Every atom has a core called a nucleus, where
the majority (99.9%, to be exact) of an atom's mass is held.
Although the nucleus contains the majority of the mass of the
atom, the nucleus is very small compared to the size of the whole
atom, because most of the atom is empty space surrounding the
nucleus. Within the nucleus are two types of smaller particles
called protons and neutrons. The
third type of particle that makes up the atom, electrons,
orbit around the nucleus.
Let's look at each particle in further detail:
Protons:
Protons are positively charged particles found inside the
nucleus of an atom. Every atom of a particular element contains
the same number of protons. In fact, the number of protons is
unique to each element. Each element has a unique atomic
number, or a unique number of protons in its nucleus.
Proton number never changes for any given element. For example,
oxygen has an atomic number of 8. That tells us that oxygen always
has 8 protons.
Neutrons:
Neutrons are the other particle found in the nucleus of an
atom. Unlike protons and electrons, however, neutrons carry no
electrical charge. Therefore, neutrons are "neutral."
Atoms of a given element do not always contain the same number
of neutrons. Atoms of an element that have a different number
of neutrons in the nucleus are called isotopes
of each other.
Electrons:
Electrons are negatively charged particles that orbit around
the outside of the nucleus. The mass of an electron is about
1/2000th of the mass of a proton or a neutron. The sharing or
exchange of electrons between atoms forms chemical bonds, which
is how new molecules and compounds are formed.
Atomic Number:
An atom's atomic number tells you how many protons are in
that atom's nucleus. For example, oxygen has an atomic number
of 8, meaning that there are 8 protons in the nucleus of an atom
of oxygen. Copper's atomic number is 29, meaning that there are
29 protons in the nucleus of an atom of copper. Later, you'll
see how the periodic table conveniently tells you each element's
atomic number.
Atomic Mass:
Because atoms are so small, their masses cannot be measured
in grams or milligrams. Instead, scientists have created the
atomic mass unit (amu) to measure mass of subatomic particles.
The mass of a proton or a neutron is about 1 amu. The mass of
an electron, however, is about 1/2000 amu. To find the
atomic mass of an atom, add the number of protons and neutrons
in the nucleus.
Example: If an atom has 3 protons, 4 neutrons,
and 3 electrons, the atomic mass is 7 amu, because you do not
count the very small mass of the atom's electrons (1/2000 amu).
Only add the number of protons and neutrons (each has a mass
of 1 amu) in the nucleus.
Question: Find the atomic mass of an atom that
has 10 protons, 8 neutrons, and 10 electrons.
Answer: 18 amu. Just add the protons and neutrons.
If you have any questions or comments, please e-mail D. Gibson, Grade 8 Science Teacher at dld1999@excite.com