fundamentals of Chemistry

1. CAPE Chemistry Unit One
Module One (1)
Fundamentals of Chemistry
Elaine Williams – Tutor
Undergrad Group of Schools
2010

2. The History of the Structure of the
Atom
The Greek Philosopher
Demecritus (460-370 BC)
believed that matter was composed of
"atoms", which are physically indivisible;
that between atoms lies empty space; that
atoms are indestructible; have always
been, and always will be, in motion; that
there are an infinite number of atoms, and
kinds of atoms, which differ in shape, and
size.

3. The History of the Structure of the
Atom
 The Irish born Englishman,
Robert Boyle (1627-1691) was
the first to Scientifically define an
element as a substance that
could not be broken down
further.
 He also hypothesized that many different
elements might exist

4. The History of the Structure of the
Atom
 English Joseph Priestly (1733-1804)
prepared and isolated the gas OXYGEN
and showed that is was the key substance
involved in combustion

5. The History of the Structure of the
Atom
French Antoine Lavoisier
(1743-1794) showed soon
after that by careful
measurements when combustion is
carried out in a closed container, the mass
of the combustion products is exactly
equal to the mass of the starting products
(The Law of Conservation of Mass and
Energy)

6. The Theory of the Conservation of
Mass and Energy
 Neither mass nor energy can be
created or destroyed in chemical
reactions but can change its form.
 The total quantity of matter and
energy available in the universe is
a fixed amount and never any more
or less.

7. The History of the Structure of the
Atom
French Chemist Joseph Proust
(1754-1826) was led based on
the premis to develop a second
fundamental principle now
known as The Law of Definite
Proportions so elements
combine chemically in specific
proportions all the time.

8. The Law of Definite
Proportions
 Different samples of a pure
chemical substance always
contains the same proportion
of elements by mass

9. The History of the Structure of the
Atom
 John Dalton (1766-1844) published a new
theory of matter now known as the Atomic
Theory from which the Law of Multiple
Proportions arose

10. The Law of Multiple Proportions
 If two elements combine in
different ways to form different
substances, the mass ratios
are small whole number
multiples of each other

11. The Atomic Theory
 Elements are made of tiny particles called
ATOMS
 Each element is characterized by the
mass of its atoms and eventually chemical
properties
 A chemical reaction consists of
rearranging atoms from one combination
to the next in whole number rations so the
atoms remain intact

12. The Atomic Theory Modified
 Some elements like Uranium can be split
 Some elements have atoms with differing
masses (isotopes)
 Some atoms may be found as fractions
especially during nuclear reactions

13. The History of the Structure of
the Atom
 These theories still did not give the
structure of the atom
 Was it a tiny compact ball?
 A square ?
 empty like a balloon?
 Like starlight?

14. The Atomic Particles
 Around the year 1900, Physicists began to find
evidence that atoms were made of smaller
particles
 The English Physicist J.J. Thompson (1856-
1940) did experiments involving the use of
cathode ray tubes to help determine what the
atom was really made of.

15. The Cathode Ray Oscilloscope
 Physicists in the 19th century found out
that if they constructed a glass tube with
wires inserted in both ends, and pumped
out as much of the air as they could, an
electric charge passed across the tube
from the wires would create a fluorescent
glow on zinc suphide coating. This
cathode ray also became known as an
‘electron gun’.

16. The Cathode Ray Oscilloscope

17. The Cathode Ray Oscilloscope
 Because the beam is produced at a
negative electrode, and deflected toward
the positive plate, Thompson proposed
that the cathode ray was made of
negatively particles he called electrons
which means “atom of electricity”

18. The Cathode Ray Oscilloscope
 Because electrons can be
emitted from electrodes made
of many different metals, it was
postulated that all these
different substances must
contain electrons.

19. The Cathode Ray Oscilloscope
 Thompson also postulated that
 The amount of deflection of an electric
beam depends on 3 factors
 the strength of the deflecting
magnetic or electric field
 The size of the negative charge on
the electron
 The mass of the electron

20. The Cathode Ray Oscilloscope
 By carefully measuring the amount of
deflection caused by electric and magnetic
fields of known strength, Thompson was
able to calculate the ratio of the electrons
electrical charge to its mass:
 e/m = 1.758819 x 108 C/g
 This does not determine if the electron
was fluid or particulate in nature.

21. Millikan’s Oil Drop Experiment
 Conclusive proof that electrons did have a
particulate nature was finally obtained by
Millikan (1868 – 1953) who used an oil
drop experiment

22.  A fine mist of oil was sprayed into a box
 The tiny droplets were allowed to fall
between two horizontal plates
 Observation of the spherical drops through
a telescopic eyepiece made it possible to
determine how rapidly they fell through the
air, which allowed their masses to be
calculated.
Millikan’s Oil Drop Experiment

23. Millikan’s Oil Drop Experiment

24. Millikan’s Oil Drop Experiment
 The drops were then given a negative
charge by irradiation with X-rays.
 By applying a voltage to the plates, with
the upper plate positive, it was possible to
counteract the downward fall of the
charged drops and keep them suspended.

25. Millikan’s Oil Drop Experiment
 Knowing voltage e/m and e, Millikan was
able to prove that charge on any given
drop was always a small whole number
multiple of e, whose modern value is
 1.602177 x 10 -19 C.
 Substituting this into Thompson charge to
mass ratio, this then given the mass (m) of
the electron as 9.019390 x 10-28g

26. Millikan’s Oil Drop Experiment
 Because matter is electrically neutral
overall, the fact that atoms in an electrode
can give off negatively charged particles,
(electrons), this must mean that these
same atoms also contain positively
charged particles.

27. Ernest Rutherford and Alpha
Particles
The positively charged particles
were proven by Ernest
Rutherford (1871-1937).
 Rutherford directed a beam of alpha
particles as a thin gold foil. He found that
most particles passed through, but a
minute amount was deflected and a few
bounced back toward the particle source.

28. Ernest Rutherford and the Gold Foil
Experiment

29. Ernest Rutherford and the Gold Foil
Experiment
 Based on his results, he was able to deduce
that the mass and positive charge must be
concentrated in a tiny fraction of the atom he
called the NUCLEUS.

30. Finding the Neutrons
 James Chatwick in 1934 found
evidence for neutral particles
within the nucleus.
He bombarded Beryllium with
Alpha particles and showed that
the nucleus was very dense
(composed of p &n).
Neutrons are almost identical in
mass to protons but carry no
charge.

31. Summary of Characteristics of Sub-
Atomic Particles
Particle Charge/C Relative
Charge
/e
Mass/Kg amu
Proton +1.6022 x 10-19 +1 1.6726 x 10-27 1.0073
Neutron 0 0 1.6750 x 10-27 1.0087
Electron -1.6022 x 10-19 -1 9.1095 x 10-31 5.4858 x 10-4