Building Scientific
Knowledge
An Understanding the theories
leading to the current model of
the atom
Observation and Inference
• Building understanding through
exploration and experimentation
• Observation is a statement that is based
on what you see, hear, taste, touch, and
smell
• Inference a judgement based on
observations and conclusions from
testing
• There are two types of scientific knowledge that
are acquired through observation and inference
• 1. empirical knowledge: information obtained
directly from what the senses observes e.g. at
100 oC water turns from liquid to vapour
• 2. theoretical knowledge: information that
attempts to explain the observations (a theory or
model) e.g. water vaporizes at 100 oC because
water molecules absorb enough energy while ….
• Or heated molecules move more quickly and
break the bonds that hold them together……
Theory
• An explanation of a large number of
related observations:
• e.g. Kinetic Molecular Theory
• Particle theory of Matter
• Observations and inferences are
integral components of qualitative
analysis
• Scientists need to infer knowledge
based on observations to gain greater
understanding of the universe and
develop testable theories.
• Theory is an explanation of a scientific
phenomenon based on a large
number of related observations.
Models of Matter
• a model is a tentative description of a
system or theory that accounts for all
of its known properties
• models are invented (usually) to
interpret the invisible and relate
experimental results
• Models can change over time as new
information is added.
Model vs Analogy
-scientific model is used to visually
communicate a theory or idea
-analogy uses everyday concepts to
relate to scientific theories that are
otherwise unrelated
The Four Elements
• An Ancient Greek model, based on logic
• 650 BC - Greek Philosophers (Empedocles,
Democritus) believed matter is made up of
four elements (and combinations)
• Matter: air, fire, earth and water
Democritus (Deep Thinker)
•
•
•
400 BC: Democritus supported an atomic
theory which stated that “all matter is
composed of atoms”
He said that atoms were indivisible and
indestructible
The word “atomos” comes from the
Greek words for “indivisible”
John Dalton’s Atomic Theory
(1803)
 used experimental methods to turn
Democritus’ ideas into theory
 Dalton’s experiments involved analyzing the
percent composition of compounds and their
molecular weights (Law of Conservation of
mass)
 Law of Multiple Proportions: when elements
combine they do so in a simple whole number
ratio
Dalton’s Atomic Theory
 Matter consists of definite particles called
atoms
 Each element is made up of its own type of
atom
 Atoms of different elements have different
properties and different sizes
 Atoms of two or more elements can combined
in constant ratios to form new substances
 Atoms are not created or destroyed in chemical
changes
Dalton’s Model
Model of the atom is a featureless
sphere with uniform charge
distribution
Dalton’s Analogy
Analogy of the atom features a group
of billard balls with some space
between them
Thomson’s Model
basic body of an atom is a spherical
object containing N electrons confined
in homogeneous jellylike but
relatively massive positive charge
distribution whose total charge
cancels that of the N electrons
Used cathode ray to demonstrate
atoms were made of smaller particles
Thomson’s Atomic Theory
(1897)
• used a cathode ray tube to energize
electrons
• experimental data showed:
– charged particles originated from the gas
inside the tube
– charged particles are deflected away from a
negatively charged object, but towards a
positively charged object.
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
• By adding an electric field
Thomson’s Experiment
Voltage source
+
 By adding an electric field, he found that
the moving pieces were negative
No matter what the gas used in the
cathode-ray tube, or the metal used for
the electrodes, the beam of particles
reacts the same way. (indicates that the
particles are the same for all atoms)
Found the electron
Atoms are usually neutral, or not
charged
Thomsons Analogy
 is a raisin bun the positively
charged sphere (the dough)
embedded with negatively
charged electrons (the raisins).
Theory: “Plum Pudding Model”
• Atoms are composed of smaller particles.
• These particles are the same for all different
types of atoms.
• These particles are negatively charged and
are called electrons.
• Electrons are embedded throughout the
uniform sphere of positive charge to make
up a neutral atom.
