Transcript The discovery of the electron

THE DISCOVERY OF THE ELECTRON
• BY 19TH CENTURY, SCIENTISTS
DISCOVERED THAT MATTER IS COMPOSED
OF INDESTRUCTIBLE BUILDING BLOCKS
CALLED ATOMS
CATHODE RAYS – IN THE LATE 1800'S AN
ENGLISH PHYSICIST NAMED J.J.THOMSON
PERFORMED EXPERIMENTS TO PROBE THE
PROPERTIES OF CATHODE RAYS.
CATHODE RAYS......
• He constructed a partially evacuated glass tube called cathode
ray tube
• Applied high electrical voltage between the two electrodes
Findings-
1. Cathode rays travel from the negatively charged electrode to
the positively charged electrode
2. The particles that compose the cathode rays travel in straight
lines
3. The are independent of the composition of the material from
which they originate
4. They carry a negative electrical charge
J.J.THOMSON CATHODE RAY EXPERIMENT
CATHODE RAY EXPERIMENT
• The electrical charge is the fundamental property of some of
the particles that compose atoms and that results in attractive
and repulsive forces
• Electric Field -
The area where the attractive and repulsive
forces exists
• J.J.Thomson measured the charge to mass ratio of the cathode
ray particle by deflecting them using electric and magnetic
fields
• The value measured was –1.76×108 Coulombs per gram
•
ROBERT MILLIKAN
Performed an “oil-drop experiment” to
determine an accurate value for the
charge (and therefore the mass also) of
the electron.
ROBERT MILLIKAN
• Atomizer produces tiny drops of oil
• Drops fall through a very small hole.
fall through at a time.
Only a few drops can
ROBERT MILLIKAN
• X rays produce charges on the oil drops.
• Charged plates above and below. Measure quantity of
charge required to stop oil drop from falling.
ROBERT MILLIKAN
• Three forces determine the motion of the
oil drops in this experiment:
• Gravitational (downward)
• Buoyant (upward) because drops
VERY small
• Electric (upward)
ROBERT MILLIKAN
• Density of the oil is known.
• Observe radii of drops with a microscope. (This
is a measure of volume.)
• With these two, mass of each drop can be
determined.
• Gravitational and buoyant forces dependent on
mass, so those known too.
ROBERT MILLIKAN
• Record electric field needed to suspend oil drop
(stop it from dropping).
• The amount of electric charge needed to stop the oil
drop is based on the total amount of charge carried
by the oil drop.
ROBERT MILLIKAN
• By repeating the experiment for many droplets, they
confirmed that the recorded charges were all
multiples of some fundamental value, and calculated
it to be 1.5924 × 10−19 Coulombs. They proposed
that this was the charge of a single electron.
• Their value was within one percent of the currently
accepted value of 1.602176487 × 10−19 coulombs.
THE STRUCTURE OF THE ATOM
• Atoms are charged neutral.
• The discovery of electron raised a question about
the presence of a positive charge, to neutralize the
negative charge of the electron.
• J.J.Thomson proposed that electrons are held inside
a positively charged sphere – Known as the plum
pudding model
PLUM PUDDING MODEL
DISCOVERY OF RADIOACTIVITY
• The discovery of radioactivity by Henri Becquerel
and Marie curie led to the experimental view of the
structure of atom.
• Radioactivity – Emission of small energetic particles
from the core of certain unstable atoms.
• Scientists discovered three types of particles
emitted during radioactivity
• They are α particles,β particles and γ particles
RUTHERFORD'S GOLD FOIL EXPERIMENT
• Rutherford worked under Thomson to confirm his plum
pudding model, conducted an experiment known as the gold
foil experiment.
• He used α particles , which is positively charged, and
passed through an ultra thin gold foil.
• Findings – Most of the α particles pass through with little or
no deflection
• Some particles were deflected
• Some bounced back
• Rutherford created a new model which suggests the
existence of a small dense nucleus
NUCLEAR MODEL - RUTHERFORD
RUTHERFORD'S NUCLEAR MODEL OF ATOM
• Rutherford observed that the mass and charge of the atom must be
concentrated in a space smaller than the size of the atom itself.
• Nuclear Theory has three basic parts 1. Most of the atom's mass and its charge are concentrated at the
center of the nucleus
2. Most of the volume of the atom is empty throughout which tiny
negatively charged electrons are dispersed
3. The atom is electrically neutral . It has positively charged as well as
negatively charged particles.
