Atoms

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Chapter 4
Atoms and Elements
Atoms - indestructible building blocks from
which are substances are constructed.
2006, Prentice Hall
CHAPTER OUTLINE
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Atoms, Elements and Symbols
Periodic Table of the Elements
Properties of Metals and Non-Metals
The Atomic Theory
The Modern Atom
Atomic Structure
Ions
Isotopes and Atomic Mass
2
Experiencing Atoms
• atoms are incredibly small, yet they
compose everything
• atoms are the pieces of elements
• properties of the atoms determine the
properties of the elements
If every atom within a pebble were the size of the pebble itself, then the pebble would be larger than Mt. Everest
(~29,000 ft)
Experiencing Atoms
•
there are about 117 elements found in nature
–
•
and 23 have been made in laboratories
each has its own, unique kind of atom
–
–
they have different structures
therefore they have different properties
If you keep dividing matter into smaller and
smaller pieces you end up with an atom.
EARLY CONCEPTS
OF THE ATOM
 The smallest particle of matter that still retains its
properties is called an atom.
 In the fifth century B.C., the Greek
philosopher Democritus proposed
that matter is composed of a finite
number of discrete particles, named
atomos (meaning un-cuttable or
indivisible)
5
DALTON’S
ATOMIC THEORY
 In
Dalton’s
1808, John
modelDalton,
represented
built on
the
ideas of
atom
as Democritus,
a featureless and
ball of uniform
formulated a precise definition
density.
themodel
building
blocks of
 of
This
is referred
to matter.
as the
“soccer ball” model.
6
DALTON’S
ATOMIC THEORY
Dalton’s atomic theory,
 explains the difference between an element
and a compound.
 explains two scientific laws, and
 predicts a new scientific law.
7
DALTON’S
ATOMIC THEORY
Postulate
1
2
Deduction
Each element consists of
indivisible, small particles
called atoms.
All the atoms of an element
are identical to one another,
but different from others.
Gives a more precise
definition for an element.
8
DALTON’S
ATOMIC THEORY
Atoms of
oxygen are
different
from
All atoms
atomsof
ofoxygen
hydrogen
are
identical to
one another
All atoms of
hydrogen
are identical
to one
another
Atoms
of consists
each element
are identical
to one
Atoms
of indivisible,
small particles.
another, but different from others.
9
5.1
DALTON’S
ATOMIC THEORY
Postulate
3
4
Atoms combine chemically
in definite whole-number
ratios to form compounds.
Atoms can neither be
created nor destroyed in
chemical reactions.
Deduction
Supports Law of Definite
Composition; predicts Law
of Multiple Proportions.
Supports Law of
Conservation of Mass.
10
LAW OF
DEFINITE COMPOSITION
As a result
Atoms
combine
compounds
in definite
always
whole-number
contain elements
ratios in
to
form
the
same
compounds.
proportions by mass.
H
2
=
O
1
H
1
=
O
1
11
LAW OF
MULTIPLE PROPORTIONS
Two or more elements may combine in different
ratios to form more than one compound.
H 2
=
O 1
H 1
=
O 1
12
Sizes of Atoms
• using compositions of compounds and
assumed formulas, Dalton was able to
determine the relative masses of the atoms
– Dalton based his scale on H = 1 amu
• we now base it on C-12 = 12 amu exactly
– unit = atomic mass unit = amu
• another term for amu is ? dalton
• absolute sizes of atoms
– mass of H atom= 1.67 x 10-24g
– volume of H atom = 2.1 x 10-25cm3
Some Notes on Charges
• Two Kinds of Charge called + and –
• Opposite Charges Attract
• Like Charges Repel
• To be Neutral, something
must have no charge or equal
amounts of opposite charges
The Atom is Divisible!
• Work done by J.J. Thomson and others
proved that the atom had pieces called
electrons
• Thomson found that electrons are much
smaller than atoms and carry a negative
charge
– the mass of the electron is 1/1836th the mass
of a hydrogen atom
– the charge on the electron is the fundamental
unit of charge which we will call –1 charge
units
DISCOVERY OF
THE ELECTRON
 Smaller
Negatively
particles
charged
than
particles
the atom
from
also
cathode
exist and
were
are called
pulled
towards
subatomic
positively
particles.
charged plate, anode,
allowed
pass through
and beexperiments
detected on
 and
In 1897,
J.J.to
Thomson
performed
a
fluorescent
screen.
with
a cathode
ray tube.
16
DISCOVERY OF
THE ELECTRON
 In
These
absence
presence
observations
ofofaamagnetic
magnetic
indicated
field,
andthat
electric
the the
cathode
cathode
fields,
rays
the
were were
cathode
rays
notrays
deflected.
negatively
were deflected
charged.
towards the positive
 plate.
These rays were later named electrons.
