Full chapter
Particles and Atoms
From changes of state to counting the particles in atoms and ions.
SEC G2 Science K223 / K225 (2027)
States and changes of state
Explain what particles do and where energy goes.
Make a guess. You are not marked.
Crushed ice at 0 degrees C is heated steadily and stirred. What does the thermometer show while the ice is melting?
Show the answer
It stays at 0 degrees C while ice melts
The energy supplied overcomes the attractions between particles instead of making them move faster, so the temperature stays constant.
Matter consists of particles. In a solid, particles remain close together in an ordered arrangement and vibrate about fixed positions. In a liquid, they remain close but move past one another. In a gas, they are widely spaced and move rapidly in random directions. A gas fills its container because its particles spread throughout the available space; compression mainly reduces the empty spaces between particles.
| State | Arrangement and movement | Observable behaviour |
|---|---|---|
| Solid | Close, regular; vibrate in fixed positions | Fixed shape and volume |
| Liquid | Close, irregular; move past one another | Fixed volume; takes container shape |
| Gas | Far apart; random movement | No fixed shape or volume; readily compressed |
- Solid to liquid: melting
Energy is absorbed to overcome enough attraction for particles to move past each other.
- Liquid to gas: boiling or evaporation
Energy is absorbed to separate particles much further. Evaporation occurs at the surface and can occur below the boiling point.
- Gas to liquid: condensation
Particles come closer and energy is transferred to the surroundings.
- Liquid to solid: freezing
Particles settle into fixed positions and energy is released. Sublimation changes solid directly to gas; deposition is the reverse.
During heating within one state, average particle kinetic energy increases and temperature rises. During the melting or boiling of a pure substance at constant pressure, supplied energy changes particle separation rather than raising temperature, so a plateau can occur. Cooling reverses these energy transfers. Particles themselves do not become larger or disappear.
Heating a pure substance
A rising line alternates with two horizontal plateaus: melting then boiling. Heat continues to enter during each plateau.
Check your understanding
Ice is melting while its temperature stays constant. Is it absorbing energy?
Show the answer
Yes. The energy overcomes the attractions between particles instead of raising their speed.
During melting, energy is absorbed to break up the fixed arrangement, not to raise the average kinetic energy. A constant temperature does not mean no energy is transferred.
Inside an atom
Use charges, masses and shells to account for an atom.
An atom has a tiny central nucleus containing protons and neutrons, collectively called nucleons. Electrons occupy shells, or energy levels, around it. Almost all the mass lies in the nucleus. A neutral atom has equal numbers of positive protons and negative electrons.
| Particle | Relative charge | Approximate relative mass | Position |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1840; negligible compared with a proton | Shells around nucleus |
A simple shell model
A lithium-7 nucleus contains three protons and four neutrons, with two electrons in the first shell and one in the second.
For the first twenty elements, use the familiar shell arrangements up to 2,8,8,2. For example, sodium has 11 electrons arranged 2,8,1 and calcium has 20 arranged 2,8,8,2. The Periodic Table supplies proton numbers. This course does not require orbital labels such as s, p, d or f.
Check your understanding
Neutral sodium has proton number 11. What are its numbers of protons and electrons, and its electron arrangement?
Show the answer
11 protons and 11 electrons, arranged 2.8.1
A neutral atom has equal numbers of protons and electrons. Eleven electrons fill the shells as 2, then 8, then 1.
Atomic numbers, isotopes and ions
Keep the identity of the element separate from its mass and charge.
Make a guess. You are not marked.
Chlorine-35 and chlorine-37 each react with sodium. How do their reactions compare?
Show the answer
They react in the same way
Isotopes have the same number of protons and electrons, and chemical reactions depend on electrons. Only the neutron number differs.
Proton number Z is the number of protons; it identifies the element. Nucleon number A is protons plus neutrons. In nuclide notation 126C, carbon has Z = 6 and A = 12, hence 6 protons and 6 neutrons. A neutral carbon atom also has 6 electrons.
- Protons = Z
The element identity fixes the proton count.
- Neutrons = A - Z
Only protons and neutrons contribute to nucleon number.
- Electrons: start with Z
Subtract electrons for a positive ion; add electrons for a negative ion.
Isotopes are atoms of the same element with the same proton number but different neutron numbers. Carbon-12 and carbon-14 both have six protons; they have six and eight neutrons respectively. Their neutral atoms have the same electron arrangement, so their chemical behaviour is similar. An ion forms when electrons are lost or gained; its nucleus does not change.
Worked exampleCount particles in ions
Find protons, neutrons and electrons in 2412Mg2+ and 3517Cl-.
Try it first. Then show the worked stepsHide the worked steps
- Magnesium: protons = 12; neutrons = 24 - 12 = 12. A 2+ charge means two electrons lost: 12 - 2 = 10.
- Chlorine: protons = 17; neutrons = 35 - 17 = 18. A 1- charge means one electron gained: 17 + 1 = 18.
Mg2+: 12 p, 12 n, 10 e. Cl-: 17 p, 18 n, 18 e.
Check your understanding
Two particles both have 8 protons, with 8 and 10 neutrons. One of them has 10 electrons. What can you conclude?
Show the answer
Both are oxygen isotopes, and the particle with 10 electrons is an O2- ion.
The same proton number (8) means the same element, oxygen. Different neutron numbers make them isotopes. Ten electrons against 8 protons gives a 2- charge, which is about electrons, not the nucleus.
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Proton (atomic) number | The number of protons in the nucleus of an atom. |
| Nucleon (mass) number | The total number of protons and neutrons in the nucleus of an atom. |
| Isotopes | Atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of electrons (the same electronic configuration). |
State changes rearrange particles; heating does not enlarge them. Z counts protons, A counts protons plus neutrons. Isotopes change neutron number; ions change electron number.
| Quantity | Rule |
|---|---|
| Protons | Z |
| Neutrons | A - Z |
| Electrons | Z minus positive charge, or Z plus magnitude of negative charge |
Scope
Learning outcomes
2. The Particulate Nature of Matter (K223 / K225). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
2.1(a) Explain states and their interconversion
- Solid, liquid and gas particle arrangements and movements
- Melting, freezing, boiling/evaporation, condensation, sublimation
- Energy changes during state changes
2.2(a) Compare subatomic particles
- Relative charges and approximate relative masses of proton, neutron, electron
2.2(b) Represent atomic structure
- Nucleons in nucleus
- Electrons in shells shown in a diagram
- No s/p/d/f classification
2.2(c) Define proton and nucleon numbers
- Proton number Z
- Nucleon number A
2.2(d) Read and write nuclide notation
- Carbon-12 notation
- Atomic and mass-number positions
2.2(e) Recognize isotopes
- Same proton number; different neutron number
2.2(f) Count particles in atoms and ions
- Use proton/nucleon numbers
- Account for positive and negative ion charges