Chapter 2
Ancient Greek and Roman concepts of Nature
1. Continuists – Aristotle
2. Atomists – Leucippus, Democritus & Lucretious
Dark & Middle Ages – Little change
Late Middle Ages – Reformation & Renaissance
• Beginnings of Modern Science – Experimentation and
questioning.
• Modern concept of elements – Robert Boyle -1661
Two new LAWS:
1. Law of Conservation of Mass ––The total mass of
substances during a chemical or physical change does
not change. Discovered by Antoine Lavoisier in late
18th Century
2. Law of Definite Proportions – Discovered by
Joseph Proust in 1799 – All samples of a compound
always have the same ratio of elements no matter
where it is obtained from.
Explained by English School Teacher, John Dalton
Modern Atomic Theory: John Dalton –1803
1. All matter made up from tiny, indestructible, indivisible
spheres called atoms.
2. Atoms of an element are all identical. Atoms of
different elements are different.
3. Compounds are formed when atoms combine
together in small, whole number ratios to form
molecules. This ratio is always the same for any
compound.
4. A chemical reaction involves a rearrangement of
atoms. No atoms are created, destroyed or broken
apart in a chemical reaction
By now the modern concept of elements was
well accepted and many attempts had been
made to arrange the known elements in
some kind of coherent manner. All attempts
had fatal flaws until a Russian chemist, Dmitri
Mendeleev, in 1869 devised an approach
which is still basically the same that is used
today. His Periodic Table was based on 2
ideas:
1. Arrange elements in order of increasing atomic mass
2. Have groups or families of elements
Predicted properties of missing elements – for
example, germanium:
Property
Predicted
Observed
Atomic mass
72
72.6
Density (g/cm3) 5.5
5.47
Color
Grayish white
Dirty gray