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Topic 3 The Atom
Section 1
Early Theories of the Atom
There are two Views of Matter
The Continuous view:
matter can be broken
down indefinitely
Ex: How many times
can a piece of chalk
be split in half?
½ to ¼
Indefinitely?
to 1/8th
to 1/16th
etc.
There are two Views of Matter
But what if there is a
smallest part that can’t
be divided further?
The Discontinuous view:
Matter can be broken
down into simple
building blocks.
(an indivisible atomos)?
Video
atoms
the building blocks
Draw a microscopic view
of a speck of chalk
from the continuous perspective
Draw a microscopic view
of a speck of chalk
from the discontinuous perspective
3.1 Atomic Theory
If no one’s ever seen an
atom, how do we know it
exists?
It’s a “theory”
Theory: a mental picture
(or concept) created to
help understand a given
phenomenon.
History of atomic theory
Democritus (400bc)– an ancient Greek
philosopher
–
–
–
–
Called them “atomos” Greek for “indivisible”
His was the “philosophical atom”
Philosophers are thinkers
He used reason, not evidence.
Check out the
Democritus clip 4 min
Dalton – an English school teacher (1800’s)
His was the Ultimate particle model of the atom
Next is John
He was the first to use symbols to represent atoms
Draw a
molecule
using the
Dalton
symbols for
atoms from
below
John Dalton (1800’s) Ultimate particle model
If all matter is made of
indivisible particles (atoms)
how can we answer the
following?
>What makes atoms of an
element the same?
Atoms of an element have
same weight.
(ex: all C atoms weigh 12)
John Dalton (1800’s) Ultimate particle model
If all matter is made of
indivisible particles (atoms)
how can we answer the
following?
>How do the atoms of
elements differ?
(eg. H vs He?)
Atoms of different elements
have different weights.
John Dalton (1800’s) Ultimate particle model
If all matter is made of
indivisible particles (atoms)
how can we answer the
following?
>How do different compounds form?
Atoms combine in simple whole
number ratios.
John Dalton (1800’s) Ultimate particle model
If all matter is made of
indivisible particles (atoms)
how can we answer the
following?
>What happens to the
atoms during a chemical
reaction?
atoms separate and
recombine in a different
arrangement
John Dalton (1800’s) Ultimate particle model
What evidence do we have
to support this theory?
Law of conservation of mass
Dalton
atomic
theory
2 min
video
How could weight (mass) be used to prove that atoms exist?
How should the mass of chemicals change during reactions?
video
LaVoisier – 1600’s heated Tin in
closed container
– Mass remain unchanged
after heating
Interpretation?:
same old atoms rearranged into a
new formula;
new ratios, but still same mass
Particle diagram Regents question:
Water reacts according to this equation:
2H2 + O2  2 H2O
–Key:
Reactants
H=
–O =
Products
Notice
Leftover
oxygen
molecule
Draw the products consistent with the law of conservation of mass
Question: If 4 grams of hydrogen reacts with 32 grams of oxygen
What mass of water is formed? 36 grams Why? Mass must be conserved!
How can the composition of a compound be used to prove
that atoms exist?
Will atoms always combine in the same ratio to form a compound?
Law of definite composition (constant composition)
Atoms of a substance always combine
in the same proportions by mass
Ex: water 11 % H to 89% O always
If water is H2O, how can this be explained?
11:88?
H weighs1
O weighs16
What is a 2:16 ratio?
H weighs1
To consider:
How does the law of conservation of mass provide
evidence for the theory of the atom?
How does the atomic theory explain the law of
definite composition (law of definite proportions)
Subatomic particles
Cathode ray tube -CRT
video
Electron JJ Thomson (1900) Cathode ray tube “CRT”
Streams of particles from the negative “cathode”
Particles were smaller than any known atom
Came from any element tested.
Where did they come from?
Inside the atom!
