THE ATOM
1
NUCLEAR MODEL OF THE ATOM
• An atom is an electrically
neutral particle
• Composed of protons,
neutrons, and electrons.
• Atoms are spherical in
shape
• Have a tiny, dense
nucleus of positive
charge surrounded by
one or more negatively
charged electrons
• Nucleus contains 99.7 %
of the mass of an atom.
2
BASIC STRUCTURE OF THE ATOM
3
SUBATOMIC PARTICLES
4
SUBATOMIC PARTICLES
• Most of the atom consists of fast-moving
electrons traveling through the space
around the nucleus.
• Atoms are neutral in charge
• The number of protons = the number of
electrons
• Atomic Number:
• The number of Protons in the nucleus
• determines the identity of an atom
• No two types of atoms have the same number of protons
• An Element consists of atoms with the same
number of protons.
5
ATOMIC NUMBER(Z): THE NUMBER OF PROTONS IN
THE NUCLEUS OF EACH ATOM OF THE ELEMENT
• Atomic Number = Protons
• Elements are arranged in the periodic table from
left to right in order of increasing atomic number.
• Atomic number identifies the element
6
ISOTOPES AND MASS NUMBER
• Atoms can have different mass.
• Naturally occurring elements are a mixture of atoms
that have different numbers of neutrons.
• Atoms with the same number of protons, but
different number of neutrons are called Isotopes.
7
HYDROGEN HAS THREE ISOTOPES:
• Protium
• Deuterium
• Tritium
1 proton
1 proton
1 proton
1 electron
1 electron
1 electron
0 neutrons
1 neutron
2 neutrons
8
ISOTOPES AND MASS
NUMBER
• Isotopes: atoms of the same element that
have different masses.
• Same number of protons
• Different number of neutrons
• Different Mass
• Atoms of different isotopes have different
masses so identity is given by name and mass.
• Mass Number = protons and neutrons
9
ISOTOPE (NUCLEAR) SYMBOLS:
Consists of three parts
1. the symbol of the element
2. the atomic number of the element
3. the mass number of the specific isotope.
10
NUCLEAR SYMBOL
• Read as Helium-4
11
ISOTOPE NAMES
Element Name-Mass Number
Helium-4
Potassium-39
Hydrogen-3
12
NEUTRONS = MASS NUMBER – ATOMIC NUMBER
Atomic Number: _________
Mass Number: __________
# of Protons: _________
# of Electrons: _______
# of Neutrons: ________
Name of Isotope: ____________
13
EXAMPLE
Atomic Number: _________
Mass Number: __________
# of Protons: _________
# of Electrons: _______
# of Neutrons: ________
Name of Isotope: ____________
14
EXAMPLE
Atomic Number: _________
Mass Number: __________
# of Protons: _________
# of Electrons: _______
# of Neutrons: ________
Isotope Name: _________
15
DRAW THE ISOTOPE SYMBOL FOR
CALCIUM WITH 21 NEUTRONS.
16
COMPARING POTASSIUM ISOTOPES
Potassium-41
• protons = ________
• electrons = ______
• neutrons = _______
Potassium-40
• protons = ________
• electrons = ______
• neutrons = _______
Potassium-39
• Protons = __________
• Electrons = _________
• Neutrons = __________
17
RELATIVE ATOMIC MASSES
• Atomic masses measured in grams
are very small.
• Example: An atom of Oxygen-16
has a mass of 2.657 x 19-23grams.
• It is most convenient to measure
mass in relative atomic masses.
18
TO SET UP A RELATIVE SCALE OF
ATOMIC MASSES:
1. One atom is chosen and assigned a
relative mass value,
2. All the other masses are expressed in
relation to this defined standard.
•
•
Carbon-12 is the chosen standard.
A single atom of C-12 is assigned a mass of
exactly 12 atomic mass units(u).
19
AN AMU
• One amu is exactly 1/12th of the mass of a carbon12 atom or 1.660 5402 x 10-24grams.
• The atomic mass of a carbon-12 atom is exactly 12
u.
• Atomic Mass: The mass of an atom expressed in
atomic mass units.
20
AVERAGE ATOMIC MASS
• Atomic Masses given for the elements on
the periodic table are weighted averages
for the naturally occurring mixtures of
isotopes, called average atomic mass.
• Average Atomic Mass depends on mass
and relative abundance of the isotopes.
21
TO CALCULATE AVERAGE ATOMIC MASS:
1.
2.
Atomic mass x relative abundance
Add results.
22
EXAMPLE PROBLEM:
Naturally occurring copper consists of 69.17% Cu-63,
mass of 62.939 598u and 30.83% Cu-65, mass of
64.927 793u
• 0.6917 x 62.939 598u =
43.535u
• 0.3083 x 64.927 793u = 20.017
23