Transcript Atoms

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Rhonda Alexander
Atomic Structure & Periodicity
Chapter 3 - Atoms
Democritus – a Greek philosopher 400 BC
* ‘Atomos’ – atoms are indivisible.
A – not
Tomos - cutting
Dalton’s Atomic Theory


John Dalton – English school teacher - 1808
He proposed an explanation for several laws
•All matter is composed of small particles called Atoms
•Atoms of a given element are identical in size, mass, &
other properties
•Atoms cannot be subdivided, created, or destroyed
•Atoms combine in simple whole-number ratios
•In a rxn, atoms are combined, separated, or rearranged
Law of Conservation of Mass
(Energy)
Matter is neither created nor destroyed.
 It only changes form.


If 5 g of element A combines with 10 g
of element B to form AB. How many
grams of AB are formed?
Law of Definite Proportion

A chemical compound contains the same
elements in exactly the same proportions
by mass regardless of the size of the
sample or the source of the compound
Table salt – NaCl 39.34 % Na & 60.66 % Cl
Law of Multiple Proportions

Two or more different compounds are
composed of the same two elements, the
masses of the 2nd element combine with a
certain mass of the 1st element can be
expressed as ratios of small whole
numbers.
Water vs. Peroxide
H2O
H2O2
Water vs. Hydrogen Peroxide
Discovery of the Electron
Cathode Ray tube
1. Different gases glow with different colors if a current
is passed through the tube.
2. Glass directly opposite the cathode glows.
3. An object placed in between will cast a shadow.
4. A paddle wheel will roll along on its rails from cathode
toward
the anode.
5. Rays are deflected away from a negative electrode.
6.
***Sir Joseph John Thomson – 1897 ***
Electrons are composed of negatively
charged subatomic particles.
Cathode Ray tube
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Development of Atomic Models
Robert A. Millikan

1909
American physicist showed that the mass
of the electron is 9.109 X 10 –31 kg
Confirmed the electrons carry a negative charge
and its mass.
Brought about more questions about the atomic structure.
1. Atoms are neutral
2. Atoms have mass
Robert Millikan: Oil Droplet Experiment
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Rutherford Experiment: Nuclear Atom
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Recording of Rutherford
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Discovery of the Atomic
Nucleus
Ernest Rutherford – 1911 Gold Foil
Experiment
‘As if you had fired a 15 – inch
shell at a piece of tissue paper
And it came back to hit you.’
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IN NUCLEUS
Protons - + charge
p+
Nuetrons – neutral
no
In The early 20th century, Rutherford
showed that most of an atom’s mass is
concentrated in a small positively
charged region called the nucleus.
Isotopes – atoms
of the same
element w/
different masses.
Electron cloud - 90% probability of
finding the electron within this space
Electron: - charge
( e-)
90 % probability of finding the
electron within this space
Bohr Model
After Rutherford’s discovery, Bohr
proposed that electrons travel in
definite orbits around the nucleus
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Structure of the Atom

Atom – is the smallest particle of an
element that retains the chemical
properties of the element
Atom has 2 regions:
Nucleus – protons & neutrons
Electron Cloud - Electrons
Weighing and Counting Atoms
Atomic number (Z) – number of protons in a nucleus
Mass Number – total number of protons & neutrons in a
nucleus
Atomic Mass – Weighted average mass of all the isotopes of the
element
Atomic Mass Units – amu 1/12 the mass of C-12
Charge = p+ - eMass # = p+ + no
Examples of PEN
Atomic number = number of protons
If atom is neutral, then the number of protons must
equal number of electrons.
Carbon Mass Number – Atomic Number = # of Neutrons
12
C
12
1. Hydrogen
4. Copper
6
-
6
= 6 neutrons
2. Sodium
5. Gold
3. Oxygen
Isotopes

Isotopes of Hydrogen include
Hydrogen -1
 Hydrogen – 2, Duterium
 Hydrogen – 3, tritium


They have the same number of protons
but different numbers of neutrons and a
different mass number
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isotopes
Average Atomic Mass – is the
weighted average of the atomic
masses of the naturally occurring
isotopes of an element.


Weighted Average of Isotopes
Copper–63 69.17%, Copper-65 30.83%,
Calculate the average atomic mass.
(63 x .6917) + ( 65 x .3083) =
43.5771 + 20.0395 = 63.6166
Carbon-12
98.90% , Carbon-13
1.1% 12.011