Democritus (460 BC 360 BC) Ancient Greek philosopher No
experiments performed! Major Contribution: The Atom He proposed
that everything was made of these atoms and they were all
indivisible Was his theory correct? No! There are subatomic
particles!
Slide 4
John Dalton (1766-1844) Major Contribution: Atomic Theory
(1808) This began the modern era of chemistry Four Principles:
Elements are composed of indivisible particles called atoms. All
atoms of a given element are identical.
Slide 5
John Dalton Compounds are composed of atoms of one or more
elements, and will form only in whole-number ratios. This is called
the Law of Multiple Proportions i.e. H 2 O exists, while H 2.35 O
does not A chemical reaction involves the combination, separation,
or rearrangement of atoms, not their creation or destruction This
is called the Law of Conservation of Mass Was his theory correct?
Mostly! Parts 1 & 2 have problems!
Slide 6
John Dalton When we attempt to conceive the number of particles
in an atmosphere [gas], it is somewhat like attempting to conceive
the number of stars in the universe; we are confounded with the
thought. But if we limit the subject, by taking a given volume of
any gas, we seem persuaded that the number of particles must be
finite - John Dalton on his approach to the theory of atoms,
1808
Slide 7
J.J. Thomson (1856-1940) Major Contribution: The Electron
Cathode Ray Tube Experiment (1897) Nobel prize (1906)
Slide 8
J.J. Thomson
Slide 9
Thomsons Atomic Model Also known as the Plum Pudding Model Was
his theory correct? No! Missing parts of atom!
Slide 10
Ernest Rutherford (1871-1937) Two Major Contributions: The
nucleus The atom is mostly empty space Gold Foil Experiment (1910)
Nobel prize in Chemistry (1908)
Slide 11
Ernest Rutherfords Gold Foil Experiment
Slide 12
Rutherfords Atomic Model Was his theory correct? Mostly!
Missing neutrons and location of electrons!
Slide 13
Niels Bohr (1885-1962) Major Contribution: Planetary Model of
the Atom Nobel Prize in Physics (1922) for spectrum of hydrogen
Atomic Line Spectra Bohr observed that when light was given off
from an atom, there were only single lines visible Bohr proposed
that each line represented an electron in a different orbit
Slide 14
Atomic Line Spectra
Slide 15
Bohrs Atomic Model Electrons Nucleus
Slide 16
Current Theory of the Atom Many scientists contributed to
developing quantum mechanics, which is the current model of the
atom Known as the electron cloud model The cloud is an area of
probability where the electron is found These electrons, moving at
extremely high speeds, effectively occupy the entire area of the
cloud, the same way that moving fan blades effectively occupy the
entire area through which they pass.
Slide 17
Current Model of the Atom: Probability cloud where electrons
found Nucleus
Slide 18
Parts of the Atom Proton Positive Nucleus Neutron Neutral
Nucleus Electron Negative Orbitting Nucleus
Slide 19
Atomic Number Atoms are identified by their number of protons
This is referred to as their atomic number Think of atomic # like a
social security card for each element
Slide 20
In atoms that have a neutral charge, the numbers of electrons
equals the number of protons When an atom gains a charge, it is
called an ion Sodium Ion (Na + ) 11 Protons10 Electrons Sodium Atom
(Na) 11 Protons11 Electrons
Slide 21
Chlorine Ion (Cl - ) 17 Protons18 Electrons Chlorine Atom (Cl)
17 Protons17 Electron
Slide 22
Mass Number The mass of an atom is the number of protons plus
the number of neutrons This is referred to as mass number The mass
of protons and neutrons are equal
Slide 23
Why is the electron not part of the mass? It takes roughly 1800
electrons to equal the mass of 1 proton, so it is left out. If we
wanted the mass of an elephant, would we weigh the flies buzzing
around it? Doesnt count!
Slide 24
The periodic table does not give the mass number, but always
the atomic number For simplicity, we round the number on the
periodic table to get the mass number Round this number to whole
number
Slide 25
Isotopes are elements that have the same number of protons, but
contains a different number of neutrons Compare: carbon-12 vs.
carbon-14 The number indicates the mass number Both contain the
same number of protons (6), so carbon-14 must have two extra
neutrons
Slide 26
The masses given on the periodic table are an average of all
the isotopes on the planet We refer to the masses on the periodic
table as the average atomic mass of an element This explains why
the atomic masses are not whole numbers it is an average!