CHEMISTRY · Stage 7 · Chapter 2: Elements, Compounds and Mixtures

Chemistry 2.12 What are you made of? — Elements in the Human Body and Earth

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Lesson summary

2.12 What are you made of?

Section 1 of 7Elements in the human body

Composition by mass
- oxygen is about 65 per cent, mostly in water, followed by carbon (18 per cent), hydrogen (10 per cent) and nitrogen (3 per cent).
- These four make up about 96 per cent of body mass.
Other elements
- calcium and phosphorus are in bones and teeth.
- Smaller amounts of sodium, potassium, chlorine, magnesium, sulfur and iron are important.
- Iron carries oxygen in blood, and sodium and potassium help nerves work.
Trace elements
- zinc and iodine are needed in tiny amounts, but a deficiency causes illness.
Example
- a person of 60 kg with 65 per cent oxygen by mass contains 0.65 × 60 = 39 kg of oxygen.

Not mixed up
- the elements are joined in hundreds of different compounds, and are rarely present as pure elements.
Tissues and organs
- skin, bones, brain and heart are compounds made mainly of carbon, hydrogen, oxygen and nitrogen.
Blood
- it is a mixture of compounds, including water, a compound of hydrogen and oxygen.
Keratin
- nails and hair are mainly this compound of carbon, hydrogen, oxygen and nitrogen, and sulfur atoms make it hard and rigid.
Vitamins
- they are needed in small amounts and are compounds made mainly of carbon, hydrogen and oxygen.

Oxygen
- the air you breathe contains 21% oxygen mixed with other gases, and your lungs separate the oxygen.
Water
- it comes from food and drink.
Proteins, fats and carbohydrates
- these compounds are in your food.
Minerals
- you need small amounts of iron and calcium, but swallowing iron nails or lumps of calcium metal would be useless and dangerous.
- You need compounds that contain these elements, called minerals.

Mineral	Symptoms of deficiency
Iron	Tiredness, lack of energy, shortness of breath
Calcium	Weak bones and frequent fractures
Zinc	Reduced growth in children, problems with senses and memory
Iodine	Swelling of thyroid gland in neck, tiredness, brain damage

Iodine in Tanzania
- in the 1980s about 25% of people had iodine deficiency disorders, so the government told salt makers to add iodine to all salt.
- In 2004 scientists tested salt from 156 000 households and found iodine added to more than 80% of samples.
- They tested 166 000 children, and deficiency fell from 25% to 7%.
- In most areas, the more people ate iodised salt, the fewer had iodine deficiency, so they advised that iodine be added to all salt.

Crust
- it is mostly oxygen and silicon (28 per cent), followed by aluminium, iron, calcium, sodium, potassium and magnesium.
  - (46 per cent by mass)
- They are combined in compounds such as silicon dioxide (sand and quartz) and silicates.
Atmosphere
- it is mostly nitrogen (78 per cent) and oxygen (21 per cent), with argon and a little carbon dioxide.
Core
- it is mostly iron and nickel.

Atoms are not created or destroyed in chemical reactions,
- so the elements in your body have cycled through rocks, water, air, plants and animals for millions of years.
Carbon
- it moves from carbon dioxide in air into plants by photosynthesis, then into animals that eat plants, and back to air in respiration.
Origin
- heavier elements were made inside stars that exploded long ago.

Ores
- most metals are obtained from rocks that contain metal compounds.
Methods
- they depend on reactivity, since very reactive metals like aluminium need electrolysis, whereas iron can be extracted by heating with carbon.
Recycling
- resources are limited, which is why recycling is important.

Last updated: 13 October 2026