.

  1. Multiply each mass by its share and add.

Answer: The relative atomic mass of chlorine is 35.5.

Try itCount the particles

Choose the atomic number, mass number and charge. See how many protons, neutrons and electrons the particle has.

Check yourself An atom has 11 protons and 12 neutrons. What is its mass number?
Check yourself Isotopes of an element have the same number of…
Check yourself Which particle has a negative charge?

Key terms

Nucleus
The tiny, dense centre of an atom containing protons and neutrons.
Isotope
An atom of the same element with a different number of neutrons.
Valence electron
An electron in the outer shell of an atom.
Mass number
The number of protons plus neutrons in an atom.

Summary

  • Dalton: matter is made of atoms that rearrange in reactions; atoms are divisible and isotopes exist.
  • Protons (+1) and neutrons (0) are in the nucleus; electrons (−1) are in shells.
  • Atomic number Z = protons; mass number A = protons + neutrons; neutrons = A − Z.
  • Isotopes have the same Z but different A; relative atomic mass is a weighted average.
Dalton's atomic theory Electronic arrangements