(A Level Chem) Atoms and molecules from O Level to A Level

When you were in upper secondary level, you learned about electronic structures of atoms whereby you fill the first energy shell with 2 electrons before you fill the second energy shell with 8 electrons. Your teacher might have led you to see the pattern that Period 1 of the Periodic Table corresponds to filling up of the first shell and therefore there are two elements in this very first period, while Period 2 corresponds to the filling up of the second shell and therefore there are eight elements in this second period. What evidence is there for the shells? Few students would ask. There are two pieces of evidence: the emission or absorption spectral lines of atoms, and successive ionization energies. Perhaps, I will in future write a separate article on these pieces of evidence but for now, I shall not go into these.


Knowing the atomic or proton number, you filled up the shells starting from the lowest energy and then end up with the last electron in the outermost shell. This last shell concerned you because the number of valence electrons that are housed here tells the Group number of the element and hence its chemical properties. The inner shells were then forgotten. That's how much is required in O Level. But when you are in A Level, the effect of the inner electrons surfaces out. It's called the shielding or screening effect of the inner electrons which is used to tell why the valence electrons of Groups I to III elements are so easy to remove, and why, across the same Period to the right, the greater nuclear charge (due to greater proton number) while keeping the shielding effect the same (due to same number of inner shell electrons) makes the atom more likely to accept electrons.

The Group number corresponds to the number of valence electrons...

In O Level, electron shells were simply represented by two-dimensional circles. Scientists call these energy shells orbits. Such is the Neil Bohr's atomic model which is an improvement from Rutherford's model of the atom based on his famous gold-foil experiment. Then there came de Broglie who hypothesized that electrons behave as waves in the atom. This is where the learning of science has become steeply abstract. Still, there's evidence for this. It's the diffraction pattern by electrons as though they are light which again is out of the scope of this article. But it explains why electrons can't spiral inwards to the nucleus.

Like an art lesson, we are then shown various three-dimensional shapes scientists call orbitals (not orbits) which are regions of space in which electrons are most probably found in. Such is called the atomic cloud model in which each energy shell consists of one or more orbitals, and each orbital rooms 2 electrons of opposite spins. The shapes of these orbitals were derived mathematically by Erwin Schrodinger who used the idea that electrons are waves in atoms. No one, however, has ever witnessed atomic orbitals. They can never be observed anyway.

Electrons are like waves and they occupy regions of space around the nucleus called orbitals...

How is the knowledge about the shapes of orbitals useful to us? Before the usefulness is obvious, you become overwhelmed by the huge weird talking about the changing of atomic orbitals to hybrid orbitals, and the overlapping of hybrid orbitals to become molecular orbitals (sigma bonds and pi bonds) sounding as though ghosts are changing forms.

Atomic orbitals interact to become hybrid orbitals before forming molecular orbitals... 

Eventually, only if you can pull your mind out of this cloud of confusion and have a glimpse of the overall picture, you will see that these work towards explaining the three-dimensional shapes of molecules from the linear CO2 to the trigonal-planar BF3 to the tetrahedral CH4 predicted using the idea of the Valence Electron Pair Repulsion Theory. Yet the long saga doesn't stop there as the concept is used again in complex ions in the chapter on Transition Elements, and in shapes of organic compounds and their reactions such as nucleophilic substitutions. The point of this article is that you make sure you clear your confusion about orbitals so that you are ready to stand for the chapters that make use of their concepts.

The type of hybridisation determines the shape of molecule... 

(Chinese translations: Absorption spectral lines 吸收谱线, Atoms 原子, Atomic orbital 原子轨道, Complex ion 络离子, Diffraction pattern 衍射图, Electronic structures 电子排布, Elements 元素, Emission spectral lines 发射谱线, Ionization energy 电离能, Hybrid orbitals 混成軌域, Hybridization 混成軌域, Hypothesized 或推测, Methane 甲烷, Molecular orbital 分子轨道, Nucleophilic substitution 亲核取代, Pi bond π键, Proton 质子, Schrodinger equation 薛定諤方程式, Sigma bond σ键, Shielding effect 屏蔽效应, Tetrahedral 四面体, Transition elements 过渡元素, Trigonal planar 平面三角形)