• Matter is naturally neutral except when it
gains or loses electrons
Rutherford Model
Used gold foil experiments to show
the positive charge and most of the
mass is located in the centre of the
atom called proton
Suggested nucleus is surrounded by
mostly empty space
Rutherford’s Gold Foil
Experiment (1911)
• Rutherford (at McGill) used very small
positive particles (alpha particles: positive
helium nuclei) and a fluorescent screen
detector to see the particles
• Since the mass is evenly distributed in
gold atoms alpha particles should go
straight through.
experimental data showed:
• Alpha particles (+ve) were shot at a thin sheet
of gold
• Most alpha particles passed through unaffected
• Some alpha particles were deflected at large
angles. Some are even reflected back.
• - Thomson's model did not explain the
deflection, so …
Lead
block
Uranium
Fluorescent
Screen
Gold Foil
What he expected
What he got
Because
Because, he thought the mass was
evenly distributed in the atom.
How he explained it
• Atom is mostly empty space
• Small dense, positive region at center.
• Alpha particles are deflected by this
region if they get close enough.
+
+
Theory
1. an atom is made of mostly empty
space
2. all the positive charge and most of the
mass are in a tiny center of the atom:
the nucleus
3. electrons surround but do not fill the
rest of the atom
Rutherford’s Analogy
Planetary analogy since the electrons
orbit the nucleus like planets orbit the
Sun
Chadwick
• 1932 James Chadwick showed that a
neutral particle with a mass about that
of a proton, was emitted when hit with
a gamma ray.
• This meant that neutrons could be
collided with atomic nuclei.
• Some of the neutrons would imbed in
a nucleus, increasing its mass.
Bohr Model
 Electrons move in orbits of fixed energy
around the nucleus
 Electrons are restricted to certain levels
(orbits) based on their quantity of
energy
 Electrons can absorb energy and
become excited and move to a higher
energy level
 Electrons cannot exist between orbits
How
• Run electricity through gaseous
elements, thus exciting the electrons
to a higher energy level
• Electrons “fall” back to their ground
state and release the absorbed energy
in the form of waves
Niels Bohr
• Explained the path of electrons
(orbits) around the positive nucleus
• These orbits are specific distances
from the nucleus (orbitals)
• Electron energy level model.
Bohr Analogy
 Electrons posses a specific amount of
energy that corresponds to their energy
level similar to rungs on a ladder.
Schrödinger’s Model
• The WAVE model
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The Wave Model
• Today’s atomic model is based on
the principles of wave mechanics.
• According to the theory of wave
mechanics, electrons do not move
about an atom in a definite path,
like the planets around the sun.
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The Wave Model
• In fact, it is impossible to determine the
exact location of an electron.
• The probable location of an electron is
based on how much energy the electron
has.
• According to the modern atomic model,
an atom has a small positively charged
nucleus surrounded by a large region in
which there are enough electrons to make
an atom neutral.
45
Electron Cloud
• A space in which electrons are likely
to be found.
• Electrons whirl about the nucleus
billions of times in one second.
• They are not moving around in random
patterns.
• Location of electrons depends upon
how much energy the electron has.
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Electron Cloud
• Depending on their energy they are
locked into a certain area in the cloud.
• Electrons with the lowest energy are
found in the energy level closest to the
nucleus.
• Electrons with the highest energy are
found in the outermost energy levels,
farther from the nucleus.
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Modern View
Atoms are made of subatomic particles;
protons, neutrons and electrons
Protons and neutrons cluster to form
nucleus
Neutral atoms protons=electrons
Electrons surround nucleus in
quantifiable energy levels called electron
cloud model (orbitals)
Shells are designated by the principal
quantum number n (n=1, n=2, etc.)
(bigger n means larger shell) {analogy
is floors in apartment building}
Each shell is divided into subshells
(called s, p, d, f, etc.) (n=1 has s; n=2
has s & p; n=3 has s, p, & d; n=4 has s,
p, d, & f) {analogy is apartments on a floor}
Symbolic representation of
elements
Name
Symbol
Charge
Mass
Location
proton
p
+1
1 a.m.u.
nucleus
neutron
n
0
1 a.m.u.
nucleus
electron
e-
-1
1/1837
a.m.u.
energy levels
(orbitals)
around nucleus
1 amu (atomic mass unit) = 1.66 X10-27 kg