Anything that takes up space is considered to be matter. Matter is most commonly found
in three states. Obviously, you can tell the differences between solids, liquids and
gases. One can easily observe that solids have a fixed volume and shape, liquids have a
fixed volume but not a fixed shape, and gases don't have fixed volumes or shapes. This is
all common sense.
Now, matter exists as a homogeneous mixture or a heterogeneous
mixture. A homogenous mixture is a mixture that cannot be identified as two different
things with the naked eye. Heterogeneous mixtures can be easily told apart (for instance,
a potato chip in a bag of pretzels).
Among mixtures, there are pure substances and not-so-pure
substances (the latter isn't a real term, I just made it up). Pure substance is
made of one thing. An example would be taking water, running it through a filter, and
getting purified water. This is a pure substance. However, if you go down to your local
creek, the water in the creek also contains dirt, various little insects, and occasionally
someone's feces. The creek water is not-so-pure substance.
Pure substances contain compounds, which are
substances with consant composition and can be broken down to their basic elements (H2O
into H2 and O2).
Elements make up compounds, and are considered the basic building
blocks of matter. You cannot break down elements into smaller parts, but you can classify
the different parts within the element. Within the element are protons, neutrons,
and electrons. The neutron, a chargeless particle, can be found in the
nucleus along with the proton, which is a particle only slightly smaller than the neutron
but positively charged. Electrons are negatively charged and are found circling the
nucleus (much like the sun and the planets in our solar system).
Within the atom, one can find a nucleus. The nucleus, which does not
move around like electrons, contains both neutrons and protons. Both neutrons and protons
have mass, and these two contribute almost 100% of the atomic mass of an element.
Electrons, on the other hand, have almost no mass. Most calculations assume a mass of zero
for electrons. Electrons, being so small, can move around very quickly around the nucleus.
Protons and neutrons can also be broken down into quarks, but you won't learn about those
petit particles in this chemistry year.
A common demonstration to show how little space the electrons and nucleus take up in
the atom is with the football field analogy. If a football stadium was considered to be an
atom, a feather on the 50 yard line would be the nucleus. That is how much empty space
there is within an atom!
As mentioned before, an element's mass is determined by its protons and neutrons. The
electrons, having almost no mass, do no contribute to the mass of an element. An element
is defined by the number of protons and electrons it has. The number of nuetrons an
element has, however, can vary from atom to atom. Each possible atom is known as an isotope.
(ex. Carbon-14) If you go look at a periodic table, you will notice that each element has
a given mass. This mass is an average of its isotopes. This does NOT mean that all carbons
will have 6 protons and 6 neutrons (don't worry about these calculations, they'll make
sense in the next chapter). Just remember that an element can have different number of
neutrons, which will change its weight.