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Atomic Structure
Chapter 4
Subatomic particles

Electrons-negatively charged,
located outside the nucleus in the
electron cloud

Protons-positively charged, located
in the nucleus

Neutrons-neutral charge, located in
the nucleus
Atomic Number Summary
 The
number of protons in the nucleus
 Also the number of electrons in a
neutral atom
 Located just above the element
symbol on the PT (whole number)
 Elements are listed on the PT
according to their atomic number
increasing from left to right
Mass Number
 Mass
number = protons + neutrons
Atomic Mass units
Atomic Mass Unit
 The atomic weight scale has traditionally
been a relative scale...
 the first atomic weight basis was
suggested by John Dalton in 1803:
 1 amu = mass of 1H (a hydrogen atom)

Number of Neutrons
of neutrons = mass number –
atomic number (protons)
#
Nuclear Symbols
A
X
Z
 A = mass number = sum of protons
and neutrons in nucleus
 Z = atomic number = # of protons
 A – Z = number of neutrons
Examples:
80
Br
A – Z = (80 – 35) = 45 neutrons
35
Z = 35 protons and 35 electrons

Atomic Structure worksheet
 Atom:
electrically neutral, number of
protons is equal to the number of
electrons
 Ion:
charged particle
 If an ion has a + charge, it has lost
electrons
 If an ion has a – charge, it has extra
electrons (has gained electrons)
What is the charge on the Fluorine
Ion?
Remember:
Atoms ONLY GAIN OR LOSE
ELECTRONS,
they do not gain or lose protons
so the charge of an ion comes from either
losing electrons (positively charged ion) or
gaining electrons (negatively charged ion)
**When atoms gain or lose electrons they
become ions!
Example:
40
K+1
19
A – Z = 40 - 19 = 21 neutrons
19 protons (# of protons can not change)
19 electrons – 1 = 18 electrons
Net charge of +1
Example:
32
P-3
15
A – Z = 32 – 15 = 17 neutrons
15 protons
15 electrons + 3 = 18 electrons
Net charge of -3
Isotopes
o Isotopes are atoms with the same
number of protons, but different
numbers of neutrons.
o Since number of protons and
electrons stays the same,
isotopes of an element are
chemically almost identical.
Representing Mass #:
Cobalt-58 has a mass number (A) of
58 and cobalt-60 has a mass
number (A) of 60
 We write:
58
60

Co
27
Co
27
Atomic Mass
 Number
located just below the
chemical symbol on the PT
 Not usually a whole number
 Isotopes of the same element have
different atomic masses (and
therefore have different mass
numbers)
To calculate:


Multiply the mass of each isotope by
its percent abundance
Add the products and divide by 100
to give total atomic mass of the
element
Calculating Average Atomic
Mass:
 Chlorine
has 2 naturally occurring
isotopes, 75.0% Chlorine – 35 and
25.0% Chlorine – 37. What is the
average atomic mass of Chlorine?
Chlorine 35 35 x 75 = 2625
 Chlorine 37 37 x 25 = 925
 Added together
= 3550/100
 Average atomic mass = 35.5 amu


An Unknown element consists of 2
isotopes. The first has a percent
abundance of 20.0% and a mass of 266
and the second an abundance of 80.0%
and a mass of 272. What is the average
atomic mass for this element?
(20.0 *266 + 80.0 *272) 100 =
 (5320 + 21760 )/100= 27080/100
 To 3 sig figs 271 amu

Isotopes WS
Periodic Table Preview
Periodic Table Preview
Arranged in order of atomic number
 Grouped in columns that have similar
properties
 A row is known as a period
 A column is known a s a group or family

Groups
All elements in a group
have similar properties.

Atom - History
The smallest particle of an element
that retains the properties of that
element
Idea dates back to ancient Greece
https://www.youtube.com/watch?v=
AiSC6nxHzCI
The structure of the nuclear atom
JJ Thomson
 credited with discovering the
electron
 Cathode ray tube experiment
Cathode Ray Experiment
 Glowing
beam when electricity flowed
through a gas.
 The “ray” was attracted toward the (+)
plate and was deflected away from
the (-) plate (magnet)

http://www.youtube.com/watch?v=7YHwM
WcxeX8&feature=related
What exactly did Thomson learn?
 Since
a beam of electrons (cathode
rays) is deflected by an electric field
toward the positively charged plate,
electrons MUST have a negative
charge.
 When the metal used in the electrodes
and the gas used in the tube were
changed, the same results were
evident. Thus, all atoms must contain
electrons.
What exactly did Thomson learn?
 Since
matter carries a neutral charge,
he deduced that the structure of the
atom is a positively charged sphere
with negative charges “floating”
around in it = “Plum Pudding” model
Plum pudding (raisin bread) model
Ernest Rutherford
 Credited with discovering the nucleus
 Gold Foil Experiment
Gold Foil Experiment
 Alpha
particles (positively charged)
were expected (according to the then
current theory of Dalton) to go
through the atoms of gold foil
 Instead some were deflected
 Evidence of a concentrated mass of
positive charge located in the center
of the atom
1 in 8000 particles were deflected
 Concluded atoms are mostly empty space.
The nucleus is 1/10,000 of the size of the
atom.
 Rest of the size of the atom is the electron
cloud
 http://www.youtube.com/watch?v=XBqHkr
af8iE&feature=related

Rutherford (Planetary) model
Protons and neutrons,
most of the mass,
is in the center, with
electrons “orbiting”
the nucleus.
Which of the following are results of
Rutherford’s work?




A. Atoms have mass
B. Electrons have a negative charge
C. The center of the atom has a
concentrated positive charge
D. The atom is mostly empty space
Perspective
If the atom was the size of football
stadium, the nucleus would be a
small marble or pea in the center of
that stadium.
Remember that most of the mass of the
atom is concentrated in the nucleus.