He Called them electrons
Old time TV, computer
monitor
Thomson’s model of the atom
We now know that the atom contains tiny
negative particles: “electrons”
We also know that the atom is neutral (its
negative and positive balance each other.
Draw a picture of Thomson's atom - A
positive sphere with negative charge
electrons embedded in it:
Draw your own model here:
A positive atom with tiny negative
electrons embedded in it.
“Plum Pudding” atom model
Negative Electrons dispersed like raisins, in
a positive matrix (pudding)
Separating radiation particles
Radiation can be of three kinds of particles:
Alpha particles : mass of 4 with +2 charge
Beta particles: mass of an electron (almost 0) with -1 charge
Gamma rays: Like x-rays. No mass or Charge (neutral)
Separating radiation particles
How do you know that beta particles are negative?
(hint: which way are they deflected?)
Why aren’t alpha particles deflected as much as betas?
(How do their masses compare?)
Rutherford gold foil experiment
He Shot alpha (+) radiation at thin gold foil
2 min Rutherford video
Results:
– Most alpha particles passed undeflected
– Few alphas were deflected from original path
Results of foil
experiment if Plum
Pudding model had
been correct.
What Actually
Happened
video
Rutherford empty space atom model
Interpretation:
Atom is mostly empty space (didn’t stop most alphas)
Atoms have a small dense positive nucleus (a few +’s were
deflected)
Electrons orbit around outside nucleus “like planets”
Subatomic Particles
Protons: Ernst Rutherford (1870’s) found positive (anode)
rays in the Cathode Ray Tube
Neutrons
H= atomic #1 (has 1 proton) …
but weighs 1
He = atomic #2 (has 2 protons) ….but weighs 4
How can we explain this difference?
0 neutrons
2 neutrons
Neutrons: Chadwick (1930’s) found neutral particle in
radiation. Explains the added weight in atoms
Learning check:
1.
Which model of the atom was the simplest? What were
its characteristics?
2. How did Thomson surmise that electrons were
subatomic particles? How did he know they were
negatively charged?
3. Why did most of the alpha particles pass through the
gold foil undeflected? Why did a few get deflected?
4. Why can’t two atoms pass through one another if there’s
so much empty space?
(Hint: what do like charges do when near each other?)
67 John Dalton was an English scientist who proposed that atoms
were hard, indivisible spheres. In the modern model, the atom
has a different internal structure.
a Identify one experiment that led scientists to develop the
modern model of the atom. [1]
b Describe this experiment. [1]
c State one conclusion about the internal structure of the atom,
based on this experiment. [1]
Section 3.2a
The Charge Cloud Model
The Charge Cloud Model
video
The atom consists of a Positive Nucleus with…
Protons (+1 charge mass =1 amu)
with neutrons (0 charge, mass = 1 amu)
and Electrons (-1 charge, 0 mass) circling rapidly outside
The “nuclear charge” Identifies the element :
a.k.a. Atomic number = the number of protons:
All atoms of an element have the same number of protons
Boron has a +5 nuclear charge
Boron is atomic number 5
The Mass Number – is the weight of the nucleus
= # protons + # neutrons
(electrons = 0 mass so they aren’t counted)
The boron-10 nucleus has a mass of 10
5 protons + 5 neutrons
The Mass Number – is the weight of the nucleus
= # protons + # neutrons
(electrons = 0 mass so they aren’t counted)
C-12: Carbon atom with 6 p+ and 6 no is the mass
standard = 12.000 atomic mass units
Mass number is NOT on the periodic table
(the table # is an average)
Isotopes
• Are atoms of the same element (same atomic
number) but different mass number
• Carbon -12 (12C) has 6 p and 6 n
• Carbon -14 (14C) has 6 p and 8 n
Isotopes
Isotopes have the same nuclear charge (protons)
but different a different number of neutrons
Representing isotopes
 Write the name of the isotope, and the mass
number in hyphen notation
ex: Sodium-23
Is the sodium with 11 protons and 12
neutrons so it weighs 23 amu’s
Representing isotopes
 The symbol shows the mass number and
atomic number in nuclear symbol form
Top: mass number
23
11 Na
Bottom: Atomic number
Isotopes?
