Elements & Compounds, Atoms, Molecules, & Mixtures
Elements are pure substances that cannot be separated into simpler substances.
Gold, silver, iron and oxygen are all examples of elements. Compounds are also pure
substances, but they are made up of more than one type of element and so can be
separated into simpler substances.
The smallest particles of matter are called atoms. Think about a bar of gold. If you
cut the gold bar into smaller and smaller pieces, eventually you would reach a point
where you would cut up the bar and have the smallest particle of gold possible, an
atom of gold. The same applies to all elements. If you continually cut up a piece of
aluminum, you would reach the smallest particle that was still aluminum. These are
aluminum atoms. An atom is the smallest particle of an element that has the
properties of that element.
Chemists use symbols to represent elements. A symbol is a letter or picture used
to represent something. Chemists use one or two letters to represent elements.
The symbol for aluminum is Al. The symbol for oxygen is O. The symbol for Iron is
Fe and the symbol for Hydrogen is H. Each element’s symbol can be found on the
Periodic Table of Elements.
Compounds
A compound is a pure substance formed when two or more different elements are
chemically joined. Water, salt, and sugar are examples of compounds. When the
elements are joined, the atoms lose their individual properties and have different
properties from the elements they are composed of. A chemical formula is used a
quick way to show the composition of compounds. Letters, numbers, and symbols
are used to represent elements and the number of elements in each compound. CO2
is an example of a chemical formula. It represents carbon dioxide.
If you were to take a sample of a compound such as salt (NaCl) and cut it
repeatedly (as in the gold example), eventually you would have a molecule of salt –
one Na bonded to one Cl. If you cut again, you would no longer have salt, but would
instead have one atom of sodium and one atom of chlorine.
Molecules
Let’s take a closer look at molecules. A molecule is two or more atoms joined
together chemically. All compounds contain two or more
different elements bonded together, so all compounds are
molecules. But, not all molecules are compounds. For
example, oxygen atoms are often found joined together in
pairs. To write a pair of oxygen atoms using symbols, we use
the symbol O and the number 2. A pair of oxygen atoms
A model of an oxygen molecule
bonded together would be written as O2. The 2 is a
subscript - "sub" means "below." A pair of oxygen atoms is a molecule of oxygen.
Most elements are found with only one of atom of that element, but oxygen, along
with nitrogen, hydrogen, and chlorine can be found in pairs of two atoms. Look at
the model of oxygen. The two balls represent the two oxygen molecules. The
oxygen molecules are bonded or stuck together.
Mixtures
Mixtures are two or more substances that are mixed together but not chemically
joined. A good example of a mixture is a salad. There are tomatoes, lettuce,
cucumbers, and salad dressing all mixed together. No chemical reactions occur
between the vegetables and the dressing. You can separate each of the vegetables
from each other. Mixtures can be made up of different elements, different
compounds, or a combination of the two.