Atoms and Molecules

Published on 10th August 2018

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The chemical organisation of the body 

Levels of organisation

Living things consist of 6 levels of structural organisation which relate to one another in various ways.

The 6 levels are:
Chemical Level
Cellular Level
Tissue Level
Organ Level
System Level
Organismic Level

The Chemical level

The chemical level is the most basic level and is made up of atoms and molecules which are essential in maintaining life. 


• Atoms are the simplest units of all matter with unique chemical properties. 
• Matter is everything around us, it is anything that takes up space and has weight. 

The Structure of an Atom:

Atoms consist of electrons surrounding a nucleus that contains protons and neutrons. Neutrons are neutral, but protons and electrons are electrically charged. Protons have a positive charge (+1), while electrons have a negative charge (-1). 

Both protons and electrons have the same size of electrical charge, a proton is positive, an electron negative and as the neutron is neutral it means overall atoms have no electrical charge. The protons and electrons balance themselves out. 

Particle     Charge 
Proton      + 1 
Neutron    0 
Electron    -1

There are over 100 different types of atoms and each type is called an element. Elements form the basic chemicals found in the Periodic Table.  Remember that an element is a single type of atom like carbon or helium. 

Some element atoms stay as single atoms and don’t join up with each other. But when atoms of the same element DO join together we get a molecule of that element. 

Oxygen is like this. Two oxygen atoms join together to make an Oxygen molecule. Most of the oxygen in the air is in this form. Hydrogen also has molecules with two atoms. Some elements have molecules with more than two atoms, sulphur atoms for example can make molecules of eight atoms joined together.

Elements essential for maintaining life are: 


Elements % of amount found in the body     Significance 
Oxygen (O)    65   Component of water and organic molecules and is essential for cellular respiration producing ATP. 
Carbon (C)  18.5  Main component of all organic molecules (carbohydrates, lipids, proteins and nucleic acids) 
Hydrogen (H) 
9.5  Component of water, all foods and most organic molecules.  It also influences pH of body fluids. 
Nitrogen (N) 
3.2  Component of all proteins and nucleic acids. 
Calcium (Ca) 

1.5  Found in bones and teeth and is also necessary for muscle contraction, nerve transmission, release of hormones and the clotting of blood.     
Potassium (K) 

0.35  Required for several chemical reactions within the cell.  Important for nerve impulse and muscle contraction. 
Sodium (Na) 

0.2  Required for many chemical reactions in the extracellular fluids.  Plays a role in water balance, nerve impulses and muscle contraction. 



As we mentioned above atoms and molecules are electrically neutral this is because the number of negatively charged electrons is exactly equal to the number of positively charged protons.  
A lot of the "normal matter" that we find around us is in this form. However, particularly when there are energy sources available (heat, electricity, radiation), atoms or molecules can gain or lose electrons and acquire an electrical charge. This process is called ionisation. 
Ionisation is when an atom or molecule gains a positive or negative charge. They have been ionised because they have either gained or lost an electron.

This can occur in one of two ways:

• When electrons are either gained or lost: - Loss of electrons converts an atom into a positively charged ion,  - Gain of electrons converts an atom into a negatively charged ion. - (an ion is an atom with a charge either positive or negative) 

• When one atom or molecule combines with another atom or molecule that already has a charge. 


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