Which of the following represent
isotopes of the same element? Which
element?
Each has a different mass # due to a different Neutron #
234
92
X
234
93
X
235
92
X
238
92
–Neptunium -234
Atomic # 92 all atoms of Uranium
X
Counting Protons, Neutrons, and Electrons
Protons: Atomic Number (from periodic table)
Electrons:
If it’s an “atom”, the protons and electrons must
be the SAME so that it is has a net charge of zero (equal
numbers of + and -)
(“ions” have diff. # of protons and electrons)
Neutrons: Mass Number (not on table) minus the number of
protons (mass number is protons and neutrons because the
mass of electrons is negligible)
= 18 n0
–Can you explain this?
What gives atoms their characteristic properties?
+4
8u
+2
nucleus
5u
nucleus
+3
6u
+4
9u
nucleus
+2
nucleus
6u
nucleus
+2
5u
nucleus
+3
8u
nucleus
+3
7u
nucleus
+4
nucleus
7u
Its not how much they weigh.
+4
7u
nucleus
+2
4u
nucleus
+3
6u
nucleus
–Can you explain this? What gives atoms their characteristic properties?
8u
9u
5u
6u
6u
5u
8u
7u
7u
7u
4u
The nuclear charge (the number of protons)
6u
Learning Check – Counting
Naturally occurring carbon consists of three isotopes, 12C, 13C,
and 14C. State the number of protons, neutrons, and electrons
in each of these carbon atoms.
12C
6
- ________
13C
6
14C
6
6
#p+ _______
_______
_______
#no _______
_______
_______
#e- _______
_______
_______
Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
D. Another isotope of this element is
1) 34X
2) 34X
3) 36X
16
14
14
3.2b AVERAGE
ATOMIC MASS
11B
10B
Because of the existence of isotopes,
the mass of a collection of atoms has an average value.
Boron is
20% 10B
and 80% 11B.
11B is 80 out of 100 atoms of Boron. 10B is only 20 / 100
10
11
11
11
11
11
10
11
11
11
3.2b AVERAGE
ATOMIC MASS
11B
10B
Calculate the average mass of these 10 atoms:
For boron atomic weight
=
20% (10 u) + 80% (11 u) =
=
20/100 (10)
+ 80/100 (11) = 2.0 + 8.8
10
11
Notice that the atomic mass (average)
comes out to be closer to the isotope of
highest abundance
(80% of the way between 10 and 11)
= 10.8 u
11
11
11
11
10
11
11
11
11B
3.2b AVERAGE
ATOMIC MASS
10B
“Formula”
Atomic mass =
( % abundance ) (mass)
100
Do you recognize the summation symbol?
Try some: calculate the atomic mass
1)
6Li
= 7.5% abundant and 7Li = 92.5%
masses: Li – 6 = 6.015, Li-7 = 7.016
2)
28Si
(27.98) = 92.23%
29Si
(28.98) = 4.67%,
30Si
(29.97) = 3.10%
3.3a The Periodic Table
Dmitri Mendeleev (1834 - 1907)
Dmitri Mendeleev was a Russian chemist, who, in 1870, looked for a way to organize the
elements. He wrote the known properties of elements on blank playing cards.
Explosive gas
1
H
He placed element cards in order of increasing mass
and looked for patterns.
Inert gas
4
Notice the formulas of the compounds formed with F and O.
