A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes
Atoms, elements and compounds
All substances are made of atoms.
Atoms contain a proton, neutron and electron.
Number of protons=number of electrons.
An atom is the smallest part of an element that can exist.
Atoms of each element are represented by a chemical symbol.
Each element has a unique number of protons (atomic number).
Compounds are formed from elements by chemical reactions. Chemical reactions always involve the formation of one or more new substances, and often involve a detectable energy change.
Compounds contain two or more elements chemically combined in fixed proportions.
Have different properties from the elements they’re made of.
Compounds can only be separated into elements by chemical reactions.
Atoms are rearranged, not created or destroyed — mass is conserved.
Mixtures
A mixture contains two or more elements or compounds that are not chemically combined.
The chemical properties of each substance stay the same because no new bonds are formed.
Technique | Used For | How It Works | Why It Works |
|---|---|---|---|
Filtration | Insoluble solid + liquid | Liquid passes through filter paper; solid is left behind | Solid particles are too large to pass through the paper |
Crystallisation | Soluble solid + solution | Heat gently, solvent evaporates, crystals form when cooled | The solid has a higher boiling point than the solvent |
Simple distillation | One pure liquid from a solution | Liquid evaporates, condenses in condenser, is collected | Liquids and dissolved solids have different boiling points |
Fractional distillation | Two or more liquids with different boiling pts | Liquids evaporate at different temperatures and are separated | Each liquid has a different boiling point |
Chromatography | Dissolved substances (e.g. dyes, inks) | Solvent moves up paper, substances travel different distances | Different attraction to the mobile and stationary phases |
Mixture | Compound |
|---|---|
Elements/compounds not chemically joined | Elements chemically joined |
Can be separated physically | Can only be separated chemically |
Keeps properties of original substances | Has new properties |
The development of the model of the atom (common content with physics)
1. Dalton’s Solid Sphere Model
Atoms are tiny, indivisible spheres.
No internal structure.
First scientific model of the atom.
2. Thomson’s Plum Pudding Model
Proposed after the discovery of the electron.
Atom = sphere of positive charge with negative electrons embedded.
Charge overall is neutral.
3. Rutherford’s Alpha Scattering (Gold Foil) Experiment
Experiment:
Positively charged alpha particles were fired at thin gold foil.
Observations:
Most alpha particles passed straight through.
Some were deflected at small angles.
A very small number bounced back.
Why this disproved the Plum Pudding Model:
If positive charge were spread out, particles would only have tiny deflections.
Large deflections and backward scattering were impossible under the plum pudding model.
Why deflections happened:
Alpha particles are positive.
Nucleus is positive.
Like charges repel → strong repulsion when close to nucleus.
Conclusions → Rutherford’s Nuclear Model:
Atoms are mostly empty space (most particles passed through).
Positive charge and most of the mass are in a tiny, dense nucleus.
Electrons orbit the nucleus.
4. Bohr’s Model
Electrons orbit the nucleus in fixed energy levels (shells).
Electrons can only exist at specific distances.
Electrons further from the nucleus have more energy.
Electrons closer to the nucleus have less energy (more stable).
Model explained atomic emission spectra.
5. Discovery of Protons
Further experiments showed the nucleus contains smaller positive particles.
Each carries the same positive charge → protons.
6. Discovery of Neutrons (Chadwick, 1932)
Evidence found for particles with no charge in the nucleus → neutrons.
Explained why atomic mass is greater than the number of protons alone.
Relative electrical charges of subatomic particles
Particle | Charge | Location |
|---|---|---|
Proton | +1 | Nucleus |
Neutron | 0 | Nucleus |
Electron | −1 | Electron cloud (orbiting nucleus) |
Atoms have no overall charge because protons have a positive charge which is equal to to electrons which have a negative charge. Therefore, they cancel out.
Atomic number = number of protons in an atom.
All atoms of an element have the same number of protons.
Atoms of different elements have different numbers of protons.
Size and mass of atoms
Particle | Relative Mass |
|---|---|
Proton | 1 |
Neutron | 1 |
Electron | 1/1836 (≈0) |
Atomic Number
Number of protons.
Defines the element.
In a neutral atom, protons = electrons.
Mass Number
Protons + Neutrons = Mass Number
Electrons are not included because their mass is negligible compared to protons and neutrons.
Used to calculate number of neutrons:
Neutrons = Mass Number – Atomic Number
Relative atomic mass
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, giving them different mass numbers.
RAM=(isotope mass × isotope abundance) / 100 + (isotope mass × isotope abundance) / 100.
RFM=To find RFM, just add up the mass numbers of all the atoms in the compound.
Electronic structure
● electrons occupy the lowest available energy levels (the shells closest to the central nucleus)
● Electronic structure of an atom tells you how many electrons are in each shell
Periods (rows):
The period number = number of occupied shells.
Example: Sodium is in Period 3 → 3 shells → 2,8,1.
Groups (columns):
The group number = number of electrons in the outer shell (for Groups 1–7).
Group 1 → 1 outer electron
Group 7 → 7 outer electrons
Group 0 → full outer shell (very stable)
Outer shell electrons determine reactivity.
Fewer outer electrons → more reactive metals (Group 1).
More outer electrons missing → more reactive non-metals (Group 7).
Atoms react to gain a full outer shell by:
losing electrons (metals),
gaining electrons (non-metals)
sharing electrons (covalent bonds).
Uses of nanoparticles
Nanoparticles have a high surface area to volume ratio, giving them different properties to bulk materials.
Nano particles have the potential to be hazardous o health and to the ecosystems, so it is important that they are researched further.