Greek Philosophers, Dalton's Atomic Theory

2. 585 B.C.
THALES of Miletus
 "Thales assures that water is the principle of all things;
and that God is that Mind which shaped and created
all things from water.“- Cicero
 Source of all things is WATER

3. 570-526 B.C.
ANAXIMENES
 Source of all things must be AIR or VAPOR

4. 490-430 B.C.
EMPEDOCLES
 Matter is made up of four elements
 EARTH, WATER, AIR and FIRE

5. 500-428 B.C.
ANAXAGORAS
 Nature is built up of an infinite number of minute
particles invisible to the eye.

6. 460-370 B.C.
DEMOCRITUS
1. All matter is composed of
“atomos”, which are bits of
matter too small to be seen.
2. There is a void, which is
empty space between atoms.
3. Atomos are completely solid.
4. Atomos are
homogeneous, with no
internal structure.
5. Atomos are different in their
sizes and their shapes

7. 384-322 B.C.
ARISTOTLE
 Rejected the idea of atomism of matter
 There is no limit to subdividing matter

8. John Dalton
1766-1844

9. DALTON’s Atomic Theory
ATOM
 Express idea of indivisibility
 Tiny indestructible sphere with mass
 Do not change during chemical reaction
 Atoms of a given element have the exactly same
properties, but atoms of different elements have
different properties.

10. DALTON’s Atomic Theory
 Each element is composed of extremely small particles
called atom.
 All atoms of a given element are identical; the atoms
of different elements are different and have different
properties, including different masses.

11.  Atoms of an element are not changed into different
types of atoms by chemical reaction; atoms are neither
created nor destroyed in a chemical reaction.
 Compounds are formed when atoms of more than one
element combine; a given compound always has the
same relative number and kind of atoms.

12.  Each element is composed of extremely small particles
called atom.

13.  All atoms of a given element are identical; the atoms of
different elements are different and have different
properties, including different masses.

14.  Atoms of an element are not changed into different types of
atoms by chemical reaction; atoms are neither created nor
destroyed in a chemical reaction.

15.  Compounds are formed when atoms of more than one
element combine; a given compound always has the same
relative number /ratio and kind of atoms.

16. Exercise
1. When copper was exposed to air and heated, it was
found out that the black compound formed has a
copper-to-oxygen ratio of 4:1.the experiment was
repeated several times and gave the same results.
Explain this finding based on Dalton’s atomic theory.

17. 2. A sample of calcium carbonate was mixed with a
certain volume of hydrochloric acid, enough to
completely react with the calcium carbonate. The
container was tightly sealed and the total mass
before and after the reaction was found constant.
Explain this observation based on Dalton’s atomic
theory.

18. 3. It was found that there are two oxides of copper. One
is black with a copper-to-oxygen mass ratio of 4:1 and
melting point of 1326 C. the other is red with copper-
to-oxygen ratio of 8:1 and melting point of 1235 C.
explain these observations based on Dalton’s
assumptions.

20. What makes compounds different?

21. Law of Constant Composition
 1799 Joseph Proust
 a chemical compound contains the same elements in
exactly the same proportions (ratios) by mass
regardless of the size of the sample or source of the
compound

22.  For example, water always consists of oxygen and
hydrogen atoms, and it is always 89 percent oxygen by
mass and 11 percent hydrogen by mass

23. Does mass change during a
chemical reaction?

24. Law of Conservation of Mass
 Lavoisier heated a measured amount of mercury to
form the red oxide of mercury. He measured the
amount of oxygen removed from the jar and the
amount of red oxide formed. When the reaction was
reversed, he found the original amounts of mercury
and oxygen.

25. Law of Conservation of Mass
 1744 Antoine Lavoisier
 matter can not be created or destroyed in ordinary
chemical or physical changes.
 the mass of the reactants (starting materials) equals
the mass of the products
2Mg (s) + O2 (g) → 2MgO (s)
48.6 g 32.0 g 80.6 g

26. Example
 10 grams of CaCO3 on heating gave 4.4g of CO2 and 5.6
of CaO. Show that these observations are in agreement
with the law of conservation

27. Law of Multiple Proportions
 1803 John Dalton
 States that when two elements combine to form
more than one compound, the masses of one
element which combine with a fixed mass of the
other element are in ratios of small whole numbers

28. Example:
 Carbon monoxide (CO): 12 parts by mass of carbon
combines with 16 parts by mass of oxygen.
 Carbon dioxide (CO2): 12 parts by mass of carbon
combines with 32 parts by mass of oxygen.
 Ratio of the masses of oxygen that combines with a
fixed mass of carbon (12 parts) 16: 32 or 1: 2

29.  Water has an oxygen-to-hydrogen mass ratio of 7.9:1.
 Hydrogen peroxide, another compound consisting of
oxygen and hydrogen, has an oxygen-to-hydrogen
mass ratio of 15.8:1.
 Ratio of the masses of oxygen that combines with a
fixed mass of hydrogen is 7.9: 15.8 or 1: 2

30. Exercise