Chemistry Unit 1-2: Atoms, Elements and Compounds S3B

Chemistry Unit 1 S3B 2026

TB DEFINITION: An atom is the smallest unit of matter that contains protons, neutrons and electrons.

→ The smallest particles, that we cannot break down further in chemical reactions, are called atoms.

→ The charge of a proton is 1+ and the charge of an electron is 1-.

→ You can identify an atom by how many protons it has.

→ Protons and neutrons form the nucleus, so they are called nucleons.

What is an atom?

The nucleon number (represented by the letter A) is the total number of protons and neutrons inside the nucleus of an atom. It is also commonly called the mass number.

What is a nucleon number?

TB DEFINITION: A substance containing only one type of atom; it cannot be broken.

What is an element?

TB DEFINITION: A substance in which two or more elements are chemically combined.

A compound is made of atoms of different elements bonded together.

What is an compound?

TB DEFINITION: A mixture contains two or more substances that are not chemically combined.

A mixture contains different elements or compounds that are NOT bonded together. You can usually separate them quite easily.

What is a mixture?

TB DEFINITION: A unit of two or more atoms held together by covalent bonds.

In many substances, the particles consist of two or more atoms joined together. These particles are called molecules.

What is a molecule?

1. Atoms - the smallest particles of matter that we CANNOT break down further by chemical means.

2. Molecules - groups of two or more atoms chemically bonded together.

3. Ions - atoms that carry a charge, negative or positive, and can conduct electricity.
   Cation → Positively charged
   Anion → Negatively charged

Types of Particles

Solid:

Arrangement of particles: close together, regular pattern (lattice)

Shape and Volume: Fixed shape and volume

Movement: Vibrate in their position

Symbol: (s)

Liquid:

Arrangement of particles: close together, random arrangement

Shape and Volume: No fixed shape (takes the shape of their container) but has fixed volume

Movement: move past eachother

Symbol: (l) or (aq) (aqueous)

Gas:

Arrangement of particles: far apart, random arrangement

Shape and Volume: No fixed shape and no fixed volume (it spreads out to take the shape of its container so its volume changes a lot)

Movement: moves quickly in all directions

Symbol: (g)

State the properties of solids, liquids and gases.

Liquids Volume:

Liquids have a fixed, definite volume that stays the same no matter what shape their container has.

Gases Volume:

Gases have no fixed volume; they expand or compress to completely fill any size or shape of container they are placed in. They can have particles added inbetween the spaces of other particles in order to fill the size or shape of their container, thus, they have no fixed volume.

Shape and Space

• Liquids: Keep the same volume. They change shape to match the bottom of a container, but do not fill the whole space if the container is large.

• Gases: Spread out. They match both the shape and the total volume of their container.

Volume of Liquids VS Volume of Gases

Yes, particles can be added into gases. Adding more gas molecules increases the total number of particles, which raises the gas pressure inside a fixed container or causes a flexible container like a balloon to expand.

→ When you heat a gas, the volume increases.

→ When you cool a gas, the volume decreases.

Can particles be added into gases?

Solid to Liquid: Melting

Liquid to Solid: Freezing

Liquid to Gas: Evaporation

Gas to Liquid: Condensation/Liquify

Gas to Solid: Deposition/Desublimation

Solid to Gas: Sublimation

→ *Water vapour is the general term for water in a gas state at any temperature, while steam specifically means hot water gas at or above the boiling point of 100°C. You can think of steam as a hot type of water vapour. Water vapour is formed from evaporation at any temperature, while steam is specifically formed from boiling.

Name the changes in states of matter.

The stronger the bonds are between atoms, the harder it is to boil/melt that substance. The energy given to the substance in melting/boiling is used to weaken and break the bonds between the substance atoms, so, it would have a higher melting/boiling point.

Relationship of bond strength with changing state.

Heating Curve:

TB DEFINITION: A heating curve is a graph showing how the temperature of a substance changes on heating, as it changes state from solid to liquid to gas.

A heating curve shows how the temperature of a substance changes as you heat it up.

→ Temperatures remain constant while water changes state. These temperatures are its melting and boiling points.

