Dorothy Hodgkin looked at how atoms fit together into very complicated molecules. She used X-ray crystallography to find out what penicillin and insulin look like. Knowing what the molecules look like has helped other scientists understand how these molecules work, and to make new medicines.

This resource looks at how the same atoms can bond together to create very different molecules.

In theory, a can of diet drink will float, while a can of regular, sugary drink will sink.  The sugar in the drink makes the can and drink more dense than water.  The diet drink floats, as its average density (the metal can, the liquid and the gas that gives it its fizz) is less than water.  There are times when this doesn't work, for instance if there is more gas in the regular drink can!

Curriculum Links


Working scientifically

Experimental skills and investigations

  • ask questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience
  • make predictions using scientific knowledge and understanding

Analysis and evaluation

  • apply mathematical concepts and calculate results
  • present observations and data using appropriate methods, including tables and graphs
  • interpret observations and data, including identifiying patterns and using observations, measurements and data to draw conclusions
  • present reasoned explanations, including explaining data in relation to predictions and hypotheses


Nutrition and digestion

  • content of a healthy human diet: carbohydrates


Atoms, elements and compounds

  • differences between atoms, elements and compounds
  • chemical symbols and formulae for elements and compounds
  • Ideas, political power, industry and empire: Britain, 1745-1901

Scottish Curriculum Links



SCN 2-08a

Forces, electricity and waves - Forces

I have collaborated in investigations to compare magnetic, electrostatic and gravitational forces and have explored their practical applications. [Note: level 2]


SCN 3-08a

Forces, electricity and waves - Forces

I have collaborated in investigations into the effects of gravity on objects and I can predict what might happen to their weight in different situations on Earth and in space.


SCN 3-15b

Materials - Properties and uses of substances

Having contributed to a variety of practical activities to make and break down compounds, I can describe examples of how the properties of compounds are different from their constituent elements.


SCN 4-15a

Materials - Properties and uses of substances

Through gaining an understanding of the structure of atoms and how they join, I can begin to connect the properties of substances with their possible structures.


TCH 4-10b

Food and textiles contexts for developing technological skills and knowledge


Dalton moleculesScientists in the 1700s began to understand that everything around us is made of tiny particles called atoms.  By the end of the century Antoine Lavoisier had discovered that water was made of hydrogen and oxygen and that air was a mixture of gases: he did this by carefully weighing different elements and compounds (where more than one element is bonded together).

Another scientist, John Dalton, used models to show how some elements bonded together to make compounds: have a look at his drawings in the 1826 trailblazing entry(  By the 1900s, chemists were able to describe what many simple compounds, or molecules, looked like.

Larger molecules were a different matter.  It was harder to work out their bonds, and two different molecules could even have the same number and type of elements in them.


Key fact: Molecules are made up of different atoms - or elements - bonded together.




Surely, if you know what elements are in something, you know all about it?

Sugar cubesA lot of the sugar we eat is made up of three different types of sugar: glucose; fructose; and galactose.  Fructose is the sweetest, while galactose is not very sweet at all.  When we eat these different sugars, our body treats them differently.  However, if we just look at the elements which make them up, they each have six carbon atoms, six oxygen atoms and twelve hydrogen atoms.

You can see the range of sugars found in foods by looking at the ingredients list on the back of the packet.

Have a look at some food packaging yourself.  You may want to choose food like chocolate biscuits and breakfast cereals.  You will probably find sucrose, common sugar (made of fructose and glucose bonded together), as well as lactose or maltose.

Types of sugar:

Sucrose - a glucose and fructose molecule bonded together
Maltose - two glucose molecules bonded together
Lactose - a galactose and glucose molecule bonded together
Honey - 20% water, 40% fructose, 30% glucose, 7% maltose, 1% sucrose.
Dextrose - this is a form of glucose
Maltodextrin - a chain of glucose molecules bonded together
Invert sugar syrup - a mixture (not bonded) of glucose and fructose

Key fact: Molecules with the same number and types of elements can have different properties.

Artificial sweeteners

IngredientsYou can test food to find out how much sugar each contains by using Benedict's Test.  However, two foods with the same amount of sugar in them will not necessarily taste equally sweet, and there is no test for sweetness.  Instead, many people have to taste a substance and scientists compare their perception of how sweet it is with something of known sweetness (like a gram of sugar).  Artificial sweeteners are hundreds or thousands of times more sweet than glucose.


Use the information below to help you find some artificial sweeteners in the ingredients lists on your food packets.  How do they compare to sugar in terms of sweetness? 

Acesulfame K - 200 times sweeter than sucrose
Aspartame - 200 times sweeter than sucrose
Neotame - this is a new sweetener and is 8000 times sweeter than sucrose
Saccharin - about 400 times sweeter than sucrose - the oldest artificial sweetener discovered 100 years ago.
Sucralose - made from sucrose, but it is 600 times sweeter than sucrose



Diet drinks - the lighter option

Take a closed can of soft drink and a diet version of the same soft drink, and place them both in a basin of water.  Does one of the cans float while the other one sinks?

Can you work out what is happening here?

You will need to think about the density of the each of the cans.  Take a look at these pictures - there is the same amount of liquid in each of the cans.

Density of drinksYou need much less sweetener than you need sugar, so which can will have a greater mass?

Both cans have exactly the same volume... by using the density equation

Density = Mass / Volume

can you work out which can will have the highest density?

Whether something sinks or floats depends on how its density compares with the density of water.  Something with a lower density than water, such as wood, will float.  Something with a higher density than water will sink.

Did the cans do what you expected?