17
ATOMIC
MODEL
 Based on these findings, Thomson proposed an
atomic model, composed of negatively charged
electrons embedded in a uniform positively
charged sphere.
 This model is called the
“plum pudding” model.
18
DISCOVERY OF
THE NUCLEUS
 In these
1910, experiments
Ernest Rutherford
he bombarded
carried out
a thin
a sheet
number
of experiments
to further
the
of
gold foil
with -particles
(large,probe
positively
nature of emitted
charged)
the atom.
from a radioactive source.
19
DISCOVERY OF
THE NUCLEUS
 Some
Few majority
The
ofofthe
theparticles
particles
of the particles
were
wereobserved
observed
were observed
totobebeturned
to
pass through
deflected
at large
un-deflected
slightly
back
towards
the angles.
directionor
they
camedeflected.
from.
20
NUCLEAR MODEL
OF THE ATOM
 The
Based
deflections
scatterings
on these were
observations,
caused
by glancing
head-on
Rutherford
proposed
collision
of a model of
particles
with
thethe
atom
consisting of a small,
nucleus.
massive positive
Deflection
center
(nucleus),
surrounded by
electrons in mostly
empty space.
Scattering
21
5.5
Structure of the Atom
• Rutherford proposed that the nucleus had a
particle that had the same amount of charge as
an electron but opposite sign
– based on measurements of the nuclear charge of the
elements
• these particles are called protons
– protons have a charge of +1 and a mass of 1 amu
• since protons and electrons have the same amount
of charge, for the atom to be neutral there must
be equal numbers of protons and electrons
Some Problems
• How could beryllium have 4 protons stuck
together in the nucleus?
– shouldn’t they repel each other?
• If a beryllium atom has 4 protons, then it should
weigh 4 amu; but it actually weighs 9.01 amu!
Where is the extra mass coming from?
– each proton weighs 1 amu
– remember, the electron’s mass is only about
0.00055 amu and Be has only 4 electrons – it can’t
account for the extra 5 amu of mass
The Must Be Something Else There!
• to answer these questions, Rutherford
proposed that there was another particle
in the nucleus – it is called a neutron
• neutrons have no charge and a mass of
1 amu
– the masses of the proton and neutron are
both approximately 1 amu
THE MODERN
ATOM
 The electrons
current model
(e-) move
of the
atom describes
rapidly
throughitthe
as aatomic
neutral spherical
volume,
held by the
entity,
composed forces
attractive
of a positively
to the
charged nucleus
nucleus.
surrounded
by negatively
 The
nucleus consists
of
charged electrons.
positively
charged protons
(p+) and neutrally charged
neutrons (n0).
25
ATOMIC
STRUCTURE
 The number
modern atom
of protons
consists
in an
of atom
3 subatomic
determines its
identity,
particles:and is called atomic number (Z).
 In a neutral atom, the number of protons (+) are
Relative
equal
to
the
number
of
electrons
(–).
Particle
Charge
Mass
 Almost all the mass of the atom rests
in the
Proton
+1
~1800
nucleus.
 Therefore
the number 0of protons and
neutrons in
Neutron
~1800
an atom is called the mass number (A).
Electron
–1
1
26
Elements
• each element has a unique number of
protons in its nucleus
• the number of protons in the nucleus of
an atom is called the atomic number
– the elements are arranged on the Periodic
Table in order of their atomic numbers
• each element has a unique name and
symbol
– symbol either one or two letters
• one capital letter or one capital letter + one lower
case
The Periodic Table of Elements
Elements:
Origins of the Names of the Elements
• Most chemical symbols are based on the English
name of the element.
• Some symbols are based on Latin names.
• The symbol for potassium is K, from the Latin kalium,
and the symbol for sodium is Na, from the Latin
natrium.
• Additional elements with symbols based on their
Greek or Latin names include the following:
lead
Pb
mercury Hg
iron
Fe
silver Ag
tin
Sn
copper Cu
plumbum
hydrargyrum
ferrum
argentum
stannum
cuprum
Elements:
Origins of the Names of the Elements
• Early scientists gave newly discovered elements
names that reflected their properties:
• Argon, from the Greek argos, means “inactive,”
referring to argon’s chemical inertness.
• Other elements were named after countries:
• Polonium after Poland
• Francium after France
• Americium after the United States of America.
• Other elements were named after scientists.
• Every element’s name, symbol, and atomic number
are included in the periodic table (inside the front
cover) and in an alphabetical listing (inside the back
cover) in this book.
Review
•
•
•
•
What is the atomic number of boron, B?
What is the atomic mass of silicon, Si?
How many protons does a chlorine atom have?
How many electrons does a neutral neon atom
have?
• Will an atom with 6 protons, 6 neutrons and 6
electrons be electrically neutral?
• Will an atom with 27 protons, 32 neutrons and 27
electrons be electrically neutral?
• Will a Na atom with 10 electrons be electrically
neutral?
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