Atomic Structure
Chapter 4
Three Subatomic Particles
Particle
Proton
Charge
+
Mass
1 amu
Location
Nucleus
Neutron
0
1 amu
Nucleus
Electron
-
0 amu
Energy levels
Subatomic Particles
Atomic Number – tells us the number
of p+
- determines the identity of an
element
Subatomic Particles
Mass # - total number of nucleons
- neutrons = mass # - atomic #
History

Two Differing Views
Continuous Theory of Matter – solid body CAN
be divided and subdivided into smaller pieces
without limit
Discontinuous Theory of Matter – there are
indestructible particles called atoms
Aristotle (400 BC)
4 elements – earth, air, fire and
water (continuous theory) – not a
scientist
Democritus (400 BC)
-
-
First to use the word “atom”
Could not break down the atom
(Discontinuous Theory)
Did not test his theories
John Dalton (1808)
John Dalton (1808) – “Modern Father of the
Atom” – Proposed a theory of matter
based on the existence of atoms
Solid Sphere Model
Similar to a golf ball or billiard
ball
John Dalton (cont’d)
4 postulates
1) All elements are composed of atoms –
indivisible and indestructible
(Not True Today)
2) All atoms of the same element are
exactly the same (ex: mass, color,
shape)
(Not True Today – isotopes)
John Dalton (Cont’d)
3) Atoms of different elements are different
(ex: color, mass, boiling point)
(Still True Today)
4) Compounds are formed by the joining of
atoms of two or more elements in a
whole-number ratio
(Still True Today)
Antoine Lavoisier
Law of Conservation of Matter
Matter can not be created nor
destroyed
Crookes (1895)
- Crookes tube (or Cathode Ray tube) led
to the discovery of the electron
- Beam of light travels from cathode
(- end) to anode (+ end)
- Applications include; TV and computer
screens, RADAR screens, and X-rays
Cathode Ray Tube
JJ Thomson (1897)
– discovered that magnetic fields deflected
cathode rays. Rays were attracted to a +
charged electrode –
- Conclusion – cathode rays must be a
stream of negatively charged particles
- He called them – ELECTRONS
JJ Thomson
JJ Thomson
Developed Plum Pudding Model
“Cookie Dough Model”
Becquerel
Studied different types of radiation:
Alpha Particles - helium nucleus and
consists of 2 p+ and 2 no
Beta Particles – high speed electron
emitted from radioactive nuclei
Gamma Rays – very high energy Xrays
Ernest Rutherford
(1909)
Gold Foil Experiment (Alpha
Scattering Experiment)
Comparison of Two Models
Thomson’s Model
Rutherford’s Model
Conclusion
Each atom contained a small, dense,
positively charged nucleus.
Proton and Nucleus were then discovered
Nuclear Model
Big Problem with
Rutherford’s Model
What is keeping the positively charged
nucleus from being attracted to the
electrons and drawing them closer???????
Bohr to the rescue…….
Niels Bohr (1913)
Proposed improvements to the
Rutherford Model
Key Idea – there are definite paths (orbits) in
which an electron can travel around a nucleus
without spiraling towards it.
Each orbit is called an Energy Level
The greater the radius of the shell, the greater
the energy of the electrons in that shell
Bohr’s Solar System Model
Ground vs. Excited
Ground State – original energy level
where an electron can be found
Excited State – When electrons gain
energy and move to a higher energy
level
- Very unstable
Ground vs. Excited State
** When an electron
moves from the excited
state, back to ground
state, they lose their
energy in the form of …..
LIGHT
Quantum – bundle of
energy
Light
Continuous Spectrum – band of
colors produced when white light
passes through a prism
Ex: ROY G. BIV
Light
Bright Line Spectrum – band of colors
produced when a gas is in the excited
state
Each element produces its own specific
line spectrum – “like fingerprints”
Charge Cloud Model
Charge Cloud Model
-
Also called Quantum Mechanical Model
-
Does not show the path of electrons
-
Instead, it shows the most probable
location of an electron
-
As you move away from the nucleus, the
chance of finding an electron decreases!