Atomic History

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Atomic History
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more simplified or more complex over time???
Models
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The existence of atoms

• The Greek philosopher Democritus (460 B.C. 370
  B.C.) was among the first to suggest the
  existence of atoms
• Atoms comes from the Greek word atomos
• He believed that atoms were indivisible and
  indestructible

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Daltons tiny indivisible, indestructible atom

• He studied the results of experiments by
• Lavoisier Law of Conservation of Matter,
• Proust Law of Definite Proportions,
• and many others.

He concluded, like the ancient Greeks, that atoms
are tiny, round hard spheres, like billiard balls.
John Dalton (1766 1844)
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Daltons Atomic Theory

• All elements are composed of tiny indivisible and
  indestructible particles called atoms
• Atoms of the same element are identical. Atoms
  of an element are different from those of any
  other element.

• Atoms of different elements combine in the same
  whole-number ratios to form compounds
• In chemical reactions, atoms are combined,
  separated, or rearranged but never changed into
  atoms of another element.

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Daltons Atomic Theory

• His theory was widely accepted at the time, since
  it supported current research and widely accepted
  ideas Democritus, Lavoisier, and Proust
• However, a new question was raised If elements
  are made up of atoms, why so many elements?

Then came J.J. Thomsons
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discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to
deduce the presence of a negatively charged
subatomic particle the electron
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Cathode-Ray Tube Experiment
J.J. Thomson

• When the negative end of a magnet was brought
  near the cathode ray tube, the subatomic
  particles repelled
• the subatomic particles then must be negatively
  charged.

• These subatomic particles were called electrons.
• The existence of the electron proved Daltons
  solid ball model was not accurate.

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J.J. Thomson
Thomson believed that the electrons were like
plums embedded in a positively charged pudding,
thus it was called the ______ _________ model
or the chocolate chip cookie dough model.
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Some ModernCathode Ray Tubes

• Cathode ray tubes pass electricity through a gas
  that is contained at a very low pressure.

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Rutherfords problem
In the following pictures, there is a target
hidden by a cloud. To figure out the shape of the
target, beams were shot into the cloud it was
recorded where the beams came out. Can you figure
out the shape of the target?
Target 2
Target 1
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Ernest RutherfordsGold Foil Experiment - 1911

• Alpha particles (positive particles) were fired
  at a thin sheet of
• gold foil

• Particles that hit on the detecting screen
  (film) are recorded

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What he expected







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What really happened

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Rutherfords Findings

• Most of the particles passed right through
• A few particles were deflected
• VERY FEW were greatly deflected

Like howitzer shells bouncing off of tissue
paper!
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Rutherfords Findings
Negative electrons move around outside the
nucleus

• Conclusions
• Atom is mostly empty space
• The nucleus is small
• The nucleus is dense
• The nucleus is positively charged
• Downfall
• It did not explain the chemical properties of the
  elements. A better description of the
• electron behavior
• was needed

Small, dense nucleus at the center of the
atom with a positive charge
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Niels Bohrs Model (1913)

• What keeps the electrons from falling into the
  nucleus?

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Bohr Model of the Atom
I pictured the electrons orbiting the nucleus
much like planets orbiting the sun.
The electrons are found in specific circular
paths, or orbits around the nucleus. Each orbit
has a distinct energy level.
Niels Bohr
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Bohrs model

• Energy level of an orbit (or electron in a orbit)
• analogous to the rungs of a ladder
• The electron cannot exist between energy levels,
  just like you cant stand between rungs on a
  ladder
• The farther away from the nucleus the energy
  level, the greater the energy

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Model of atom
An atom can only be represented by a model since
the electrons are in constant random motion.
3rd energy level _____ (can hold 8 electrons)
2nd energy level _____ (can hold 8 electrons)
1st energy level _____ (can hold only 2
electrons)
Nucleus
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Model Atom of Sulfur (S)
Atomic number 16 Determine total number of
electrons
4th ____
3rd 6e-
2nd 8e-
1st 2e-
Nucleus
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Model Atom of Potassium
Try one on your own Atomic number ___ Total
number of electrons ___
4th ____
3rd ____
2nd ____
1st ____
Nucleus
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Electrons

• The number of electrons in the highest
  (outermost) energy level determines the behavior
  and characteristics of the element
• Electrons in the highest energy level (valence
  energy level) are valence electrons

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Electron Dot Diagrams

• show valence e- as dots
• distribute one dot on each side of symbol (N, E,
  W, S) traveling clockwise around the element
  symbol
• Distribute second dot on each side of symbol (N,
  E, W, S) traveling clockwise around the element
  symbol
• Dots should not exceed two on each side ? for a
  total of 8

Ex. Sulfur
Try these
S
C
Al
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Bohrs model Absorption and Emission

• Electrons cannot exist between energy levels, but
  they can jump from one level to another.

