When you begin to delve into the world of H2 Chemistry and transition from Sec 4 to JC Chemistry, you could realise that some of the things you discover are in direct opposition to the information you have previously acquired. It is essential to be aware that the discipline of Chemistry is founded on models developed using the exacting scientific process. What we now know is not reality but only beliefs that data and experimentation have supported. Consequently, when we go to higher levels of Chemistry, we frequently need to unlearn and relearn material. Several significant distinctions exist between the Chemistry taught in high school and H2 Chemistry.
Atomic Model
You were taught to write electronic configurations using the 2.8.8 format in secondary school. Therefore, this is the format that you should use. On the other hand, JC will teach you all about the SPDF system and the option of increasing the octet configuration! You will also learn about the Aufbau Principle, Pauli’s Exclusion Principle, and Hund’s Rule. Pauli’s Exclusion Principle states that two electrons occupying the same orbital must have opposing spins. Hund’s Rule states that electrons are introduced to orbitals of the same energy level individually at first and in parallel spins until pairing happens. (electrons fill orbitals of lowest energy levels first).
Also, the circle is no longer an essential component of dot-and-cross diagrams; you may exclude it entirely.
The Acids and the Bases
In secondary school, you were taught that when acids dissolve in water, they release hydrogen ions (H+), but when bases dissociate in water, they release hydroxide ions (OH–). When studying JC, you will find out that this is only part of the picture. Only the Arrhenius hypothesis can explain that! You will also learn about Bronsted-Lowry acids and bases, known as proton acceptors and donors, and Lewis acids and bases, known as lone pair acceptors and donors.
In addition, H+ is not produced by the simple dissociation of acids and bases. Instead, we need to add water to create the hydronium ion, which is denoted by the symbol H3O+.
Bonding
You learned in secondary school that ionic bonds may form between metals and non-metals, but covalent bonds can only form between non-metals and other non-metals.
On the other hand, you will discover in junior college that non-metals and metals can form covalent bonds (AlCl3). The Al3+ cation has a relatively modest size for such a significant positive charge. As a result, the charge density of Al3+ will be rather high, as will its capacity to polarise. The electrons in AlCl3 are shared because Al3+ polarises the electron cloud of Cl– to a greater extent than otherwise. In point of fact, you will conclude that bonds do not represent a binary choice between ionic and covalent, but rather a spectrum that includes the possibility of covalent bonds with ionic character and ionic bonds with covalent character.
If you are having trouble, be sure to look into JC or Sec 4 chemistry tuition. Best of luck to you!
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