In reference to an article I have recently read
(http://www.fromquarkstoquasars.com/do-you-believe-in-molecules/), yes, I do believe in molecules.
And this article gave me the idea for one of my own here in Science At Hand.
I still remember how difficult it was at the
university an important subject during the first semester. A subject named
something like “Chemical bond and structure of the matter”. It consisted in
hours and hours of lessons about the atomic and molecular structures, the
different chemical bonds, and things like that. But we had to learn that if we
wanted to get one day the BSc in Chemistry, so there was no chance…
How could I explain a few things about the
atoms, the molecules and the structures of those molecules? I don’t want to
spend days talking about it, and I want to give just some clues about the topic…
in a comprehensive way if possible!
So let’s start for what we know about atoms
(and be assured that I will not talk about the atomic bomb!). The word atom comes from the ancient Greek
(again!) and means “indivisible”. And this is the first problem we find when
trying to understand them, you will see why in a minute. The term “atom” as the
smallest unit forming the matter was proposed by early Indian and Greek
philosophers, and it was rescued by chemists in the 18th and 19th
centuries after showing that those atoms could not be broken down by chemical
methods. Then, by the late 19th century and the early 20th
century, physicists discovered substructures inside the atoms, but even with
the word atom not being suitable
anymore the name was kept. Hey, I cannot help it!
The point is, atoms are made of protons,
neutrons and electrons. Each atom has a nucleus with protons and neutrons, and
this nucleus is surrounded by electrons. I am pretty sure you have seen an
image like this:
Keeping aside the inaccuracy of this model of
the atomic structure, it fits with the concept: the atoms have a nucleus surrounded by
electrons. Let’s stick to this idea, then.
All matter is made of atoms. OK, this simple
sentence is well accepted by the public, which is a good thing. But when we
talk about the oxygen that we need to breathe, or about the ozone layer, to put
examples commonly used outside the scientific world, we are not talking about
atoms, we are talking about molecules. Or when we mention the carbon dioxide
which causes a greenhouse effect when accumulated in an excessive quantity in
the atmosphere, carbon dioxide is a molecule. And water is probably the most
important molecule for us!! (Logically, when we talk about these molecules, we refer to incredibly huge amounts of molecules of the said substance grouped together -thousands of millions of them, actually-... otherwise we would die of thirst if each time we drank water we were drinking just one molecule of water!!!).
So we should go to the next sentence which
should be accepted and understood: atoms combine between themselves to form molecules.
Therefore, water is H2O, a molecule formed by two atoms of hydrogen
and one atom of oxygen. Looks very basic, but this simple fact allows the
wonder of water, with all of its properties. Carbon dioxide is CO2,
one atom of carbon and two atoms of oxygen. The oxygen we breathe, molecular
oxygen, is O2, two atoms of oxygen. And ozone is O3,
three atoms of oxygen.
How are the atoms able to combine with each
other to form all these substances? Well, that’s where the periodic table comes
into the game. Yes, that image which causes nightmares to many just when they
see it! But the periodic table is a very useful tool, believe it or not. Mr.
Dmitri Mendeleev did a really good job, no wonder.
(First thing for you to know: why periodic table? Well, we see the
elements arranged into rows and columns. The rows are called periods, and the columns are named groups. Now we can move on.)
The atoms of each chemical element have
intrinsic properties according to their individual structures. Let’s take hydrogen and have a look on the increasing complexity as
we move along the periodic table:
Hydrogen is the simplest atom. It consists of a
proton and one electron. Yes, you read well, only one proton and one electron.
Simple, ain’t it? The atomic number
-which is the number of protons in the atomic nucleus- of hydrogen, then, is 1.
(Just to mention it, though I am not going to talk in detail about it, the
elements have isotopes, which are the
same elements but with a different number of neutrons in the nucleus, and
therefore they have a different atomic
weight: protons and neutrons are the components of atoms which have a
weight, while electrons are weightless for they are too small, thus the atomic
weight is determined by the protons and the neutrons in the nucleus. Hydrogen
has three isotopes, protium -without
neutrons-, deuterium -with one
neutron- and tritium -with two
neutrons-. The term protium is rarely
used, though, as we generally assimilate hydrogen to this one isotope.)
What is the next element, in terms of
complexity? Helium, of which the atomic number is 2. Its nucleus has 2 protons
and one or two neutrons (there are two isotopes of helium, one with just one
neutron in the nucleus and another one with two), and then it has two electrons
around the nucleus. And as we go along the periodic table, the elements are
arranged by increasing order of atomic number, and they are organised in periods and groups. But there is an asymmetry in this table, why? We will see it in a
future article -the explanation is not short-.
Each element has specific properties that are caused by the atomic number. And more specifically by the number of electrons and
how these electrons are organised around the nucleus. Let’s imagine each element as a citizen in a
city, as an individual with a specific personality. There is one thing that all
the elements like doing, as it was written in their DNA: they arrange the
electrons in different “orbits” (actually, they are called orbitals) around the nucleus, at different levels of energy, and
each of these orbits can accommodate a specific maximum number of electrons…
the elements love having the most external orbit filled with the corresponding
number of electrons, and this is what gives each of them its properties. Here
are some examples:
-Hydrogen just needs a second electron to feel
happy. With 2 electrons he feels fine, like a pair of shoes. Or if he loses the
electron he had and becomes a positively charged proton, he is quite OK too. (By the
way, when an element loses or wins electrons and becomes electrically charged,
it becomes an ion).
-Helium already has 2 electrons so he doesn’t
show much interest in the world outside. From the chemical point of view,
helium is inert. When in 1903 it was discovered there was a big amount of this
gas (it had been first isolated in 1895) in a mine in Kansas, they tried to burn it in vain. That's because helium is
not flammable.
-And there are elements with a craving for 8
electrons on their external layer (in these cases, they follow the octet rule), and elements with high
atomic number who might look for 12 or 18 electrons.
So the elements lose, share or steal electrons
from their environment when necessary, in order to achieve the suitable number
of electrons they need. Some elements share or exchange the electrons without
too much noise, but others can be really aggressive. Any element not having the
external orbit duly filled will steal, beg, arrange deals, whatever is
necessary, to get the correct number of electrons. If they need 8 electrons and
they have less than those 8 they look for, they will try to grab them from
their neighbours or convince neighbouring elements to share electrons with them;
if they need 8 electrons and they have more than those 8 electrons, they will tend
to give away the extra ones.
Many times, life is beautiful and the conflicts
are solved by sharing or exchanging electrons so different elements can live in
harmony. When this happens, different atoms form a molecule which is a little bit like a happy family where an atom
that likes giving away an electron gladly shares it with an atom that likes
getting an extra electron. But then sometimes two different molecules react, as in an interchange between
couples, so molecules which seemed to live in harmony decide to go for a change
in their lives (the different elements are attracted by the exciting electron
exchange which will generate new molecules). And this is the base of the chemical structures of molecules, and of chemical reactions.
More chemistry to follow in a future article!!
