Chapter 11 Atomic Theory - all matter is made
of tiny particles called atoms
I. An atom is the smallest particle of an element
which still has the same characteristics.
A. One type of atom makes an element
(all are the same!)
One atom.
Here we have several atoms that are the
same – all of the same element!
(Question: is this a solid, liquid or gas?)
B. Molecule is the smallest part of an
element or compound which contains
more than one element chemically
combined. Some atoms don’t like
“being on their own” so they chemically
bind themselves to an atom of the same
type to make a molecule.
A molecule (since there
is more than one atom) of the same
element.
A single water
molecule – smallest part of a
compound
Lots of water molecules
together.
C. Mixtures can have atoms of elements
and molecules of compounds or both.
Notice that there
is no repeating pattern. This would
indicate that the substance is a mixture.
Review
CHECK YOUR
THINKING!
1. Which picture shows atoms of the same element?
2. Which picture shows molecules and atoms of the same
element?
3. Which picture shows molecules of the same compound?
4. Which picture shows a mixture of atoms of different
elements and molecules of one compound?
Composition of an atom
A.Nucleus – center of an atom – small, dense, positively
charged, contains most of the mass
1. proton – positively charged particles in the nucleus of
an atom, weighs
2. neutrons are particles in the atom that have no charge)
3. atomic mass unit (amu) equals mass of protons &
neutrons (usually 1 amu) each
a. actual mass on periodic table shows the average of
masses of all
naturally occurring atoms
B. Electron clouds – where electrons travel
1. electrons are negatively charged particles
with very little mass
2. Valence electrons are those electrons located
in the very last electron cloud. They react by
either combining with or repelling other valence
electrons from other atoms during a chemical
reaction.
C. Number of atomic particles
1. Atomic number - Each type of atom has a
different number of protons. The
number of protons determines its atomic
number.
The atomic number for Fluorine is 9 which
means that Fluorine has 9 protons.
Quick check!
What is the atomic number of Neon? How
many protons does it have? How many positive
charges does it have?
2. Usually number of protons = number of
electrons. If not the atom will have a positive or
negative charge.
Since fluorine has an atomic number of 9, it has
9 protons. How many electrons should it have?
How many positive charges does fluorine
usually have? How many negative charges does
fluorine usually have?
3. Atomic mass – protons (or atomic number) =
number of neutrons
1. What is the atomic mass of fluorine? Neon?
2. How many neutrons does fluorine have?
Neon?
D. Different types of atoms
1. More or less electrons than usual results in an
atom being positively or negatively charges. This is
called an ion, which is represented by a positive or
negative superscript such as Na+ or Cl-
2. more or less neutron than usual – isotope,
represented by the number of neutrons found in
this changed atom. Carbon 14
a. represents a change in neutrons and a
change in mass
Carbon 14 has 8 neutrons which gives it an atomic
mass of 14 rather than 12 like regular carbon atoms.
E. Forces in atoms
1. gravity – acts between all objects all of the
time. The amount of gravity depends on its
mass and distance between them. Gravity pulls
objects together. ALL MATTER HAS MASS,
THEREFORE ALL MATTER HAS GRAVITY
NO MATTER HOW SMALL.
2. electromagnetic force – some are the same
and repel each other, different forces attract
each other due to electromagnetic force.
3. strong force – holds the positive charges of
the protons together in an atom
4. weak force – associated with radioactivity
(you’ll study that in high school)
Review
Protons
Equals atomic
number
Positive Charge
Weighs 1 amu
Neutrons
Equals atomic
mass minus
atomic number
No Charge
Weighs 1 amu
Protons +
neutrons =
atomic mass
Located in
nucleus
Protons +
neutrons =
atomic mass
Located in
nucleus
Determines the
identity of the
element
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Electrons
Equals atomic
number
Negative charge
Relatively no
weight
Same as atomic
number
Located in
electron shells,
electrons in last
shell called
valence electrons
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Quick check.
1. What types of matter do not have gravity?
2. What do atoms need “strong forces”?
3. What do atoms need electromagnetic forces?