Download Chapter 4 Notes

Chemistry – Atomic structure &
Nuclear Chemistry
Chapter 4 & 25
History of the Atom
• Democritus (400 BC) – Named the atom as
indestructible and indivisible.
– His ideas of the atom did not explain chemical
behavior.
• John Dalton (1766-1844)
– Studied the ratios in which elements combine.
– Discovered the real nature of atoms and the
connection between observable changes.
Dalton’s Atomic Theory
Dalton’s Atomic Theory
• 5 parts (in 1906)
1. All matter is made of atoms
2. **All atoms of the same element are identical in
mass, volume, and properties.
3. Atoms can’t be created, destroyed, or divided.
4. Atoms combine in small, whole number ratios
5. In chemical reactions, atoms are combined,
separated or rearranged, NOT turned into
another element.
Sizing up the atom
• A coin the size of a
penny has 2.4 x 1022
atoms of pure copper.
(all atoms act like copper)
• Earth’s population is 6 x 109.
• Despite their small
size, atoms are
observable with
scanning tunneling
microscopes.
More History
• In 1897, Thompson discovers the electron
using a Cathode Ray Tube.
– A Cathode Ray Tube charge is negative (-). A
magnetic field pushed the stream away.
– The electron is discovered!!!!
• Atoms redefined: Smallest particle of an
element that retains its properties.
More History
• Discovery of the Nucleus
– Rutherford (1909) discovered the nucleus in his
Gold foil Experiment.
– He shot alpha particles (Helium) through gold and
determined:
• The nucleus is a very densely packed bundle of matter
with a positive charge.
• That atoms are mostly made of empty space.
Atomic Structure
• Electrons: Negatively charged, outside the
nucleus of the atom.
– Mass is 1/1840 the mass of a hydrogen atom.
• Nucleus- Tiny central core of the atom
consisting of Protons & Neutrons.
• Protons- Positively charged particles.
– Each proton is about the size of a hydrogen atom.
• Neutrons – Particles with no charge, but equal
to the size of a proton.
Distinguishing Among Atoms
• Atomic Number
– Equal to the number of protons in the nucleus in
an atom of that element.
• Almost all elements are electrically neutral,
therefore the number of electrons equals the
number of protons.
• Mass Number
– The total number of protons and Neutrons.
– Always a whole number. Never a decimal.
Distinguishing Among Atoms
• Determining # of Neutrons
# of neutrons = mass number – atomic number
• Example: Gold has a mass number of 197 and
an atomic number of 79. How many neutrons
does Gold have?
Whole Numbers of e-, n◦, p+
• There can only be a whole number of electrons,
neutrons, and protons in a substance.
• The elements are written in either of these 2
ways:
108
𝐴𝑔 : 108 = Mass #;
47 = Atomic #
47
– Neutrons = (mass # - Atomic #) = (108 – 47) = 61
– Ag – 108. Means the same as above.
–
• Silver always has 47 protons, so it is unnecessary to write it
in the above format.
Isotopes
• Isotopes:
– Atoms that have the same number to protons, but
different numbers of neutrons. This will also
create different mass numbers.
• Examples:
108
109
111
–
Ag ,
Ag ,
Ag or Ag-108, Ag-109, Ag-111
47
47
47
– These are still all Silver and have all of the same
properties and characteristics as Silver.
AMU’s
An Atomic Mass Unit (amu) is defined as 1:12
the mass of a carbon-12 atom.
Any proton or neutron is equal to 1 amu’s.
The numbers on the periodic table are written
as amu’s in decimal form.
Atomic Mass
• Atomic mass of an element is a weighted
average mass of the atoms in a naturally
occurring sample of the element.
• Example: Silver has an atomic mass of 107.87.
– This means that a sample of Silver will have some
Ag-107, Ag-108, Ag-109, etc., and that the average
of those atoms is 107.87
Calculating Atomic Mass
• In order to calculate the atomic mass of an
element you must know:
a) The number of stable isotopes
b) The mass of each isotope
c) The natural abundance (percentage) of each
isotope.
Calculating Atomic Mass
• Carbon has two stable isotopes: Carbon-12,
which has a natural abundance of 98.89%, and
carbon-13, which has a natural abundance of
1.11%.
• Atomic Mass of carbon =
=(12.000 amu x 0.9889) + (13.003 amu x 0.0111)
= 12.011 amu
• This is why there are decimals on the periodic table.
Periodic Table
• Elements are listed in order by Atomic #, which
is the # of protons.
• The # of protons defines the element.
• The mass of elements can change: Isotopes.