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.
Periodic Table
