Chemistry: Atoms
Atoms and the
Periodic Table
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Early Ideas About Matter- The ancient Greeks tried to explain matter, but the scientific
study of the atom began with John Dalton in the early 1800’s.
Defining the Atom-An atom is made of a nucleus containing
protons and neutrons; electrons move around the nucleus
of an atom.
An atom is the smallest quantity of matter that still retains the
properties of matter.
An element is a substance that cannot be broken down into two or
more simpler substances by any means.
 Examples: gold, oxygen, helium
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Particles in the Atom
Electrons
(-) charge
no mass
located outside the nucleus
Protons
(+) charge
1 amu
located inside the nucleus
1 amu
located inside the nucleus
Neutrons
no charge
Properties of Subatomic Particles
Electrons
The particles that Thomson detected were
later named electrons.
An electron is a negatively charged
subatomic particle that is found in the space
outside the nucleus. Each electron has a
charge of 1.
Properties of Subatomic Particles
Protons
Based on experiments with elements other
than gold, Rutherford concluded that the
amount of positive charge varies among
elements.
A proton is a positively charged subatomic
particle that is found in the nucleus of an
atom. Each proton is assigned a charge of
1+. Each nucleus must contain at least one
proton.
Properties of Subatomic Particles
Neutrons
In 1932, the English physicist James
Chadwick carried out an experiment to show
that neutrons exist. Chadwick concluded that
the particles he produced were neutral
because a charged object did not deflect their
paths.
A neutron is a neutral subatomic particle that
is found in the nucleus of an atom. It has a
mass almost exactly equal to that of a proton.
Comparing Subatomic Particles
Everything scientists know about subatomic
particles is based on how the particles behave in
experiments. Scientists still do not have an
instrument that can show the inside of an atom.
Comparing Subatomic Particles
Here are some similarities and differences between
protons, electrons, and neutrons.
• Protons and neutrons have almost the same mass.
About 2000 electrons equal the mass of one proton.
• An electron has a charge that is equal in size to, but
the opposite of, the charge of a proton. Neutrons have
no charge.
• Protons and neutrons are found in the nucleus.
Electrons are found in the space outside the nucleus.
How Atoms Differ- The number of protons and the mass number
define the type of atom.
Atomic Number
• The atomic number of an element is the
number of protons in an atom of that
element.
• Atomic number = number of protons =
number of electrons.
• Mass number = atomic number + number
of neutrons.
• All atoms of any given element have the
same atomic number. Each hydrogen atom
has one proton in its nucleus. Hydrogen is
assigned the atomic number 1.
• Each element has a unique atomic number.
Atomic Number and Mass Number
Each element has a different atomic number.
A The atomic number of sulfur (S) is 16.
B The atomic number of iron (Fe) is 26.
C The atomic number of silver (Ag) is 47.
Atomic Number and Mass Number
Atoms are neutral, so each positive charge in
an atom is balanced by a negative charge.
That means the atomic number of an element
also equals the number of electrons in an
atom of that element.
• Hydrogen has an atomic number of 1, so a
hydrogen atom has 1 electron.
• Sulfur has an atomic number of 16, so a sulfur
atom has 16 electrons.
Atomic Number and Mass Number
Mass Number
The mass number of an atom is the sum of
the protons and neutrons in the nucleus of
that atom. To find the number of neutrons in
an atom, you need the mass number of the
atom and its atomic number.
The atomic number of aluminum is 13. An
atom of aluminum that has a mass number of
27 has 13 protons and 14 neutrons
Structure of the Atom
There are two regions
The nucleus
• With protons and neutrons
– Positive charge
– Almost all the mass
Electron cloud
– Most of the volume of an atom
– The region where the electron can be found
Subatomic Particles
ATOM
ATOM
NUCLEUS
NUCLEUS
PROTONS
PROTONS
ELECTRONS
ELECTRONS
NEUTRONS
NEUTRONS
NEGATIVE
CHARGE
Negative
Charge
POSITIVE
Positive
CHARGE
Charge
NEUTRAL
Neutral
CHARGE
Charge
equal in a
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QUARKS
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
Subatomic Particles
• Proton - positive electrical charge (1)
and mass of 1 amu
• Neutron - no electrical charge and mass
of 1 amu
• Electron - negative electrical charge (-1)
and negligible mass
Crude Atom Model
Atomic Structure (Refined)
• The electrons move continuously
through differently shaped clouds of
space (electron energy levels) that lie at
specific distances from the nucleus (see
figures)
Figure 3.5A
What are atoms made of?
Protons: positively charged particles that are housed in the nucleus
of an atom and have significant mass
Neutrons: neutral particles that are housed in the nucleus. They act to
hold the protons in place since like charges repel each
other. Neutrons have significant mass
Electrons have negligible mass, have a negative charge and are allowed
to roam freely in the electron cloud so they take up significant
volume in the atom
Taylor 2010
Atomic Number, and Mass Number
All atoms can be identified by the number of protons and neutrons
they contain.
The atomic number (Z) is the number of protons in the nucleus.
 Atoms are neutral, so it’s also the number of electrons.
 Protons determine the identity of an element. For example,
nitrogen’s atomic number is 7, so every nitrogen has 7 protons.
The mass number (A) is the total number of protons and neutrons.
 Protons and neutrons are collectively referred to as nucleons.
Mass number
(number of protons + neutrons)
Atomic number
(number of protons)
A
Z
X
Elemental symbol
I can identify an atom and the number of its subatomic particles using a periodic
table.
Complete the chart below. Use your periodic table and the following reminders:
Atomic mass = Number of protons and neutrons in a nucleus.
Protons = Atomic number
Neutrons = Atomic mass – atomic number
Electrons = Atomic number