RLO Transcript: Atomic Bonding
This is a transcript of the Reusable Learning Object (RLO) entitled "Atomic Bonding", online at:

http://www.nottingham.ac.uk/nursing/sonet/rlos/science/atomic_bonding/
Molecules
Although there are only about 100 different types of atoms, most things around us, including our bodies are made up of groups of atoms stuck , or bonded  together to form  molecules.

Some examples that you will be familiar with include:

…. the salt molecule made up of sodium and  chloride .  See how the two atoms are written together to denote bonding. 

…….the oxygen molecule.  This is made up of two oxygen atoms bonded together and here a small 2 below the atom indicates that 2 oxygen atoms are present.

…. and carbon dioxide.  – This is made up of one carbon atom and two oxygen atoms.  You can see how this is represented.

Molecules can have very different properties from the atoms they are made from.  Take the example of water.  This is made up of two atoms of hydrogen and one atom of oxygen.  At room temperature oxygen and hydrogen are gases, but when they come together, a liquid is formed.  When you start to think of the various ways different atoms can combine, you begin to understand the diversity of the physical world around and within us.   

Electrons

The ability of an atom to form bonds essentially depends on the arrangement of electrons in the outermost portion of the atom.  As we shall see atoms react together in an attempt to completely fill, or completely empty these outermost shells

Some atoms such as argon already have a complete outer shell and so are inert.  But the atoms of most elements do not and these will react with other atoms to try to remedy this. 

There are two basic ways that this can be achieved known as ionic bonding and covalent bonding.

Ionic bonding

In forming an ionic bond, atoms exchange electrons with one another.  Let's look at sodium chloride again.  The chlorine atom has 7 electrons in its outermost shell, and thus requires 1 more electron to fill it, whereas the sodium atom has only one electron in its outermost shell and can stabilise itself by losing this solitary electron..  So, when atoms of sodium and chlorine come together, the sodium atoms donate an electron to the chlorine atoms. Both now have a complete outer shell.  

The electron that is transferred takes its negative charge with it,  so the resultant chlorine atom now has a net negative charge, whilst the sodium atom that lost it has a positive charge. These charged atoms are called ions.  The charges act as an attractive force that holds the ions together in an ionic bond.  Ionic bonding is important within the salt and mineral components of the body.  Ionic bonds tend to be weaker and the component ions can move apart sometimes, such as when they dissolve in water.

Covalent Bonding

In a covalent bond, electrons are not exchanged, but are shared between the atoms.  Let's look at oxygen again.  As we have seen this molecule consists of two oxygen atoms bonded together. Each oxygen atom has 6 electrons in its outermost shell and requires 2 more electrons to achieve stability.  To achieve this, the two oxygen atoms actually share two pairs of electrons which now orbit both atoms effectively completing their shells.  This is known as covalent bonding.  

Covalent bonding is very important in biology as the molecules formed by them are very tightly held together.  It is the bond found in the water molecule H2O.  It also holds together the carbon backbones of most biological molecules such as fats, sugars and proteins.  You might like to have a try at drawing this bonding for yourself.
Glossary

•	Atom: an electrically-neutral combination of electrons, protons and neutrons, the "smallest unit of an element to retain all the chemical properties of that element" (Wikipedia). 
•	Electron: an almost weightless particle carrying a negative charge. 
•	Electron shells: the orbits in which electrons in an atom reside. Only the electrons in the outermost, or valence, shell are involved in chemical reactions. 
•	Molecule: a coherent particle comprising two or more atoms, "the smallest particle of a pure chemical substance that still retains its chemical composition and properties" (Wikipedia). 
•	Ionic bonding: chemical bonding between two oppositely-charged ions, one negative and one positive.
•	Ion: an atom or group of atoms with a net electrical charge. 
•	Covalent bonding: chemical bonding where electrons in outer shells are shared between two atoms.