| The two main ways in which atoms can be combined to form molecules are
by electrovalent bonds and covalent
bonds. Some molecules contain both
electrovalent and covalent bonds, but many have just one or the other
type. When you study enzymes, you will also need to know something about
much weaker attractions between atoms in molecules of proteins: these are hydrogen
bonds.
Electrovalent chemicals are held together
by an electrical attraction between positively charged cations and
negatively charged anions. You can read more about ions
to refresh your memory if you need to. Cations are formed when an atom of
a metallic element loses one or more of its electrons. Hydrogen is the
only non-metal to form a cation. Anions are formed when an atom or group
of atoms gains one or more electrons. This means that cations have a
positive charge and anions have a negative charge.
When an electrovalent chemical is dissolved in water it will
dissociate. That means that the cations and anions separate. These
chemicals are called electrolytes because they will conduct electricity.
You should have a look at my animations of atoms to see how they are
composed of protons, neutrons and electrons. Group
I metals have a single outer electron which they lose to form cations
with a single positive charge. Group
II metals have two outer electrons and form ions with a double
positive charge. The halogens
have seven outer electrons, Group VII, and can gain one electron to form
anions with a single negative charge.
Crystals of electrovalent compounds consist of a lattice of ions.
Sodium chloride (ordinary table salt) consists of sodium and chloride
ions. Each sodium ion is surrounded by six chloride ions, and each
chloride ion is surrounded by six sodium ions. Have a think about that:
you should realise that a crystal will contain exactly the same number of
each kind of ion.
Covalent bonds are formed when atoms share
electrons. These substances do not conduct electricity and do not
dissociate into ions if they dissolve in water. The simplest covalent
molecule is that formed between two atoms of hydrogen. A molecule of
hydrogen consists of two atoms bonded together by a pair of electrons.
This pair of electrons orbits the nuclei of both atoms so holding them
together. This is the most common way in which non-metallic atoms are
combined into molecules.
Many substances have both kinds of bonds. The most obvious ones are the
acids. Sulphuric acid contains atoms of hydrogen, sulphur and oxygen.
Covalent bonds hold the atoms of oxygen and sulphur together. However you
should know that acids are substances that release hydrogen ions in
solution, so sulphuric acid must have electrovalent bonds. These hold
positively charged hydrogen ions (cations) onto negatively charged
sulphate ions (anions). Again, you can have a look at this on my ions
page to refresh your memory.
In your biology lessons you will learn about
enzymes. Enzymes are biological catalysts. They are affected by
temperature and pH. They must have the correct 3D shape to do their jobs
(the lock and key model), and will not work if their shapes are changed by
pH or temperature. Enzymes are protein molecules. They contain atoms of
carbon, hydrogen, oxygen, nitrogen and sometimes sulphur and even
phosphorus. These are all chemically combined by covalent bonds into a
long chain. However, some of the hydrogen atoms are attracted to atoms of
oxygen or nitrogen by a much weaker kind of bond called a "hydrogen
bond".
Hydrogen bonds are caused by attractions between the positively charged
nucleus of a hydrogen atom and a pair of electrons in the outer shell of
another atom, either oxygen or nitrogen. These are very weak bonds and
easily affected by temperature. Hydrogen bonds help to hold an enzyme into
its proper 3D shape, and this will change if the enzyme is heated or
cooled.
Proteins are also held into their correct shapes by weak electrical
attractions between positive and negative charges on some of the atoms.
Some of the amino acids in a protein contain an extra amino group that can
gain a hydrogen ion becoming positively charged. Other amino acids have an
extra acid group that can lose a hydrogen ion becoming negatively charged.
Attractions between these extra positive and negative regions of a protein
molecule also help to hold it into its correct shape. These attractions
are upset if the pH of the solution is changed.
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