RUTHERFORD'S MODEL - INCOMPLETE
• Rutherford's model seemed as incomplete – Compare H and He . H
has one proton and He has 2. But mass of H seems to be ¼ th of
mass of He.
• Rutherford's student James Chadwick discovered that the uncounted
mass is due to the presence of neutrons within the nucleus.
• Mass of neutron is similar to that of a proton but it doesn't have a
charge
• The He atom is 4 times massive than H atom, since it contains 2
protons and 2 neutrons.
SUBATOMIC PARTICLES
• Subatomic particles – Protons, neutrons and electrons compose
atoms and the are called subatomic particles
• Protons and neutrons has almost the same mass
• Mass of Proton = 1.67262* 10 -27kg
• Mass of neutron = 1.67493* 10 -27kg
• This mass can be expressed in atomic mass units (AMU)
• AMU - 1/12 th the weight of 1 carbon atom containing 6
protons and 6 neutrons
PROTONS, NEUTRONS AND ELECTRONS
• The mass of proton and neutron is approximately 1 amu
and that of an electron is 0.00055amu.
• Both protons and electrons have a charge. Protons are
assigned to have a charge of +1 and electrons are
assigned to have a charge of –1.
• The charge of proton and electron are equal in
magnitude but opposite in sign.
• Neutron has no charge
MATTER IS CHARGED NEUTRAL
• Matter is usually charged neutral
- protons and electrons are
present in equal number
• When matter acquires some charge imbalances, it tries to equalize
quickly
• Example : The shock you receive by touching a door nob during
dry weather is an equalization of charge that you developed as
walked across the carpet.
• Can you imagine matter as a sample with only protons or only
electrons?
- Matter would have been unstable with extraordinary repulsive
forces
WHAT MAKES THE ELEMENTS DIFFERENT FROM ONE
ANOTHER?
THE NUMBER OF PROTONS DEFINE THE ELEMENT
• Example: An atom with 2 protons in its nucleus is He
atoms
• An atom with 6 protons is C atom
• An atom with 92 protons is U atom
SUBATOMIC PARTICLES
Particle Symbol Relative Relative Location
Mass
Charge
Proton
p+
1 amu
1+
Inside
Nucleus
neutron n
1 amu
0
Inside
Nucleus
Electron e -
1/2000
amu
1-
Outside
Nucleus
ATOMIC NUMBER AND MASS NUMBER
•“Z” Atomic Number-
This is equal to the
number of protons in your atom.
•“A” Mass Number-
Protons + Neutrons
ELEMENTS
• Each element is identified by its atomic number
• Each element is represented by a chemical symbol
• He – Helium
• C – Carbon
• Th – Thorium
• Elements derived their names from Latin, Names of
places, Names of scientists etc
ELEMENTS AND THEIR NAME ORIGIN
• Sodium – Latin word Natrium – Na
• Tin – Latin word Stannum – Sn
• Chlorine – Greek word Chloros meaning pale green – Cl
• Europium , Polonium , and Berkilium are derived from the
places where they discovered or the place of the discoverers
• Curium, Einsteinium and Rutherfordium are named after name
of scientists
ISOTOPES
Isotopes: naturally occurring atoms of the same
element that vary in their number of neutrons
Examples:
35Cl
& 37Cl
Isobars- atoms with the same mass number but
different atomic numbers
Example C-14 and N-14
RELATIVE ABUNDANCE
Relative (percent or natural) abundance: how
often the isotope occurs in nature; expressed in a
percentage
Example: 35Cl has an abundance of 75% & 37Cl
has an abundance of 25%
THE PERIODIC TABLE
All elements up to #118 have been
synthesized in lab by research scientists.
CONTENTS OF EACH BOX
Atomic number
Symbol
Element name
Atomic mass
Different periodic tables provide different
amounts of info and in different orders
Who developed the Periodic Table?
• Dimitri Mendeleev: used atomic mass to
order the elements
• Henry Mosley: current arrangement by
atomic number
Vertical columns are called
groups or families.
Horizontal rows are called periods.
WHY THE NAME?
Properties of elements change as you move across a
period. The same pattern of properties repeats
when you move from one period to the next.
So the properties occur “periodically”.
Elements with similar physical and chemical
characteristics end up in same family.
Ex: Group 1A elements are all very reactive with
water.