He
HF H2O
Active metal
7
Li
LiF Li2O
Active metal
9
semimetal
11
Be
B
BeF2 BeO
BF3 B2O3
nonmetal
12
unreactive gas
14
Explosive gas
16
active gas
18
Inert gas
20
C
N
O
F
Ne
CF4 CO2
NF3 N2O3
OF2 O2
F2 F2O
Active metal
23
Active metal
24
metal
27
semimetal
28
nonmetal
31
nonmetal
32
Active gas
35
Inert gas
40
Na
Mg
Al
Si
P
S
Cl
Ar
MgF2 MgO
AlF3 Al2O3
SiF4 SiO2
PF3 P2O3
SF2 SO
ClF Cl2O
NaF Na2O
Active metal
39
After fluorine, he noticed that sodium was similar to lithium, and the pattern repeated.
K
After chlorine, he noticed that potassium was similar to sodium, etc.
KF K2O
Hydrogen starts the table since it’s the smallest, and the noble gases, once
discovered took their place at the end of the line.
TED
Link
Notice how elements line up into columns with similar properties?
Also, notice how the properties repeat each row?
That’s “periodic”!
Mendeleev’s Table
–Video clip
Organized first table based on increasing
mass (but…. allowed elements to group by
similar properties)
Found that properties of elements repeated
periodically (in a regular pattern)
Put elements in vertical groups
with similar properties
Left spaces for elements that weren’t yet
discovered
–Eka-silicon - aka germanium
Mass dilemma:
mass sometimes
decreases
Henry Moseley (1913) Did X-ray studies
Found that the positive nuclear charge increased by one
for each element  called it “atomic number”
Modern Periodic Law: “Properties of the elements are
periodic functions of their atomic numbers”
Ie. Properties of atoms depend on number of protons
Mass dilemma:
mass sometimes
decreases
Modern table:
Organized into groups (vertical columns) of
elements with similar properties
 have same # of outer shell electrons
Organized into periods (horizontal rows) –
properties change systematically from metals to
nonmetals to inert (unreactive) gases
Properties are
 similar 
Group 1
Group 2
Etc.
Period 1
Period 2
Period 3
Etc.
Metals
Properties change
 systematically 
To Non- Metals
Regions of the Periodic Table
Nonmetals
Metals
Noble gases are a special
group of stable unreactive
elements on the extreme right
Noble Gases
Metals on the left side and middle
Non-Metals
on the
right side
H
Nonmetals
Metals
Noble Gases
Hydrogen is actually a nonmetal, but starts the
table since it is the smallest.
Its is more at home on the right side of the table
with the other non-metals
H
Nonmetals
Metals
Si
Ge As
Sb Te
Po
Noble Gases
Metalloids are elements that lie
along the border. They can
behave like either metals or
nonmetals depending on how
you find them.
B
Types of elements
Metals
Solids
(one
liquid)
Shiny
Luster
Malleable
Ductile
Good
conductors
of Heat
and
electricity
Nonmetals
Solids and
gases
(one liquid)
Metalloids
Noble
gases
Solids
Gases
Semiconductors
Properties
of
both M & NM
Inert
or
Unreactive
Dull
Luster
Brittle
Poor
conductors
Good
insulators
video
Learning check:
1. State the modern periodic law: “Properties of elements are a
periodic function of ….
2. What are “groups” on the periodic table? How are the members
related to one another?
3. What are the periods on the periodic table?
how do the properties change across a period (what pattern)?
4. What are the 3 + 1 types of elements?
Where is each located on the table?
How do their properties differ?
