Fundamental Particles
Objectives
 Understand how the model of the atom has changed over time
 Understand the key facts about each subatomic particle
I.
History of the atom
 5th Century BC (Ancient Greece) – Philosophers propose matter is composed of
indivisible particles called atoms (from the Greek, atomon = “indivisible”)
 1805 – John Dalton expands on the theory of atoms and suggests that different
substances have different atomic weights and sizes.
 1897 – JJ Thompson discovers the electron, proposes the “plum pudding” model
 1909 – Ernest Rutherford discovers the nucleus and proposes that it must have a
positive charge by using the gold foil experiment
 1913 – Neils Bohr suggests the Bohr model of the atom
 1924-Present – Quantum theory suggests wave-particle duality
II.
Plum pudding model
In JJ Thompson’s plum pudding model, electrons are embedded in a large cloud of positive
charge, much like a Christmas plum pudding has raisins embedded in the cake. Note that the
overall charge is neutral, as the positive charge in the cloud is cancelled out by the electrons.
Image obtained from http://askeyphysics.org/
III.
Rutherford gold foil experiment
Rutherford fired positively charged particles called α-particles at a thin sheet of gold foil. He
found that most particles passed straight through the foil and hit the detector, meaning atoms
are mostly empty space. A very small number of particles were reflected back, meaning that the
nucleus of the atom is positively charged, and is relatively small. This leads to the current model
of the atom, with a positively charged nucleus and negatively charged electrons orbiting.
Image obtained from http://imgkid.com/
IV.
The Atomic Model
The modern structure of the atom is a version of the Bohr model consisting of a nucleus
composed of protons and neutrons, with electrons orbiting around the nucleus. Once again the
atom is neutral; the positive charge on the protons is cancelled out by the negative charge on
the electrons.
You can see that protons and neutrons are much larger than electrons. Therefore electrons
have relatively almost no mass.
Particle
Charge
Relative Mass
Neutron
0
1
Proton
+1
1
Electron
-1
0
Since atoms are neutral in their default state, the number of electrons will equal the number of
protons to give an overall net charge of 0.
Questions
1. In what year was the Plum Pudding Model proposed?
a) 1982
b) 1909
c) 1897
d) 1927
2. Who proposed the Plum Pudding Model?
a) Rutherford
b) JJ Abrams
c) John Dalton
d) JJ Thomson
3. In Rutherford’s experiment, why did most of the α-particles pass through the foil?
a) They were pushed by the magnetic field
b) They were attracted to the negative charge on the other side of the foil
c) Atoms can pass through neutrons without being deflected
d) Atoms are mostly empty space
4. Explain one difference between the Plum Pudding Model and the modern Bohr model of
the atom
a) The Plum Pudding Model had electrons embedded in the nucleus, whereas the Bohr
model did not
b) The Plum Pudding Model had no electrons
c) The Bohr Model used more accurate data
d) Bohr atoms are bigger
5. Which are biggest, neutrons, protons or electrons?
a) Neutrons and electrons are the same size and bigger than protons
b) Protons, neutrons and electrons are all the same size
c) Electrons
d) Protons and neutrons are the same size and are bigger than electrons
6. The nucleus consists of which 2 particles?
a) Atoms and electrons
b) Protons and electrons
c) Protons and neutrons
d) Atoms and neutrons
7. What is the charge and relative mass of a neutron?
a) +1 charge, no mass
b) 0 charge, no mass
c) -1 charge, no mass
d) 0 charge, 1 mass
8. What is the total relative mass of oxygen? (note: oxygen has 8 neutrons, 8 protons and 8
electrons)
a) 18
b) 16
c) 8
d) 24
9. What is the relative charge of one electron?
a) +1
b) -1
c) 0
d) +1/2
10. A neutral atom has 12 protons. How many electrons will it have?
a) 14
b) 12
c) 6
d) 0