GCSE AQA Chemistry Revision Notes

Chemical Equations and Balancing

Chemical equations are like recipes for reactions - they show exactly what goes in and what comes out. Reactants (the starting materials) appear before the arrow, whilst products (what you end up with) come after it.

The golden rule is that equations must be balanced - you need the same number of each type of atom on both sides. Think of it like a see-saw that needs to be perfectly level. To balance equations, add numbers in front of the chemical formulas, but never change the formulas themselves.

Here's how to tackle balancing: Start with magnesium + oxygen → magnesium oxide, which becomes Mg + O₂ → MgO. First, balance the oxygen by adding a 2 in front of MgO, giving you Mg + O₂ → 2MgO. Then balance the magnesium by adding a 2 in front of Mg, resulting in 2Mg + O₂ → 2MgO.

Top Tip: Count atoms of each element separately and balance one at a time - trying to do everything at once will leave you confused!

State Symbols and Common Ions

State symbols tell you the physical state of each substance in a reaction. You'll use $s$ for solid, $l$ for liquid, $g$ for gas, and (aq) for aqueous (dissolved in water). These symbols go in brackets right after each chemical formula.

A complete equation with state symbols looks like this: 2Na$s$ + 2H₂O$l$ → 2NaOH(aq) + H₂$g$. This shows solid sodium reacting with liquid water to produce an aqueous solution and hydrogen gas.

Common ions have predictable charges based on their position in the periodic table. Group 1 metals like sodium always have a +1 charge, whilst Group 2 metals like magnesium have +2. Important negative ions include hydroxide (OH⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), and sulfate (SO₄²⁻).

Remember: Aqueous simply means "dissolved in water" - so when you see (aq), think of a solution you could actually pour!

Elements and Compounds

Everything around you is made of atoms - the smallest possible units of matter that can exist. When atoms of the same type group together, they form elements, like pure oxygen or carbon. Elements are organised in the periodic table by atomic number.

Compounds form when two or more different elements chemically combine in fixed proportions. Water (H₂O) always has exactly 2 hydrogen atoms for every oxygen atom - never more, never less. This is completely different from just mixing elements together.

Creating compounds involves making and breaking chemical bonds through electron sharing, transfer, or rearrangement. This process usually releases or requires energy, which is why some reactions feel hot whilst others feel cold. Covalent bonds form when electrons are shared, whilst ionic bonds form when electrons transfer from metals to non-metals.

Key Point: The difference between elements and compounds is fundamental to all of chemistry - make sure you've got this distinction crystal clear!

Early Atomic Models

Scientists' understanding of atoms has evolved dramatically through experiments and evidence. The journey began with John Dalton's Billiard Ball Model in the early 1800s, which suggested atoms were solid, indivisible spheres that could be rearranged during chemical reactions.

Thompson's discovery of electrons completely revolutionised atomic theory. His Plum Pudding Model proposed that atoms were spheres of positive charge with negatively charged electrons embedded randomly throughout, like plums in a pudding. The mass was thought to be evenly distributed across the entire atom.

This model seemed logical at the time - it explained why atoms were generally neutral (positive and negative charges balanced out) and accounted for the newly discovered electrons. However, it was about to be completely overturned by a famous experiment.

Study Smart: Learn each model's key features systematically - exam questions love testing the differences between these early theories!

Rutherford's Revolutionary Discovery

Rutherford's alpha particle scattering experiment changed everything we thought we knew about atoms. He fired tiny, positively charged alpha particles at extremely thin gold foil, expecting them all to pass straight through based on the plum pudding model.

The results were shocking. Whilst most alpha particles did pass through undeflected, some were dramatically deflected or even bounced back completely. This was like firing bullets at tissue paper and having some bounce back at you!

These observations led to three crucial conclusions: most of the atom is empty space (explaining why most particles passed through), the nucleus must be positively charged (to repel the positive alpha particles), and the nucleus is incredibly tiny compared to the atom's total size (since only a few particles were deflected).

Rutherford's Nuclear Model placed a tiny, dense, positive nucleus at the atom's centre, with electrons orbiting in the surrounding empty space - completely revolutionising atomic theory.

Exam Focus: Understanding what each observation proved is crucial - questions often ask you to link specific results to conclusions about atomic structure!