Chapter 3 Notes

1
Chapter 3Notes
2

• Atoms- Smallest particle of an element that
  retains the chemical identity of
• that element.
• Principles of chemical behavior
• Lavoisier Law of Conservation of Matter
• Proust Law of Constant Composition
• a compound always contains the same elements in
  the same proportions by mass.

3
John Daltons Atomic Theory of Matter

• elements are composed of atoms
• all atoms of an element are identical, but
  different from atoms in other elements
• atoms are neither created nor destroyed
• a given compound always has the same relative
  number and kind of atoms.

4
Michael Faraday

• atoms contain particles that have an electrical
  charge

5
Ben Franklin

• studied electricity
• he determined the following
• there are 2 kinds of charge positive and negative
• 2 like charges repel each other
• opposites charges attract each other
• excess negative charge can be discharged as
  static electricity

6
Ben Franklin

• Do you remember Ben Franklins famous kite
  experiment?

7
J.J. Thomson

• called the negative particles electrons
• determined the charge to mass ratio of an
  electron
• The Plum Pudding Model is Thomsons name for his
  model of the atom

8
JJ Thomsons Plum Pudding Model of the Atom

• www.tamucc.edu

9

• Cathode Ray Tube (CRT)
• negative end is the cathode
• positive end is the anode
• A cathode ray is radiation streaming from a
  cathode to an anode in a CRT
• it is a stream of particles
• a magnet can deflect the ray
• cathode ray particles have a
• negative charge

10

• http//www.chem.uiuc.edu/clcwebsite/cathode.html

11
Robert Millikan

• measured the charge of an electron using the oil
  drop experiment.
• x-rays gave the oil a negative electron
• 1.60x10 -19 coulomb is the charge of an electron
• using Thomsons charge to mass ratio, he
  determined the mass of the electron is 9.11x10
  -28 g

12
Robert Millikans Oil Drop Experiment

• Robert Millikan
• received the Nobel
• Prize for his work
• www.68pair.com

13
Henri Becquerel

• discovered that uranium
• exhibits radioactivity
• the chemical properties
• of an element change as
• it gives off radiation

14
Ernest Rutherford

• alpha particles have a 2 charge
• beta particles are high
• speed electrons
• gamma rays are not
• composed of particles

15
Rutherford

• Gold Foil Experiment (alpha scattering)
• he determined that an atoms positive charge, and
  most of its mass, was concentrated in the core
• (most of the atom is empty space)
• he named the core of the atom
• the nucleus

16
Rutherfords Gold Foil Experiment

• Over 98 of the particles went straight through
• Approx. 2 of the particles were deflected
• Approx. 0.01 of
• the particles bounced
• off the gold foil
• www.sci.tamucc.edu

17
Rutherfords Nuclear Model

• The atom contains a tiny dense center called the
  nucleus
• The nucleus is essentially the entire mass of the
  atom
• The nucleus is positively charged
• The amount of positive charge of the nucleus
  balances the negative charge of the electrons
• The electrons move around in the empty space of
  the atom surrounding the nucleus
• www.sci.tamucc.edu

18
Modern Atomic Theory

• atoms are composed of protons, neutrons and
  electrons
• Recently, scientists discovered these particles
  contain particles (i.e. quarks, gluons)
• A nucleus is a positively charged central core
  containing protons and neutrons
• An individual proton has a charge that is equal
  to the charge of an electron, but the charge is
  positive
• In a neutral atom the number of protons
  equals the number of electrons
• a proton is much more massive than an electron

19
Charge and Mass

• a protons charge is 1
• an electrons charge is -1
• mass is expressed in atomic mass units (amu)
• the mass of a proton is approx. 1 amu
• the mass of a neutron is approx. 1 amu
• the mass of an electron is
• approx. 0 amu

20
Henry Mosely

• an atoms identity comes from the number of
  protons in its nucleus
• the number of protons is called the atomic number
• every element has a unique atomic number
• the atomic number is written above the chemical
  symbol on the periodic table
• since atoms are electrically neutral, the number
  of positive charges equals the number of negative
  charges
• the number of neutrons equals the mass number
  minus the atomic number
• of no mass - atomic

21
IONS

• when an atom loses or gains one or more electrons
  it acquires a net charge and is called an ion
• if there are more electrons than protons, the ion
  has a negative charge
• charge of ion ( of p) - ( of e-)
• charge can be written as 2 or 2
• place the charge to the upper right corner of the
  chemical symbol

