The particulate nature of matter is the idea that all matter is made up of tiny particles in constant, random motion. This topic covers the kinetic particle theory and the three states of matter, changes of state and heating and cooling curves, diffusion, atomic structure and electronic configuration, and isotopes and ions.
Chemical bonding, the mole concept, the Periodic Table and rates of reaction all assume you can picture what the particles are doing. The topic is tested in both multiple-choice and structured questions, usually by asking you to describe the particles in a substance or to explain a property using the particle model.
What these notes cover
- •Kinetic particle theory
- •The three states of matter compared
- •Bulk properties from the particle model
- •The six changes of state
- •Heating and cooling curves
- •Purity and sharp melting points
- •Diffusion and factors affecting its rate
- •Atomic structure and nuclide notation
- •Electronic configuration and the Periodic Table
- •Isotopes and relative atomic mass
- •Ions and counting sub-atomic particles
- •Worked examples and common mistakes
Key definitions
- Kinetic particle theory
- All matter is made up of tiny particles that are in constant, random motion.
- Diffusion
- The net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their constant random motion.
- Isotopes
- Atoms of the same element that have the same number of protons but different numbers of neutrons.
- Relative atomic mass, Aᵣ
- The weighted average mass of the isotopes of an element compared with one-twelfth of the mass of a carbon-12 atom. It is a ratio of two masses, so it has no unit.
- Ion
- A charged particle formed when an atom (or a group of atoms) loses or gains electrons.
Key formulas
Number of neutrons
number of neutrons = A − Z
A is the nucleon number (protons + neutrons) and Z is the proton number. The charge on an ion does not affect this.
Electrons in a neutral atom
number of electrons = number of protons = Z
Applies to a neutral atom: each proton is +1 and each electron is −1, so equal numbers give no overall charge.
Electrons in an ion
number of electrons = Z − charge on the ion
Include the sign of the charge: a 2+ ion has Z − 2 electrons; a 2− ion has Z − (−2) = Z + 2 electrons.
Relative atomic mass from isotopes
Aᵣ = Σ(isotopic mass × percentage abundance) ÷ 100
Σ means add up: multiply each isotope's mass by its percentage abundance, add the results for all isotopes, then divide by 100.
Period number
number of occupied electron shells = Period number
Always use the configuration of the atom, not the ion.
Group number
number of valence electrons → Group number
Groups 1 and 2: Group number = number of valence electrons. Groups 13 to 18: Group number = number of valence electrons + 10. Helium has 2 valence electrons but is in Group 18.
Worked example
Naturally occurring chlorine consists of 75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass of chlorine.
- 1Multiply each isotopic mass by its percentage abundance and add: Aᵣ = [(35 × 75) + (37 × 25)] ÷ 100.
- 2Evaluate the products: 35 × 75 = 2625 and 37 × 25 = 925.
- 3Add and divide by 100: Aᵣ = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5.
- 4Check: the answer lies between 35 and 37 and is closer to 35, because chlorine-35 is the more abundant isotope.
Watch out: Do not confuse Aᵣ with the nucleon number. The nucleon number belongs to one isotope and is always a whole number; Aᵣ is a weighted average for the element and is often not a whole number. No single chlorine atom has a mass of 35.5.
Common mistakes
Saying the particles shrink when a gas is compressed
Wrong: When the gas is compressed, the particles are compressed or shrink.
Right: The particles do not get smaller. Only the spaces between the particles get smaller.
Saying no heat is absorbed on a flat section of a heating curve
Wrong: No heat is absorbed during melting, or heating has stopped.
Right: Heat is absorbed throughout. During the change of state it is used to overcome the forces of attraction between the particles rather than to increase their kinetic energy, so the temperature stays constant.
Placing the white ring nearer the ammonia end
Wrong: The white ring of NH₄Cl forms nearer the ammonia end of the tube.
Right: The ring forms nearer the HCl end. NH₃ (Mᵣ = 17) diffuses faster than HCl (Mᵣ = 36.5), so it travels further in the same time.
Questions students ask
What is the difference between boiling and evaporation?
Boiling occurs only at the boiling point, throughout the liquid, quickly and with bubbles. Evaporation occurs at any temperature below the boiling point, at the surface of the liquid only, slowly and without bubbles. Both change a liquid to a gas and both absorb energy.
Why does the temperature stay constant when a substance is melting?
During the change of state, the heat absorbed is used to overcome the forces of attraction between the particles rather than to increase the kinetic energy of the particles. Hence the temperature remains constant until all of the solid has melted.
Why do isotopes have the same chemical properties?
Chemical reactions involve only the electrons, and isotopes of an element have the same number of electrons and the same electronic configuration. Their physical properties, such as density and rate of diffusion, differ slightly because they have different numbers of neutrons and therefore different masses.
How do you tell if a substance is pure from its melting point?
A pure substance melts at one sharp, fixed temperature, shown as a flat section on a heating or cooling curve. An impure substance melts over a range of temperatures, with a lower melting point and a higher boiling point than the pure substance. For example, pure water melts at 0 °C and boils at 100 °C, while sea water freezes below 0 °C and boils above 100 °C.
In the full PDF
- •Labelled diagrams of particles in the three states, heating and cooling curves, the ammonia and hydrogen chloride tube, and the shell structure of a sodium atom
- •A table of electronic configurations for the first 20 elements
- •Further worked examples on deducing states from melting and boiling points, reading cooling curves, and identifying atoms and ions
- •Model full-mark answers with mark allocations for diffusion and heating-curve questions
- •The remaining common mistakes, including mixing up isotopes and ions
- •Exam phrasing to memorise and a quick recap for each section