• When energy is added, the atoms will absorb
  energy to excite the electrons to higher energy
  levels.
• As the excited electron returns to the normal
  ground state, a particle of light called a
  photon is emitted

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Bohrs model Absorption and Emission

• Both the absorption and emission of energy
  (photons) are quantized
• only a discrete amount of energy packet can be
  released or absorbed

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Electromagnetic Spectrum

• The emitted photon determines the color of the
  light observed when energy is added

?--------------------------Increasing Energy
-------------------------------- -----------------
----------Increasing Wavelength
-------------------------?
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Flame Tests

• Every element emits a characteristic light
• The wavelength of light given off by the electron
  can be used to identify an element

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Flame Tests
?Strontium?Potassium?Barium?Copper
Downfalls of flame tests?
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Spectral Lines

• When the emitted light is observed through a
  diffraction grating/prism (funky glasses), the
  light is separated into the component wavelengths
  

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Flame tests and Spectral Lines
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Chadwicks Neutron

• In 1932, he discovered the neutron.
• First suggested by Rutherford

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The Quantum Mechanical Model

• Energy is quantized - It comes in chunks.
• A quantum is the amount of energy needed to move
  from one energy level to another.
• Since the energy of an atom is never in between
  there must be a quantum leap in energy.
• In 1926, Erwin Schrodinger derived an equation
  that described the energy and position of the
  electrons in an atom

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Subatomic Particles
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Atomic Number

• Atomic number (Z)
• on periodic table increasing by 1
• The atomic number of an element is ALWAYS
  equal to the number of protons

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Mass Number (A)
Mass number is the number of protons and neutrons
in the nucleus of an isotope
Mass p n0
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8
8
18
Arsenic
75
33
75
Phosphorus
16
15
31
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Complete Symbols

• Contain the symbol of the element, the mass
  number and the atomic number.

Mass number
X
Superscript ?
Atomic number
Subscript ?
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Symbols

• Find each of these
• number of protons
• number of neutrons
• number of electrons
• Atomic number
• Mass Number

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Br
35
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Symbols

• If an element has an atomic number of 34 and a
  mass number of 78, what is the
• number of protons
• number of neutrons
• number of electrons
• complete symbol

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Symbols

• If an element has 91 protons and 140 neutrons
  what is the
• Atomic number
• Mass number
• number of electrons
• complete symbol

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Symbols

• If an element has 78 electrons and 117 neutrons
  what is the
• Atomic number
• Mass number
• number of protons
• complete symbol

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Sizing up the Atom

• Elements are able to be subdivided into smaller
  and smaller particles these are the atoms, and
  they still have properties of that element
• If you could line up 100,000,000 copper atoms in
  a single file, they would be approximately 1 cm
  long

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Isotopes

• Dalton was wrong about all elements of the same
  type being identical
• Atoms of the same element can have different
  numbers of neutrons.
• Thus, different mass numbers.
• These are called isotopes.

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Naming Isotopes

• We can also put the mass number after the name of
  the element
• carbon-12
• carbon-14
• uranium-235

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Isotopes are atoms of the same element having
different masses, due to varying numbers of
neutrons.
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Isotopes
Elements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that
differ in the number of neutrons.
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Atomic Mass

• How heavy is an atom of oxygen?
• It depends, because there are different kinds of
  oxygen atoms.
• We are more concerned with the average atomic
  mass.
• This is based on the abundance (percentage) of
  each variety of that element in nature.
• We dont use grams for this mass because the
  numbers would be too small.

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Measuring Atomic Mass

• Instead of grams, the unit we use is the Atomic
  Mass Unit (amu)
• It is defined as one-twelfth the mass of a
  carbon-12 atom.
• Carbon-12 chosen because of its isotope purity.
• Each isotope has its own atomic mass, thus we
  determine the average from percent abundance.

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To calculate the average

• Multiply the atomic mass of each isotope by its
  abundance (expressed as a decimal), then add the
  results.
• If not told otherwise, the mass of the isotope is
  expressed in atomic mass units (amu)

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Atomic Masses
Atomic mass is the average of all the naturally
occurring isotopes of that element.
Carbon 12.011
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- Page 117
Question
Knowns and Unknown
Solution
Answer
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The Periodic TableA Preview

• A periodic table is an arrangement of elements
  in which the elements are separated into groups
  based on a set of repeating properties
• The periodic table allows you to easily compare
  the properties of one element to another

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The Periodic TableA Preview

• Each horizontal row (there are 7 of them) is
  called a period
• Each vertical column is called a group, or family
• Elements in a group have similar chemical and
  physical properties
• Identified with a number and either an A or B
• More presented in Chapter 6

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Mass of the Electron
Mass of the electron is 9.11 x 10-28 g
The oil drop apparatus

• 1916 Robert Millikan determines the mass of the
  electron
• 1/1840 the mass of a hydrogen atom
• has one unit of negative charge
• Charge on an electron (qe) 1.602 x 10-19 C