AT THE MELTING POINT: The heat energy that is taken in is used to overcome the forces holding the particles; together in the lattice; the temperature will not rise again until all the ice has melted.

AT THE BOILING POINT: The heat energy that is taken in is used to overcome the forces between the particles; the temperature will not rise again until all the water has turned to gas.

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Cooling Curve:

TB DEFINITION: A cooling curve is a graph showing how the temperature of a substance changes, while it is being cooled from gas to liquid to solid.

A cooling curve shows how the temperature of a substance changes as you cool it down.

→ When the gas condenses into a liquid, it reaches another flat, constant line as the temperature doesn’t change. This is because the particles in the gas do not have enough energy to bounce apart when they collide; the forces of attraction between them take over; the gas condenses to a liquid; this releases heat, so even though you keep on cooling the substance, the temperature does not fall again until all the gas has liquified. (The heat released balances the cooling effect.)

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Melting and Boiling Points in the Presence of an Impurity:

→ When there is an impurity, the melting point will decrease.

THEORY: When there are particles of a impurity in the lattice, it is easier to break up. So the solid melts at a lower temperature — and less sharply. The more impurity, the bigger the effect.

→ When there is an impurity, the boiling point will increase.

THEORY: In the liquid, the impurity makes it HARDER for particles to leave the liquid and form a gas. So the boiling point is higher — and less sharp.

What is a heating and cooling curve?

The pressure P of a gas has an inverse relationship with its volume V and a direct relationship with its absolute temperature T.

→ When volume increases, pressure decreases.

(Why would decreasing the pressure increase the volume of a gas? Decreasing the pressure of a gas increases its volume because gas particles have less force pushing them inward. With less pressure, the fast-moving particles spread out more and take up a larger space until the outward push equals the new lower pressure.)

→ When temperature increases, volume increases at a constant rate.

→ When temperature increases, pressure increases at a constant rate. (If temperature goes up, pressure goes up.) Heating a gas gives particles more energy so they hit the container walls harder and faster, raising the pressure.

→ If volume decreases, pressure increases. (If volume goes down, pressure goes up. ) Shrinking the space (volume) makes particles hit the walls more often, which raises the pressure.

What is the relationship of the pressure with the temperature and volume of a gas?

When the temperature of a gas decreases at a constant rate in a closed, fixed container, the gas pressure also decreases at a constant rate. Cooling the gas makes its particles slow down. These slower particles hit the container walls less hard and less often.

Why Pressure Drops

• Slower motion: Gas particles lose heat energy and move much slower.

• Less force: Slower particles hit the walls of the box with less force.

• Lower pressure: Fewer hard hits mean the overall pressure goes down.

What happens to the pressure of gas when the temperature decreases at a constant rate?

Physical Properties:

Physical properties can be observed or measured without changing the chemical identity of a substance. They can be the appearance, dimensions, colour, density or electrical conductivity of a substance. Other physical properties can be found during physical changes, such as the melting and boiling point, which is found during changes in state of matter of substances.

Chemical Properties:

Chemical properties describe how a substance changes into a completely different substance through a chemical reaction. They can be observed when a chemical reaction takes place. They can be flammability, toxicity, corrosiveness/rusting, pH levels (acidity or basicity) or reactivity with water or oxygen.

Physical Properties VS Chemical Properties

Physical Changes

Physical changes are changes that are reversible, like a change in state of matter of a substance. If no new substance is formed. a change is a physical change.

Eg. Solid → Liquid

Chemical Changes

Chemical changes are changes that are irreversible. They are usually the product of chemical reactions, such as sodium and chlorine reacting to form sodium chlorine or table salt. A chemical change is usually called a chemical reaction.

Eg. Oxygen + Hydrogen gas → Water

How to know if its a chemical change?

1. One or more new chemical substances are formed.

2. Energy is taken in or given out, during the reaction.

3. The change is usually difficult to reverse.

Physical Change VS Chemical Change

TB DEFINITION: Diffusion is the process in which particles mix of colliding randomly with each other, and bouncing off in all directions.

The mixing process where the particles mix and spread by colliding with other particles and bouncing off in all directions is called diffusion. Diffusion takes place in liquids and gases. Diffusion is when the particles of a substance move from where there is a higher concentration of the substance to a region where there is a lower concentration of the substance. The particles spread from where they are more concentrated until all the particles are evenly mixed.

