Transcript History of the Atom

The Atom, in a nutshell…
1
Democritus
(c. 460—370 BC)
Greek philosopher
The name atom was his idea
Proposed matter is made up of small particles
Atoms comes from atomos=indivisible
Atoms differ in size and shape and combine
in different ways, & constantly moving
2
John Dalton
(1766—1844)
English chemist & physicist, self educated
published Atomic Theory:
1. Each element made of really small particles called atoms
2. All atoms of a given element are identical
3. Atoms of different elements have different properties, like mass
and chemical reactivity
4. Atoms aren’t changed by chemical reactions, just rearranged
3
J.J. Thomson
(1856—1940)
 Identified the electron using cathode ray
tube, determined negative charge
 Atom is neutral, with a positive sphere
and negative electrons embedded called
the “plum pudding” model. Yum.
 The television and computer monitors
use this technology even today.
4
Ernest Rutherford
(1871—1937)
Gold Foil Experiment—bombarded gold foil with
radioactive particles, most passed through,
some bounced back!
Conclusions:
 Some parts of atom must be more dense.
 New picture of the atom: dense at the
center, electrons on the outside
 Atom mostly empty space, most of mass
found in the center—nucleus with positively
charged protons
Hyperlink to demo
5
Niels Bohr
(1885—1962)
 Used light and observed the way light interacts
only with specific energies: determined the
electrons are in specific levels
 Electrons moving around nucleus in fixed
orbits w/fixed energy
 Sometimes called the planetary model
 Not completely accurate with how electrons
are truly arranged
6
Other Scientists
 Louis de Broglie (1892—1987): mathematical
description of electrons in atoms
 James Chadwick (1891—1974): discovered
the neutron around 1932, another massive
particle in the nucleus
7
Basics on the atom:
 Three kinds of subatomic particles are
electrons, protons, and neutrons.
 The nucleus is the center area of an
atom and contains most of its mass
 Protons and neutrons are found in the
nucleus. Each has a mass of 1 amu
 Electrons are found in levels outside
of the nucleus
8
Summary of particles
9
The number of protons are equal to the
number of electrons in an atom so…
Put a number where there
is a letter.
10
Answers:
All of the numbers have
to be the SAME!
19
11
Answers:
19
B
5
5
16
23
16
23
12
Finding the amount of
neutrons is a bit more work…
7
3
Li
This is called the isotope notation.
13
The notation means:
Lithium - 7
Mass number, A
Protons and
neutrons added
together
Atomic number, Z
Number of protons
only
7
3
Li
7
3
4
This particular atom has
4 neutrons.
14
The notation means:
Lithium - 6
Mass number
Protons and
neutrons added
together
Atomic number
Number of
protons only
6
3
Li
6
3
3
This particular atom has
3 neutrons.
15
7
3
Li
6
3
Li
Both of these atoms would have the same
chemical properties, but one is slightly heavier
than the other. They are Isotopes.
16
Hint: always find the
atomic number FIRST
then make protons and
electrons equal to that!
17
9
9
Neutrons = mass – atomic
[
]
=
?
–
[
]
18
9
9
Neutrons = mass – atomic
[ 10 ]
=
?
- [ 9 ]
What does the “?”
equal?
19
19
9
9
F
20
14
22
25
19
29
9
22
9
14
25
30
25
F
Si
Ti
Mn
21
Other demos Link:
Fun facts about atomic particles
electron
Modern view of atom:
neutron
proton
22