If chemistry is a study of how objects in the material world change their identity – how one kind of thing turns into another - it will be helpful to establish at the outset what we mean by "one kind of thing". Chemists call one kind of thing a pure substance. (A pure substance is a sample of matter with properties like melting point and density that does not vary when subjected to physical processes in tended to purify the substance.)
An example of a pure substance would be a metal like sodium. It is an element. It has a unique set of properties and we call it one kind of thing. If we took a sample and broke it in half each piece would have the same identity – would still be sodium. If we keep repeating this cutting process the same would hold - no matter how small the pieces, we would still have sodium. However, conceptually there is a lower limit to this process for two reasons. First, sodium, like other elements, is made of a unique kind of atom. When we break a sample of pure sodium we are separating groups of atoms from each other. Each group has the same properties. However, once we have broken apart every group, we are left with separated atoms. These can be further broken down, but doing so would destroy the basic building block of the sodium and it would no longer be classified as the same substance – as the element sodium. Second, when we break things down too small, we cannot reasonably talk about properties like melting or boiling – properties which require groups of atoms.
Chlorine is another pure substance that is an element. Like sodium it has a unique set of properties and is made of a unique kind of atom.
Sodium and chlorine can be mixed in all possible ratios. The mixture that results is not a new substance or kind of thing. The properties of sodium and chlorine are still present in the mixture. However, under the right conditions, sodium atoms and chlorine atoms can combine. If this takes place, the result is a new kind of thing – a new substance. We call it a substance (not a mixture) since it has its own unique properties – not those of either sodium or chlorine – although it is made of these alone. The new way that the atoms group together as sodium chloride, give the sample a unique set of properties – so we classify it as a pure substance – but a compound rather than an element, since it is made up of more than one element.
From the point of view of chemistry, all of matter can be classified as either a substance ( an element or a compound), or a mixture of substances.
Example 1: Hydrogen
Below is a picture of hydrogen gas at the atomic level.
Note that it is made up of only one type of atom, so we call hydrogen an element. It cannot be broken down into two or more simpler substances.
You might find it confusing that the atoms are not separate from each other but attached to each other in pairs. Each pair makes up a molecule of hydrogen. Although the pairs could be split apart, if we tried to break down the sample, we would find that we could not break it down into simpler substances. If we broke the hydrogen molecules apart into atoms we would find that the atoms are far more reactive than the molecules, but it would not result in two or more simpler substances.
Of all the elements, hydrogen is the simplest because it has the simplest arrangement of electrons and protons which we will study later. It is the most abundant element in the universe.
Example 2: Helium
Below is a picture of helium gas at the atomic level. It does not display the more confusing pattern of existing as pairs of atoms as a pure element. Unlike atoms of hydrogen, atoms of helium are stable in isolation. In terms of electron and proton structure it is more complex than hydrogen. Helium is the second most abundant element in the universe.
There are about 100 different kinds of atoms, so 100 or so different kinds of elements. Here is a link to a periodic table showing then elements in increasing order of complexity – with each atom type containing an extra proton and electron.
As you know from your summer reading there are many interesting facts to learn about each element. Here are just two.
90% of the mass of the human body is made up of just three elements: oxygen (65%), carbon (18%), and hydrogen (10%) – bound up in a large variety of molecules, including the most abundant – water.
The Earth's crust, where life evolved, does not have the same make up of elements. It is made of oxygen (49.5%), silicon (25.7%), aluminum (7.5%), iron (4.7%) and calcium (3.4%).
Example 1: Water
When different types of atoms combine in unique ways (we will say they form a chemical bond), they make molecules which are the smallest units of new substances. For example, when hydrogen atoms and oxygen atoms combine they can make water molecules. The properties of water do not resemble those of hydrogen or oxygen. Once incorporated in the water molecules, there are no groups of hydrogen or oxygen molecules to display the properties of the pure elements hydrogen or oxygen.
Water as a compound is made up of more that one type of atom chemically combined. It can be broken down into two or more simpler substances, hydrogen and oxygen.
Example 2: Methane
When hydrogen atoms and carbon atoms combine they can make methane molecules. The properties of methane do not resemble those of hydrogen or carbon.
Methane as a compound is made up of more that one type of atom chemically combined. It can be broken down into two or more simpler substances, hydrogen and carbon.
Mixtures are combinations of substances – some combination of different elements and/or compounds – but unlike compounds, they retain the properties of the substances that make them up. The atoms from the substances that make them up do not form unique partnerships, but are simply mixed together.
Example 1: A rock
The rock has regions that differ from each other in color and texture and as such are made up of different materials. The random arrangement of materials makes it clear that the mixture is not uniform.
Example 2: Air
Air is a mixture of primarily two elements – nitrogen and oxygen.
In any sample of air other substances are present as well. For example, argon, water vapor and carbon dioxide. So besides the elements of nitrogen and oxygen that make up most of the air, there are also elements like argon (made of independent atoms rather than molecules) and compounds like water and carbon dioxide.
The examples above bring up a last point. When elements and compounds are mixed, they may be like the rock with clusters of one type of substances randomly separated from clusters of other types, or the mixed substances might be uniformly distributed as in the case of air. (Note that the drawing of air shown contains too few molecules to show the uniform distribution.) When a mixture has uniformly distributed parts it is called a homogeneous mixture or solution. When a mixture does not have uniformly distributed parts it is called a heterogeneous mixture.