Download The Evolution of Atomic Theory

The Evolution of Atomic
Theory
Chapter 3
Law of conservation of Matter
• When a chemical reaction takes place,
matter is neither created nor destroyed
Law of Constant Composition
• Multiple Samples of any pure chemical
compound always contain the same percent by
mass of each element making up the compound
– Example: if a 50 g sample of pure water is
decomposed into it’s component elements, you will
obtain 5.6 g of hydrogen and 44.4 g oxygen gas
– The percent by mass of these elements is therefore:
Dalton’s Atomic Theory
 All matter is made of tiny indivisible
particles called atoms.
2 Atoms can neither be created nor
destroyed (they persist unchanged for all
eternity)
3 Atoms of the same element are identical
(in size, mass, and properties), those of
different atoms are different.
Dalton’s Atomic Theory
4 A chemical reaction involves either the
union or the separation of individual
atoms.
Law of Multiple Proportions
•
A law proposed by Dalton
which states that when
elements combine, they
do so in the ratio of small
whole numbers. For
example carbon and
oxygen react to form CO
or CO2, but not CO1.8.
Parts of an Atom
• J. J. Thomson - English physicist. 1897
• Made a piece of equipment called a
cathode ray tube.
• It is a vacuum tube - all the air has been
pumped out.
Thomson’s Experiment
Voltage source
+
-
Vacuum tube
Metal Disks
Thomson’s Experiment
-
Voltage source
+
Thomson Experiment
Voltage source
+
-
Passing
an electric current makes a beam
appear to move from the negative to the
positive end
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
What Did Thomson
Demonstrate?
• Cathode rays:
• Travel in straight lines
• Are negatively charged
• Are deflected by electric and magnetic fields
Thomson’s Model
• Found the electron
• Said the atom was
like plum pudding
• A bunch of positive
stuff, with the
electrons able to be
removed
Rutherford’s Experiment
• Ernest Rutherford English physicist. (1910)
• Believed in the plum pudding model of the
atom.
• Used uranium to produce alpha particles.
Rutherford’s Experiment
Lead
block
Uranium
Florescent
Screen
Gold Foil
What he expected
Why ??
• The alpha particles
to pass through
without changing
direction very much
• The positive
charges were
spread out evenly.
Alone they were
not enough to stop
the alpha particles
What he got
How He Explained It
• Atom is mostly empty
• Small dense, positive
piece at center
• Alpha particles are
deflected by it if they
get close enough
How He Explained It
+
Density and the Atom
• Since most of the particles went through, it
was mostly empty.
• Because the pieces turned so much, the
positive pieces were heavy.
• Small volume, big mass, big density
• This small dense positive area is the nucleus
Modern View
• The atom is mostly
empty space
• Two regions
• Nucleus- protons and
neutrons
• Electron cloud- region
where you might find
an electron
Structure of Atom
• There are two regions:
– The nucleus: with protons and neutrons
• Almost all the mass
– Electron cloud- Most of the volume of an
atom
• The region where the electron can be found
Subatomic Particles
Name
Symbol
Charge
Electron
e-
-1
Proton
p+
+1
Electron
n0
0
Size of Atom
• Nucleus tiny compared to atom
• IF the atom was the size of a stadium, the
nucleus would be the size of a marble.
• Radius of the nucleus near 10-15m.
• Density near 1014 g/cm
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in their nuclei
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
U
(T)
Isotopes
• Dalton was wrong.
• Atoms of the same element can have
different numbers of neutrons
• different mass numbers
• called isotopes
The Periodic Table
The Periodic Table
•Each column is called a group.
•Each row is called a period.
Noble Gas
Halogen
Group
Alkali Metal
Alkali Earth Metal
Period
2.4
Non-metals
• Not conductors of heat or electricity
• Brittle cannot be rolled into wires or pounded into sheets.
• Exist in two of the three states of matter at room temperature:
• Gases- Oxygen
• Solids- Carbon
• No metallic luster and do not reflect light
• You are to know the names (correct spelling) and symbols for
these elements.
Metalloids
• Properties of both metals and non-metals
• Silicon and Germanium are semiconductors.
Alkali Metals
IA
Li
Na
K
Rb
Cs
Fr
Alkali Metals
• Very react-active metals that do not occur freely in
nature.
• They are Malleable, ductile and good conductors of heat
an electricity
• Softer than most other metal
• Explode if they are exposed to water
• Color flames
• Li-red Na- Yellow K- lilac Rb- red Cs-blue
Alkaline Earth Metals
Be
Mg
Ca
Sr
Ba
Ra
Alkaline Earth Metals
• Very reactive but not as reactive as the
alkali metals
• All are found in the earth’s crust but not in
elemental form. Instead they are widely
distributed in rock structures
Halogens
F
Cl
Br
I
At
Halogens
• Halogen means “salt-former” therefore compounds
containing halogens are called “salts.”
• All halogens exist as diatomic molecules, X2
• The halogens exist, at room temperature, in all
three states of matter
• Solid-Iodine, Astatine
• Liquid- Bromine
• Gas- Fluorine, Chlorine
• The halogens are too reactive to occur free in
nature.
Transition metals
Sc Ti V Cr Mn Fe Co Ni Cu Zn
Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
La Hf Ta W Re Os Ir Pt Au Hg
Ac
Transition Metals
• Ductile
• Malleable
• Conductors of heat and electricity
• Iron, Cobalt and Nickel are they only elements
known to produce a magnetic field
Noble Gases
He
Ne
Ar
Kr
Xe
Rn
Noble Gases
• Very un-reactive
• Very small quantities in the atmosphere
• Monatomic
• Low boiling points
• He, Ne, and Ar form no known compounds
Rare Earth Metals
Rare Earth Metals
Lanthanide Series
Actinide Series
High Luster
Conductors of electricity
Tarnish rapidly in air
Many ignite and burn
vigorously at elevated
temperatures
All isotopes of all actinides
are radioactive
Actinides are very dense
Typically react with:
air (tarnishing)
boiling water or dilute acid
most non-metals in direct
combination
Atomic Mass
•
•
•
•
How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
More concerned with average atomic mass.
Based on abundance of each element in
nature.
• Don’t use grams because the numbers
would be too small
Measuring Atomic Mass
• Unit is the Atomic Mass Unit (amu)
• Each isotope has its own atomic mass
we need the average from percent
abundance.
Calculating averages
• You have five rocks, four with a mass of 50
g, and one with a mass of 60 g. What is the
average mass of the rocks?
• Total mass =
4 x 50 + 1 x 60 = 260 g
• Average mass = 4 x 50 + 1 x 60 = 260 g
5
5
Calculating averages
• Average mass = 4 x 50 + 1 x 60 = 260 g
5
5
5
• Average mass = .8 x 50 + .2 x 60
• 80% of the rocks were 50 grams
• 20% of the rocks were 60 grams
• Average = % as decimal x mass +
% as decimal x mass +
% as decimal x mass +
Atomic Mass
• Calculate the atomic mass of copper if
copper has two isotopes. 69.1% has a mass
of 62.93 amu and the rest has a mass of
64.93 amu.
Atomic Mass
• Magnesium has three isotopes. 78.99%
magnesium 24 with a mass of 23.9850 amu,
10.00% magnesium 25 with a mass of 24.9858
amu, and the rest magnesium 25 with a mass of
25.9826 amu. What is the atomic mass of
magnesium?
• If not told otherwise, the mass of the isotope is the
mass number in amu