The Building Blocks of Matter
All matter, from a rock to an animal to the magma at the
center of the Earth, is made from different combinations of 92 naturally
occurring substances known as elements. The smallest
quantity of an element that still exhibits the characteristics of
that element is known as an atom. One atom of carbon,
for example, is the smallest piece of matter that still retains
the chemical and physical characteristics of carbon.
Atoms are made up of even smaller particles called electrons, protons,
and neutrons. Each of these particles has a different
electrical charge. Protons are positively charged, neutrons have
no charge, and electrons are negatively charged. The protons and
neutrons of an atom reside in a central body called a nucleus. Electrons
appear around the nucleus within orbitals of varying energy. Overall,
the atom is neutrally charged with equal numbers of positively charged
protons and negatively charged electrons.
Elements are distinguished by the number of protons in
their nuclei. All atoms containing six protons are called carbon.
Any element with one proton is called hydrogen. Only the number
of protons—and not the number of neutrons or electrons—distinguishes
elements from each other.
Isotopes and Ions
Though the number of neutrons and electrons in an atom
won’t change the atom’s status as a particular element, it can affect
the properties of an element in subtle ways. An atom that contains
a larger or smaller number of neutrons than usual is called an isotope.
Carbon usually has six protons and six neutrons and can be called
carbon-12 because the number of its protons and neutrons add up
to 12. But some carbon atoms have seven or even eight neutrons.
These two isotopes are called carbon-13 and carbon-14. Isotopes
do not have charge, because the numbers of positive and negative
particles remain balanced. Even though they have different masses,
isotopes of the same element all have similar chemical properties,
because the number of electrons (not the number of neutrons or protons) determines
the way an atom will interact with other atoms.
Ions are atoms that either lack or have extra electrons.
Because these atoms have unequal numbers of electrons and protons,
they are charged particles and are often quite chemically interactive
with other atoms. Though the SAT II Biology Test rarely asks direct questions
about ions, ions do play an important role in many biological processes
and phenomena, so understanding the basics of ions can help you
understand the processes that the test covers.
Molecules and Compounds
Atoms combine with each other in chemical reactions to
create molecules, unique substances with physical and chemical properties
distinct from those of their constituent elements. Combining two
hydrogen atoms with one oxygen atom creates water, which has very
different characteristics than hydrogen or oxygen do alone. Molecules
such as water containing more than one type of element can also
be called compounds. A water molecule made up of oxygen and hydrogen
can be called a compound; a hydrogen molecule, which contains only
two hydrogen atoms, cannot be called a compound.
You may have heard water referred to as H2O.
This notation is the standard way of representing molecules and
compounds by shorthand. The “H” and “O” stand for the elements hydrogen
and oxygen, and the subscript indicates that water contains two
parts hydrogen for every one part oxygen. You can create the formula
for any compound by writing down the letter symbol of each of its
constituent elements and using subscripted numbers to indicate how
many atoms of each element are present.