3.3b Families of the Periodic Table
Nonmetals
H
Noble Gases
Alkali metals
Alkaline earth metals
Metals
Group 1 and 2 (S block)
alkali metals and alkaline earth metals
–2 video
–clips
Group 1 Alkali Metals
and 2 Alkaline earth metals
–Both groups very reactive
–Found in nature only in compounds
–Obtained from breakdown of salts
Families of the Periodic Table
Nonmetals
H
Transitional metals
Rare earth elements
Noble Gases
Alkali metals
Alkaline earth metals
Metals
Transitional metals
Transition
Element
video
Actinide video
Lanthide video
–Rare Earth metals
Transitional Metals - d block
•
•
•
•
•
•
Group 3 to 12
Less reactive than s-block metals
Generally similar properties
Many form colored ions
Some found in elemental state in nature (ex Au)
Transition from group 2 to group 13
• Includes rare earth elements of the f block
Found in certain rare minerals
Group 13 to 16
Nonmetals
–Video
–clip
semimetals
–Video
–clip
poor
metals
Groups 13 to 16
• Nonmetals at top change to
metals at bottom
• Properties change going
down the groups
• Nonmetals are gases and
brittle solids
• Reactivity increases toward
oxygen
• “poor” Metals are fairly
unreactive
• Aluminum, tin, lead
Families of the Periodic Table
Nonmetals
H
Transitional metals
Rare earth elements
Noble Gases
Alkali metals
Alkaline earth metals
Metals
Metaloids
–Video
–clip
Metaloids (Semimetals)
• Along stepped line
• B, Si, Ge, As, Sb, Te, Po
• Properties of both metals
and nonmetals
• Ex: computer
“semiconductors” made of
silicon
Families of the Periodic Table
Nonmetals
H
Rare earth elements
Noble Gases
Transitional metals
Halogens
Alkali metals
Alkaline earth metals
Metals
Group 17 Halogens
–Video
–clip
Group 17: The Halogens
( “salt makers” ) F, Cl, Br, I
• Highly reactive nonmetals
• Found in nature only in
compounds
• F gas - most active
nonmetal (reactive)
• Gas to liquid to solid
–due to increasing intermolecular (sticky)
forces
Group 18 Noble gases
Video
clip
Group 18:
The Noble (Inert) Gases
He, Ne, Ar, Kr, Xe, Rn
• “Neon” signs
• Very Unreactive
because they have
full electron shells
Learning check:
1.
What families of elements are so reactive that they are only found in
nature combined in compounds?
2. Where are the transitional elements located?
What kind of elements are they? Other traits?
3. Which groups on the table contain both metals and nonmetals? Explain
4. Which halogen is most reactive?
What trend occurs in melting and boiling points for elements in group
17? Why does this trend occur?
5. List the 7 semimetals (metalloids):
Why are they named as such?
6. What elements exist as diatomic molecules when in their pure,
uncombined state?
6 An element that is malleable and a good
conductor of heat and electricity could have an
atomic number of (1) 16 (2) 18 (3) 29 (4) 35
–Hint: It’s a metal
3 In which list are the elements arranged in order
of increasing atomic mass?
(1) Cl, Ar, K (2) Fe, Co, Ni (3) Te, I, Xe (4) F, Ne, Na
–Hint: mass usually increases left to right and top to bottom
–(4) But there are at least 3 exceptions!
37 Which list of elements contains two metalloids?
(1) Si, Ge, Po, Pb (2) As, Bi, Br, Kr (3) Si, P, S, Cl (4) Po, Sb, I, Xe
–Tricky! Along the stepped line, doesn’t include Al or At
In the 1920s, paint used to inscribe the numbers on watch dials was
composed of a luminescent (glow-in-the-dark) mixture. The powdered-paint
base was a mixture of radium salts and zinc sulfide. As the paint was mixed,
the powdered base became airborne and drifted throughout the workroom
causing the contents of the workroom, including the painters’ clothes and
bodies, to glow in the dark.
The paint is luminescent because radiation from the radium salts strikes
a scintillator. A scintillator is a material that emits visible light in response to
ionizing radiation. In watch dial paint, zinc sulfide acts as the scintillator.
Radium present in the radium salts decomposes spontaneously,
emitting alpha particles. These particles can cause damage to the body when
they enter human tissue. Alpha particles are especially harmful to the blood,
liver, lungs, and spleen because they can alter genetic information in the cells.
Radium can be deposited in the bones because it substitutes for calcium.
72 Why does radium substitute for calcium in bones? [1]
–Need a hint? Where is Ra located relative to calcium on the per. Table?