Download 1.2 Developing a Model of the Atom

1.1 Nature of Chemistry and
1.2 Atomic Structure
• Empirical knowledge: based on
observable phenomena that is observed
directly and is capable of being
experimented on.
• Theoretical knowledge is created to
explain observations based on ideas. It is
an attempt to rationalize things we cannot
see.
• Theories are constantly being revised to
make them fit the evidence that is being
evaluated.
• Theories aim to describe, explain, and
predict in the most concise manner
possible.
IUPAC
• International Union Pure and Applied
Chemistry
– Established in 1919
– Role is to help regulate standards and
procedures in chemistry
Early Models of
Matter
Democritus
• matter can be divided until you arrive at
the smallest piece possible
• called it the atom
• atoms are in constant motion
• empty space between atoms
Aristotle
• thought Democritus was out to lunch!
• believed all matter was derived of four
substance: earth, water, fire and air
• idea lasted for 2000 years
John Dalton
• “pool ball” model
• all matter is composed of tiny indivisible spheres
called atoms
• all atoms of an element have the same
properties
• atoms of different elements will have different
properties
• atoms of two or more elements can combine in
constant ratios to form new compounds
• atoms cannot be created, destroyed or
subdivided in a chemical change
J. J Thomson
• “raisin bun” model
• first to come up with the idea that an atom
consisted of positive and negative parts
• negatives were embedded in a positive
sphere
Hantaro Nagaoka
• He proposed a “Saturian or planetary
model" of the atom (1904).
• The model was based on principles
used to explain the stability of Saturn’s
rings.
– the rings are stable because the planet
they orbit is very, very massive.
Hantaro Nagaoka -Predictions
– a very massive nucleus (analogous
to a massive planet)
– electrons revolved around the nucleus,
bound by electrostatic forces (analogous
to the rings revolving around Saturn,
being bound by gravitational forces)
Ernest Rutherford
• “beehive” model
• gold foil experiment
• proved that the positive part of an atom
was at the centre
• there was a lot of empty space in an atom
• that some electrons could be found
amongst the empty space
James Chadwick
• proved the existence of neutrons
• neutrons are heavy particles with no
charge
Neils Bohr (1885- 1962) Danish
Physicist
Bohr’s Model
• Based on the element Hydrogen
• electrons orbit the nucleus at discrete
energy levels
• these energy levels are at a fixed
distance from the nucleus
• the electron travels along a three
dimensional path called an orbit
• these energy shells are designated by
the principal quantum number n
• n=1,2,3 …
• electrons can transition between levels
• in order to jump it must receive that
specific amount of energy that would
put it to the next energy level
• when an electron loses its gained
energy, it falls back to its original orbit
“ground state”
Assumptions
• an electron can travel indefinitely within
an energy level without losing energy
• that the greater the distance between
the nucleus of the atom and the energy
level, the greater the energy required for
an electron to travel in that energy level
• that an electron cannot exist between
orbits, but can move to a higher unfilled
orbit if it absorbs a specific quantity of
energy and to a lower unfilled orbit if it
loses energy
Experimental Evidence
• The basis for Bohr’s Theory came from
observing line spectra
• line spectrum: a pattern of distinct
lines, each of which corresponds to light
of a single wavelength, produced when
light consisting of only a few distinct
wavelengths passes through a prism or
spectroscope
• How spectroscopy works!
• This is the bright line spectrum for
hydrogen
Particle Location Charge
Mass
Symbo
l
Electron
Orbiting the
nucleus
Negative
9.11 × 10-31 kilograms
e-
Proton
Nucleus
Positive
1.67 × 10-27 kilograms
p+
Neutron
Nucleus
Neutral
1.67 × 10-27 kilograms
n0
Atomic Number and Mass Number
What is an atom's atomic number
(Z)?
• number of protons in the nucleus
• each element has a unique number that
identifies the number of protons
• Oxygen atoms contain 8 protons and
has an atomic number of 8
What is an atom’s mass number (A)?
• derived from the number of protons and
neutrons in an atoms nucleus.
• (N) = A- Z
The Mass of an Atom
• The mass of an atom is very small.
Scientists compare the mass of an atom to
that of a standard atom.
• As the mass of an atom is compared to that
of another reference atom it is referred to as
a relative atomic mass (Ar).
• The reference atom used to compare all
other atomic masses is the stable and
abundant isotope of carbon . . . carbon-12.
• The C-12 atom is said to be 12 unified
atomic mass units (u). Consequently,
one atomic mass unit is equivalent to the
mass of a nucleon (proton or neutron).
• Therefore, the atomic mass unit is defined
as 1/12 the mass of a carbon-12 atom