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As you explore the realm of H2 Chemistry and if you compare Sec 4 vs JC Chemistry, you may realise that some things you learn are in direct contradiction to what you have learnt in the past. It is important to recognise that Chemistry is built on models established by the rigorous scientific method. What we know now is not fact, but simply theories backed up by experimentation and evidence. As such, as we pursue higher levels of Chemistry, we must often unlearn and relearn. There are some key differences between secondary school Chemistry and H2 Chemistry. This post will aim to bring them to your attention.

1. Atomic model

When writing electronic configuration, in secondary school you learnt to write it in the 2.8.8 way. Whereas in JC, you will learn about the spdf system and the possibility of expanding octet configuration! You will also find out about Hund’s Rule (electrons are added to orbitals of the same energy level singly first and in parallel spins until pairing occurs), Pauli’s Exclusion Principle (the two electrons in the same orbital must have opposite spins) and the Aufbau Principle (electrons fill orbitals of lowest energy levels first).

Also, when drawing dot-and-cross diagrams, you don’t have to include the circle anymore!

2. Acids and Bases

You learnt in secondary school that acids dissociate in water to give H⁺ whereas bases dissociate in water to give OH^–. In JC, you will discover that it isn’t the full picture. That is only the Arrhenius theory! You will also learn about Bronsted-Lowry (proton acceptor/donor) and Lewis (lone pair acceptor/donor) acids and bases!

Also, acids and bases do not simply dissociate to form H⁺. Instead, we need to add water to form the hydronium ion H₃O⁺. For example:

HA + H₂O → H₃O⁺ + A^–

3. Bonding

In secondary school, you learnt that ionic bonds exist between metal and non-metals, while covalent bonds exist between non-metals and non-metals.

However, in JC you will learn that covalent bonds can exist between metals and non-metals (AlCl₃)!  Al³⁺ has a large positive charge and small cation size. Thus, Al³⁺  will have a high charge density and a high polarising power. Al3+ polarises the electron cloud of Cl^–  to a greater extent, resulting in electron sharing in AlCl₃! In fact, you will realise that bonds are not a dichotomy of either ionic or covalent, but a spectrum, with the possibility of covalent bonds with ionic character and ionic bonds with covalent character.

It’s time to let go of your past notions of Chemistry and move on to embrace a more comprehensive understanding of Chemistry. All the best!