Introduction to Atoms:
Structure of an Atom
 Atoms are the building blocks of matter and
are made of even smaller particles called
protons, neutrons, and electrons.
Particles in an Atom:
 An atom consists of a nucleus surrounded by
one or more electrons.
 The nucleus is the very small center core of an
atom. The nucleus is a group of smaller
particles called protons and neutrons.
 Protons have a positive electric charge
(indicated by a plus symbol, p+).
 Neutrons have no charge. They are neutral. (n)
 The third type of particle in an atom moves in
the space outside the nucleus. Electrons move
rapidly around the nucleus and have a negative
electric charge. An electron is shown by the
symbol e-.
 In an atom, the number of protons equals the number
of electrons. As a result, the positive charge from the
protons equals the negative charge from the electrons.
 Example: Boron has 5p+ and 5e- = no charge
A Cloud of Electrons:
 Electrons move around the nucleus, which
contains the protons and neutrons. This area
is called an electron cloud because electrons
may move anywhere within it.
Comparing Particle Masses:
 Although electrons occupy most of an atom’s
volume, they don’t account for much of its
mass.
 A proton and a neutron are about equal in
mass. Together, the protons and neutrons make
up nearly all the mass of an atom, this is called
the Atomic Mass = p + n. (Mass Number)
 Atoms are too small to be measured in
everyday units of mass, such as grams or
kilograms. Instead, scientists use units known
as atomic mass units (amu).
 A proton or a neutron has a mass equal to
about one amu.
 The mass of an electron is about 1/2,000 amu.
Atoms and Elements
 An element can be identified by the number
of protons in the nucleus of its atoms.
Atomic Number:
 Every atom of an element has the same
number of protons.
 For example, the nucleus of every carbon atom
contains 6 protons. Every oxygen atom has 8
protons.
 Each element has a unique atomic number—
the number of protons in its nucleus.

6

C
Carbon
12.011
 Carbon’s atomic number is 6, telling us the
number of protons.
 The number of protons is also equal to the
number of electrons, so it will have 6 electrons
Mass Number (Atomic Mass):
 The mass number is the sum of the protons
and neutrons in the nucleus of an atom. (p + n)
 Carbon’s mass number is 12.011 amu
 *Atomic Mass is usually a larger, decimal
number.



6
C
Carbon
12.011
Review Questions:
1. What are the 3 main particles in an atom?
- proton, neutron, electron
2. How do the particles in an atom differ in
electrical charge?
- proton = positive charge +
- neutron = neutral (no) charge
- electron = negative charge 3. Why do atoms have no electrical charge even
though most of their particles have charges?
- Each atom has equal numbers of positively
charge protons and negatively charged
electrons.
Carbon has +6p and –6e = 0, no charge
4. What is the atomic number of an element?
- The number of protons in each atom of that
element.
5. How can atomic numbers be used to tell one
element from another?
- Each element has a unique atomic number
because it tells its number of protons. Adding or
taking away protons changes the element.