Ancient Models of Matter
- Empedocles (490 – 430 BC) – a Greek philosopher who proposed that matter is made up of a combination of earth, air, fire and water
- Democritus (460 – 370 BC) – another Greek philosopher who proposed that matter could be divided until an indivisible particle is reached; this particle was called the atom
Dalton’s Atomic Theory
In
the 16th century, experimentation became popular. John
Dalton came up with a new atomic theory, which included the following
ideas:
(a)
Matter consists of definite particles, called atoms.
(b)
Each element is made up of its own type of element.
(c)
Atoms of different elements have different properties.
(d)
Atoms of two or more elements can combine in constant ratios
to form new substances. (Law of Constant
Composition)
(e)
Atoms cannot be created, destroyed or subdivided in a
chemical change. (Law of Conservation of
Mass)
The Subatomic Particles
As
technology became available in the 18th and 19th
centuries, many discoveries were made.
- J. J. Thomson discovered that there were particles in the atom – a positively charged mass that had negatively charged particles embedded in it (raisin bun model).
- In 1911, Ernest Rutherford tested Thomson’s model and found that the atom is a positively charged nucleus surrounded by negatively charged electrons.
- Rutherford found that the nucleus contained positively charged particles, he called protons
- In 1932, James Chadwick modified Rutherford’s model to include a neutral particle in the nucleus – the neutron.
This
led to the introduction of the idea of isotopes
– atoms of the same element with different atomic masses (carbon-12 has 6 protons, 6 electrons and 6 neutrons; carbon-13 has 6 protons, 6 electrons and 7 neutrons).
The Bohr Atom
Rutherford’s
model, however did not explain why the negatively charged electrons did not
collapse in to the positively charged nucleus.
- Neils Bohr proposed that the electrons traveled in fixed orbitals around the nucleus.
Subatomic Particles
Standard Atomic Notation (AZX)
So, fluorine (F) has 9 protons, 9 electrons and 10 neutrons.
Bohr – Rutherford Diagrams
 |
| Remember that the first shell has a maximum of two electrons and all subsequent shells have a maximum of eight electrons. Also recall that you do not pair up electrons at any position in a shell until there is one at each position. |
Lewis Diagrams
- G.N. Lewis proposed the use of the element symbol with the appropriate number of dots to represent valence electrons.
 |
| Like Bohr diagrams, do not pair up electrons until there is one electron at each position around the element symbol. Also, recall that as you proceed across the periodic table (only using the "tall columns"), the number of valence electrons (and hence the number of dots) is equal to 1, 2, 3, 4, 5, 6, 7, 8. There is one exception to this rule - who's the rebel? |
Homework
Draw
the Bohr-Rutherford diagrams for H, He, Li, Al, O, Mg, P, Ca, C, S, Ne
Draw
the Lewis dot diagrams for the above elements + I, K, Ar, Sn, Rb
Review,
p.21 # 3-5