22
ISOTOPES

• atoms that have the same number of protons, but
  different numbers of neutrons
• the chemical properties of an element depend
  primarily on its electrons and protons
• isotopes of the same element, with different
  numbers of neutrons have essentially the same
  chemical properties
• the major difference between 2 isotopes is their
  mass
• isotopes with more neutrons have higher mass than
  those with fewer neutrons
• mass (isotopes of p) (isotopes of
  n0)
• to identify an isotope you add the mass number
  after the elements name (i.e. chlorine -37)
• number of neutrons mass - atomic

23
MASS OF AN ATOM

• amu can be defined as the mass of a proton or
  neutron
• Generally mass of an atom
• number of protons number of neutrons
• an amu is defined in terms of an arbitrary
  standard
• a carbon-12 atom
• scientists set the mass of a carbon-12 atom to
  be exactly equal to 12 amu
• therefore, one amu is 1/12 of the mass of a
  carbon-12 atom
• 1 amu 1/12 (mass of 126C atom) 1.66 x 10-24
  g

24
MASS OF AN ATOM

• when determining the mass of a large number of
  atoms
• remember that elements have different isotopes
• determine the mass of an average elements atom
• the isotopes of some elements are found in
  constant ratios
• these rations are called fractional abundances
• atomic mass is the average mass of an elements
  atoms
• the atomic mass for each element is written on
  the periodic table below the element symbol

25
RADIOACTIVITY

• nuclear reactions change the composition of an
  atoms nucleus.
• alpha, beta and gamma radiation are produced by
  nuclear reactions
• alpha and beta radiation consist of particles
  that are emitted from the nucleus
• Radioactivity is the spontaneous emission of
  radiation from an atom
• atoms with stable nuclei are not radioactive

26
PURPOSE OF NEUTRONS

• not all combinations of protons and neutrons
  create a stable nucleus
• protons in the nucleus should repel each other,
  but there is a strong nuclear force (that is an
  attractive force) that holds the nucleus together
• the nuclear force is only strong between
  subatomic particles that are extremely close
• the presence of neutrons in the nucleus adds a
  net attractive force to the inside of the nucleus
• think of neutrons as the glue that holds the
  nucleus together

27
STABLE OR NOT?

• elements with
• atomic number 1-20 stable nuclei have almost
  equal numbers of protons and neutrons
• atomic number 21-83 nuclei need more neutrons
  than protons to be stable
• atomic number gt83 no number of neutrons is
  sufficient to hold the nucleus together
  indefinitely.
• All nuclei with atomic numbers greater than 83
  are radioactive.

28
UNSTABLE NUCLEI

• nuclei are unstable if they contain too few or
  too many neutrons
• nuclei that have excess neutrons are likely to
  emit beta radiation
• Generally, isotopes that are much heavier
  (contain more neutrons) or much lighter (contain
  fewer neutrons) than the most common isotope are
  likely to be radioactive.

29
TYPES OF RADIOACTIVE DECAY

• radioactive elements emit different kinds of
  radiation (three types are alpha, beta and gamma)
• these types of radiation can be distinguished
  by their
• - charge
• - mass
• - penetrating power

30
ALPHA PARTICLES

• consist of a stream of high-energy alpha
  particles
• alpha particle consists of 2 protons and 2
  neutrons
• (it is identical to the helium-4 nucleus)
• these particles do not have much penetrating
  power
• these particles travel only a few centimeters
  in air and can be stopped by paper or clothing
• not normally a health hazard

31
BETA RADIATION

• consists of a stream of high-speed electrons
• the electrons come from changes in the nucleus
• in the process that produces beta radiation, a
  neutron changes into a proton and an electron
• the proton remains in the nucleus and the
  electron (now the beta particle) is propelled out
  of the nucleus at high speed
• the mass number for a beta particle is zero
  because an electron has a very small mass
  compared with a proton or neutron
• beta radiation is approx. 100 times more
  penetrating than alpha
• it can penetrate clothing and harm skin

32
GAMMA RAYS

• gamma rays are not particles
• they are a very energetic form of light that we
  cannot see
• it can penetrate deeply into solid material (it
  can only be stopped by concrete or lead)

33
RADIOACTIVE DECAY

• Radioactive Decay occurs when an atom emits alpha
  or beta particles or gamma rays.
• The term decay is used because the original
  nucleus decomposes (decays) to form a new
  nucleus, releasing radiation in the process.
• Use a nuclear equation to show the radioactive
  decay.