The frozen planet
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# 4 The energy balance

In Section 2 it was noted that temperature differences between the higher and lower latitudes are large, but before continuing, it is necessary to define what is actually meant by temperature. Your experience will be from the sensations of warmth and coldness, but what is temperature from a scientific perspective? The temperature of a material is actually an indication of its energy, and to understand the relationship between temperature and energy, one needs to know the make-up of the material. All substances are made of small constituent parts called atoms (see Box 3). For example, humans contain a large amount of carbon, hydrogen and oxygen atoms, and breathe in air which contains mostly nitrogen atoms.

## Box 3 The chemical elements

Every object on Earth is made of chemical elements. Most elements were created in the nuclear furnaces of exploding stars in the early Universe, so you and the Earth around us are made literally from stardust. There are 109 named elements, of which 92 occur naturally on Earth. Many will be familiar, such as carbon and oxygen. Of the known elements, up to 30 are believed to be essential to the survival of living organisms. Each element can be represented by a chemical symbol of one or two letters, a shorthand form of its current name or, sometimes, its Latin name. The smallest recognisable part of a chemical element is called an atom.

Some common examples of elements are shown in the table. C stands for carbon and Ca for calcium, Fe refers to ferrum, the Latin name for iron, and Na to natrium, which is Latin for sodium. Few elements exist as single atoms and so they are usually found combined together either with other atoms of the same element, or with atoms of other elements to make compounds. There is an enormous range of possible combinations of different elements and therefore an enormous number of chemical compounds. The smallest amount, or building block, of any substance – compound or element – comprising two or more atoms, is called a molecule.

##

Element | Symbol
--- | ---
carbon | C
hydrogen | H
oxygen | O
nitrogen | N
calcium | Ca
phosphorus | P
sulfur | S
sodium | Na
iron | Fe

The symbols of the elements are put together to give the formula of a compound. For example, hydrogen and oxygen combine to make water, which has the formula H2O. The number 2 indicates that two atoms of hydrogen combine with one atom of oxygen to make one molecule of water. The subscript number refers to the element it follows, so for H2O the 2 refers to hydrogen and not oxygen. In contrast, a number before the formula refers to the number of individual molecules. For example, 3H2O indicates three molecules of water, but these three molecules contain six atoms of hydrogen and three atoms of oxygen in total.

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