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1 Jan 2026

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7 pages

AQA Triple Chemistry: Topic 1a - Understanding Atoms, Elements, Compounds, and Mixtures

Somaia S

@somaia

Ready to master the building blocks of chemistry? This revision... Show more

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CHEMISTRY REVISION
Atoms, Elements, Compounds and Mixtures.
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Atoms, Elements and Isotopes

Understanding elements is dead simple - they're just substances made from one type of atom only. Think of them as the pure ingredients of chemistry, like having a jar filled with only carbon atoms or only oxygen atoms.

Nuclear symbols might look scary, but they're actually straightforward. The big number (mass number) sits on top, showing protons plus neutrons. The small number below (atomic number) tells you how many protons there are, which is always equal to the number of electrons in a neutral atom.

Isotopes are like identical twins with different weights - same element, same number of protons, but different numbers of neutrons. Carbon-12 and Carbon-13 are both carbon, but Carbon-13 has one extra neutron making it slightly heavier.

Relative atomic mass averages out all the different isotopes of an element. Use this formula: multiply each isotope's abundance by its mass number, add them up, then divide by the total abundance (usually 100 if given as percentages).

Quick Tip: Remember that proton number = atomic number = electron number in neutral atoms!

Compounds, Chemical Equations and Mixtures

Compounds form when different elements chemically bond together in fixed proportions - like a chemical marriage that's hard to break. Once formed, you can't separate the elements without a proper chemical reaction.

There are two main types: ions $formed when metals react with non-metals$ and molecules $formed from non-metals only$ . Chemical formulas like CO₂ or H₂SO₄ show exactly which elements are present and in what ratios.

Chemical equations are chemistry's way of showing what happens during reactions. Word equations are simple $magnesium + oxygen → magnesium oxide$ , whilst symbol equations show the exact amounts $2Mg + O₂ → 2MgO$ .

Mixtures are completely different - they're just substances mixed together without chemical bonding. The brilliant thing about mixtures is that you can separate them using physical methods like chromatography, filtration, or distillation.

Remember: Compounds need chemical reactions to separate them, but mixtures can be separated physically!

Paper Chromatography and Filtration

Paper chromatography is perfect for separating mixtures of different coloured liquids, like separating the dyes in ink. It works because different substances travel up the paper at different speeds.

Here's the method: draw a pencil line near the bottom of filter paper $never use pen - it's not insoluble!$ , spot your ink on the line, then dip the paper in solvent. The solvent carries the ink up the paper, separating the different dyes into distinct spots.

Filtration separates insoluble solids from liquids - think sand from water. Fold your filter paper into quarters, make a cone, and pop it in a funnel. Pour your mixture through, and the solid stays behind whilst the liquid passes through.

Evaporation and crystallisation both remove soluble salts from solutions. Evaporation uses heat to completely dry out the solution, whilst crystallisation involves gentle heating followed by slow cooling to form lovely crystals.

Exam Tip: You might be asked which separation technique to use for specific mixtures - make sure you know what separates what!

Crystallisation and Distillation Setup

Crystallisation gives you much better results than simple evaporation because you get proper crystals instead of just dried-out powder. Heat your solution gently until crystals start forming, then remove from heat and let it cool slowly.

The secret to great crystals is patience - the longer you leave the solution to cool after heating, the larger and more perfect your crystals will be. Always use tongs when handling hot evaporating dishes!

Both evaporation and crystallisation use similar equipment: evaporating dish, tripod, gauze mat, and Bunsen burner. The key difference is that crystallisation involves controlled cooling, whilst evaporation continues heating until everything's dry.

For crystallisation, you'll need to filter out your crystals once they've formed and leave them somewhere warm to dry completely. This gives you pure, solid crystals that look much more impressive than the crusty residue from evaporation.

Pro Tip: Remove the evaporating dish as soon as crystals start forming - don't keep heating or you'll end up with evaporation instead!

Simple and Fractional Distillation

Simple distillation is brilliant for getting pure liquids from mixtures - like extracting pure water from seawater. It works by heating the mixture so the liquid with the lowest boiling point evaporates first.

The clever bit happens in the condenser - the vapour travels through this cooled tube where it condenses back into liquid and drips into your collection container. Meanwhile, everything with a higher boiling point stays behind in the original flask.

Use simple distillation when the boiling points are very different from each other. The thermometer helps you monitor the temperature so you know which component is currently evaporating.

Fractional distillation is the advanced version - perfect when you need to separate liquids with very similar boiling points. It's essentially the same process but with extra equipment to make the separation more precise.

Key Point: Simple distillation works best when boiling points are far apart - fractional distillation handles similar boiling points!

The History of the Atom

The plum pudding model by J.J. Thomson in 1897 suggested atoms were like a sphere of positive charge with electrons dotted throughout - imagine a Christmas pudding with raisins scattered inside.

Everything changed with Rutherford's alpha particle experiment in 1909. He fired positively charged particles at thin gold foil, expecting them to pass straight through. Instead, some bounced back dramatically!

This shocking result led to the nuclear model - a tiny, dense, positively charged nucleus at the centre with electrons floating around it in mostly empty space. It completely revolutionised how we think about atoms.

Bohr's model in 1913 refined this further by suggesting electrons orbit in fixed shells around the nucleus, rather than just floating randomly. Later experiments discovered that the nucleus itself contains protons (positive) and neutrons (neutral particles).

Historical Fact: Each new discovery built on previous work - science is all about testing ideas and improving them!

Electronic Structure

Electronic structure (also called electron configuration) shows how electrons are arranged in shells around the nucleus. It's actually much simpler than it sounds once you get the hang of it.

The shell filling rules are straightforward: shell 1 holds maximum 2 electrons, shell 2 holds maximum 8 electrons, and shell 3 holds maximum 8 electrons. Electrons always fill the innermost shells first.

Noble gases in Group 0 have completely full outer shells, which makes them incredibly stable and unreactive. This is why helium has 2 electrons (filling shell 1) and neon has 2,8 (filling shells 1 and 2).

You can write electron configurations as numbers separated by commas - for example, 2,5 means 2 electrons in the first shell and 5 in the second shell. This tells you it's nitrogen, which has 7 electrons total.

Quick Check: Always make sure your electron numbers add up to the atomic number!

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Chemistry

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209

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1 Jan 2026

•

7 pages

AQA Triple Chemistry: Topic 1a - Understanding Atoms, Elements, Compounds, and Mixtures

Somaia S

@somaia

Use simple distillation when the boiling points are very different from each other. The thermometer helps you monitor the temperature so you know which component is currently evaporating.

Key Point: Simple distillation works best when boiling points are far apart - fractional distillation handles similar boiling points!

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The History of the Atom

The plum pudding model by J.J. Thomson in 1897 suggested atoms were like a sphere of positive charge with electrons dotted throughout - imagine a Christmas pudding with raisins scattered inside.

Historical Fact: Each new discovery built on previous work - science is all about testing ideas and improving them!

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Improve your grades

Join milions of students

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Electronic Structure

Electronic structure (also called electron configuration) shows how electrons are arranged in shells around the nucleus. It's actually much simpler than it sounds once you get the hang of it.

Quick Check: Always make sure your electron numbers add up to the atomic number!