Atomic Orbital Essentials
Atomic orbital are regions of space where the electrons are located.
There are numerous possible orbitals within an atom, and each can accommodate up to two electrons.
There are several types of orbital. These differ in their shape. And they differ in which shells they may be found in.
Types of Orbitals
As we already alluded to, there are several types of orbital. But you are not expected to know them all at this level.
The names given to orbital types are simply a letter. The first four orbital types are s, p, d and f.
For A-Level it is necessary to know about s, p and d orbitals. The requirement for Advanced Higher students is to know s, p, d and f orbitals.
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Which Orbitals in Which Shells?
There are rules regarding which orbitals exist in which shells. These rules are our way of describing the observations we have about atoms and electons.
In short, there is one type of orbital possible in the first shell (closest to the nucleus). For each shell further out there is an additional orbital type possible.
In the first shell that possible orbital type is the s orbital.
In the second shell we can have s orbitals and p orbitals.
In the third shell we add possibility of d orbitals. So we may have s, p and d orbitals in the third shell.
The possibility of f orbitals starts from the fourth shell.
| Shell | Possible Orbitals Types |
|---|---|
| 1 | s |
| 2 | s, p |
| 3 | s, p, d |
| 4 | s, p, d, f |
How Many Orbitals?
Like so many questions, the answer is “it depends”. It depends on the type of orbital. Each orbital type has a different number of orbitals, here is a summary of those rules:
| Orbital Type | Number of Orbitals (per shell) |
|---|---|
| s | 1 |
| p | 3 |
| d | 5 |
| f | 7 |
Shapes of Atomic Orbitals
Earlier in this article we defined orbitals a being regions of space where electrons are found. And, as you can no doubt imagine, these regions have defined shape. They’re not just random, their shape is well defined – just like everything else in quantum mechanics is well defined by rules.
You are expected to know and recognise the shapes of some of the orbitals we have been looking at.
The Shape of s Orbitals
s orbitals are spherical and therefore the simplest shape of all the atomic orbitals. The sphere is centred on the exact centre of the atom.
Is there a difference in shape between s orbitals in different shells? Yes and no. The shape of a 1s and a 2s orbital, for example, are the same – both spherical and both centred at the centre of the atom. They are just different sizes.
The 2s orbital is larger than the s orbital in shell 1. The s orbital in each subsequent shell is larger again, so the 3s orbital is larger than the 2s and so on.
The Shape of p Orbitals
When considering the shape of p orbitals it is important to remember that there are three p orbitals in each shell from shell 2 onward. The shape of p orbitals is often described as dumbbell as there are two symmetrical lobes.
Each of these orbitals is oriented along one of the x, y or z axes, and to differentiate between them we include the axes in the name. The p orbital aligned along the x axis is called the px orbital. Similarly, the py is aligned along the y axis, and the pz orbital is aligned along the z axis. The axes are imaginary, of course, and exist in order to locate the centre of the atom, and the positional relationship between orbitals in the same shell.
As with s orbitals, p orbitals in different shells are different sizes. When comparing the size of p orbitals, the ones in the shell furthest from the nucleus are larger than those nearer to the nucleus – a 3p orbital is larger than a 2p orbital, for example. p orbitals within the same shell are the same size as each other.
The Shape of d Orbitals
When it comes to d orbital shapes we jump a level in complexity. There are five of these d orbitals per shell (from the third shell onward) and they don’t all have similar shapes. I’m just going to show the shapes rather than describe them.
You’re not going to be expected to reproduce these shapes. In fact, if you are studying A-Level you don’t need to know them at all. For Advanced Higher students you need to be aware of these orbital shapes, to recognise them.
As with other orbital types, the size of these orbitals in outer shells is larger than in shells nearer to the atom’s nucleus.
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