What is diffusion?

If the relative molecular mass of a substance is smaller, the substance will diffuse quicker.

If the relative molecular mass of a substance is smaller, it will diffuse quicker with another substance.

Rate of Diffusion and relative molecular mass.

*Atomic number = number of protons = number of electrons

*Atomic mass = number of protons + number of neutrons

*Relative Mass of 1 Proton = 1

*Relative Mass of 1 Neutron = 1

*Charge of 1 proton = +1

*Charge of 1 electron = -1

Info about Atoms and their component numbers.

Isotopes are forms of the same element with the same atomic numbers (same number of protons) but different mass numbers (different numbers of neutrons).

Relative atomic mass is the AVERAGE of all the masses of all isotopes. The relative atomic mass of the element A is the average mass of all its isotopes compared to 1/12 of the mass of an atom of carbon (with amu 12).

The atomic mass of elements on the periodic table are decimals because they are the weighted average of the mass of all their isotopes.

Relative atomic mass (Ar) = [ (mass 1 x %1) + (mass 2 x %2) ] / 100

All isotopes of an element behave in the same way because they all have the same number of valence electrons (same number of electrons). The electrons dictates how an element reacts, so there will be no difference as only the electrons react in chemical reactions. They will all have the same chemical properties.

What are isotopes?

The maximum number of electrons per shell is 2(n)².

The valence electron shell MUST have 8 electrons. It CANNOT exceed 8 electrons. All other shells can have their maximum number of electrons but the outermost shell must have 8.

Example of calculating electrons per shell:

Calcium - 20 electrons.

20 - 8 = 12 electrons

12 - 2 - 8 - 2 = 0. So, it would be 2, 8, 8, 2.

Ca → 2, 8, 8, 2 (you can’t directly put 10 in shell 3 because there must be 8 valence electrons!)

Electron orbits and calculating the number of electrons per shell.

Mixtures can be easily separated if there are physical differences between them.

1. Settling: Letting the solids rest and collect at the bottom of the container. (insoluble solid and liquid)

2. Decanting: Used to separate an Insoluble solid and a liquid (eg. Sand and Water). In decanting the liquid is poured gently so as not to disturb the sediment. (insoluble solid and liquid)

3. Filtration: Filtration is a method used to separate small insoluble solids from a liquid. It uses a funnel and filter paper to trap the solid particles. In filtration, the solid parts left behind is called the residue solid and the part collected in the beaker is called the filtrate. (insoluble solid and liquid)

4. Evaporation: Is a method used to separate soluble solids from a solution by evaporating off the liquid to leave the solid. (soluble solid and liquid)

5. Crystallisation: Occurs when crystals appear from a solution which has been evaporated. If cooled slowly, large crystals will form. If cooled quickly, small crystals will form. (Solid and liquid)

6. Distillation: Used to separate a Soluble solid and a liquid. Distillation can be used to separate two liquids with different boiling points. (Soluble solid and liquid OR liquid and liquid)

7. Chromatography: Is a technique used to separate very small quantities of substances which vary in solubility in different solvents. (Liquid and liquid, Dyes in Ink, Gases)

8. Separating Funnel: A device which operates on the difference in density of immiscible liquids. The denser liquid sinks to the bottom and drains out through the tap. (immiscible liquid and liquid)

State the Methods of Filtration.

This is because the components have different attractions to the paper and different solubilities with the solvent used.

• Parts that DISSOLVE WELL in the solvent moves up and carries dissolved substances along.

• Parts that STICK STRONGLY to the paper will move SLOW and stay LOW.

• Parts with higher density will move slowly and STAY LOW.

Why does the components travel different distances in paper chromatography?

Homogeneous: The same state of matter in a substance regarding a mixture. It is hard to differentiate which substance is which in the mixture.

→ eg. Milk, salt water

Heterogeneous: The different state of matter in a substance regarding a mixture. It is easy to differentiate which substance is which in the mixture.

→ eg. Sand + water, oil and water.

What is the difference between Heterogeneous mixture VS Homogeneous mixture?