NOTICE THAT THERE ARE TWO NUMBERING
SYSTEMS FOR THE FAMILIES:
A AND B GROUPS DISTINCTION
IUPAC CONSECUTIVE NUMBERING SYSTEM
“A” Columns
1A
8A
2A
3A
4A
5A
6A
7A
“B” Columns
3B
4B
5B
6B
7B
8B
9B 10B 1B
2B
IUPAC Consecutive Numbering System
1
18
2
13
3
4
5
6
7
8
9
10
11
12
14
15
16
17
THERE ARE THREE TYPES OF ELEMENTS ON
THE PERIODIC TABLE.
METALS
- elements to the left of the stairstep
- EXCEPT FOR HYDROGEN!
PROPERTIES OF METALS
• Lustrous
• Conduct electricity
• Ductile (can be drawn into wires)
• Malleable (can be pounded into sheets)
• High boiling/melting points
• Usually solids
NONMETALS
- elements to the right of the stairstep
and hydrogen
PROPERTIES OF NONMETALS
• Dull appearance
• Non or poor conductors of electricity
• Usually gases or liquids
• Not malleable or ductile
METALLOIDS
- elements ON the stairstep
- EXCEPT FOR ALUMINUM!
PROPERTIES OF METALLOIDS
Properties are more variable.
Properties are intermediate between those of metals and those of nonmetals.
KNOW THE NAMES OF THE FOLLOWING
REGIONS ON THE PERIODIC TABLE
ALKALI METALS
Alkali Metals
ALKALINE EARTH METALS
HALOGENS
Alkali Metals
Alkaline Earth Metals
Halogens
Alkali Metals
Alkaline Earth Metals
NOBLE GASES
Halogens
Alkali Metals
Alkaline Earth Metals
TRANSITION METALS
Noble Gases
Halogens
Alkali Metals
Alkaline Earth Metals
Transition Metals
Noble Gases
INNER TRANSITION METALS
PLEASE NOTE: THE INNER TRANSITION
METALS ARE PART OF THE PERIODIC TABLE.
To see where they fit in, look at the atomic numbers.
To see an “intact” periodic table, go to http://www.ptable.com/
Then click on the box beside “Wide”
One reason the periodic table
is drawn with the inner
transition metals separate is so
the table fits better onto a
single piece of paper.
Halogens
Alkali Metals
Alkaline Earth Metals
Transition Metals
Noble Gases
INNER TRANSITION METALS
Halogens
Alkali Metals
Alkaline Earth Metals
“Other” Metals
Transition Metals
Noble Gases
INNER TRANSITION METALS
SOME ELEMENTS ARE REFERRED TO AS
“REPRESENTATIVE ELEMENTS”
• Most of the A-Group Elements
• Their properties very clearly illustrate the periodic law
REPRESENTATIVE ELEMENTS
All of Column 1.
REPRESENTATIVE ELEMENTS
All of Column 1.
All of Column 2.
REPRESENTATIVE ELEMENTS
All of Column 1.
All of Column 2.
All the nonmetals.
Representative Elements
All of Column 1.
All of Column 2.
All the nonmetals.
And Aluminum.
REPRESENTATIVE ELEMENTS
• Properties very clearly represent the periodic law
• Number of valence electrons can be determined by
looking at group number
• Valence electrons- electrons in the highest
occupied energy level of an atom
• Elements in a group have similar properties
BECAUSE they have the same number of electrons
SIGNIFICANCE OF % ABUNDANCE
In any sample of Cl, 75.77% of the atoms have a mass
number of 35 and 24.23% of the atoms have a mass number
of 37
These isotopes are considered when average atomic mass is
calculated
ATOMIC MASS
• The mass reported in the periodic table is the
weighted average of all naturally occurring
isotopes
• Remember they are usually written as 1735Cl with
the atomic number on the bottom and the mass
number on the top.
AVERAGE ATOMIC MASS
To Calculate Average Atomic Mass:
1) Change the percent into the decimal form.
2) Multiply the percent abundance by the mass for each
isotope.
2)
Add these numbers together to determine the average
atomic mass
EXAMPLE
Magnesium has two naturally occurring isotopes:
24Mg
and
25Mg. 24Mg has a mass of23.985042 amu and a percent
abundance of 79% and 25Mg has a mass of24.985837 amu
and a percent abundance of 21%. Determine the average
atomic mass.
PRACTICE PROBLEM
The three naturally isotopes of neon, their percent
abundances, and their atomic masses are: neon-20,
19.99244amu, 90.51%; neon-21, 20.99395amu,
0.27%; neon-22, 9.22%, 21.99138amu. Calculate
the weighted average atomic mass of neon.