What is an atomic
orbital?
Orbitals and orbits
When a planet moves
around the sun, you can plot a definite path for it which is called an orbit. A
simple view of the atom looks similar and you may have pictured the electrons
as orbiting around the nucleus. The truth is different, and electrons in fact
inhabit regions of space known as orbitals.
Orbits and orbitals
sound similar, but they have quite different meanings. It is essential that you
understand the difference between them.
The impossibility of
drawing orbits for electrons
To plot a path for
something you need to know exactly where the object is and be able to work out
exactly where it's going to be an instant later. You can't do this for
electrons.
The Heisenberg
Uncertainty Principle says - loosely - that you can't know with
certainty both where an electron is and where it's going next. (What it
actually says is that it is impossible to define with absolute precision, at
the same time, both the position and the momentum of an electron.)
That makes it impossible to
plot an orbit for an electron around a nucleus. Is this a big problem? No. If
something is impossible, you have to accept it and find a way around it.Hydrogen's electron - the 1s orbital
Suppose you had a single
hydrogen atom and at a particular instant plotted the position of the one
electron. Soon afterwards, you do the same thing, and find that it is in a new
position. You have no idea how it got from the first place to the second.
You keep on doing this
over and over again, and gradually build up a sort of 3D map of the places that
the electron is likely to be found.
In the hydrogen case,
the electron can be found anywhere within a spherical space surrounding the
nucleus. The diagram shows a cross-section through this spherical space.
95% of the time (or any
other percentage you choose), the electron will be found within a fairly easily
defined region of space quite close to the nucleus. Such a region of space is
called an orbital. You can think of an orbital as being the
region of space in which the electron lives
What is the electron
doing in the orbital? We don't know, we can't know, and so we just ignore the
problem! All you can say is that if an electron is in a particular orbital it
will have a particular definable energy.
Each orbital has a name.
The orbital occupied by the
hydrogen electron is called a 1s orbital. The "1"
represents the fact that the orbital is in the energy level closest to the
nucleus. The "s" tells you about the shape of the
orbital. s orbitals are spherically symmetric around the nucleus - in each
case, like a hollow ball made of rather chunky material with the nucleus at its
centre
The orbital on the left
is a 2s orbital. This is similar to a 1s orbital except that the
region where there is the greatest chance of finding the electron is further
from the nucleus - this is an orbital at the second energy level.
If you look carefully,
you will notice that there is another region of slightly higher electron
density (where the dots are thicker) nearer the nucleus. ("Electron
density" is another way of talking about how likely you are to find an
electron at a particular place.)
2s (and 3s, 4s, etc)
electrons spend some of their time closer to the nucleus than you might expect.
The effect of this is to slightly reduce the energy of electrons in s orbitals.
The nearer the nucleus the electrons get, the lower their energy.
3s, 4s (etc) orbitals
get progressively further from the nucleus.
p orbitals
Not all electrons
inhabit s orbitals (in fact, very few electrons live in s orbitals). At the
first energy level, the only orbital available to electrons is the 1s orbital,
but at the second level, as well as a 2s orbital, there are also orbitals
called 2p orbitals.
A p orbital is rather like
2 identical balloons tied together at the nucleus. The diagram on the left is a
cross-section through that 3-dimensional region of space. Once again, the
orbital shows where there is a 95% chance of finding a particular electron
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