1. The History of the Atom

2. 1.3 Atomic Theory Early ideas about matter
Greek philosophers believed that matter was made of atomos that were the smallest pieces of matter.
Aristotle believed matter was made of
different combinations of earth, air,
fire, and water.
Alchemists experimented with matter and
tried to turn common metals into gold.
Their activities marked the beginning of
our understanding of matter.
See pages

3. Development of Atomic Theory I
John Dalton ( )
Credited with developing a theory that was a new way of explaining matter.
He studied gases that make up Earth’s atmosphere.
Based on his studies, he suggested that:
matter is made of small, hard spheres
that are different for different
elements
the smallest particle of an element
is called an atom
This is the basis for Dalton’s Atomic Theory.
See page 29

4. John Dalton and his ‘Atomic Theory’
chemical elements are made of atoms.
the atoms of the same element are identical in their masses. Different atoms have different masses.
3) Compounds are created when atoms of different elements link together in definite proportions.
4) atoms cannot be created, destroyed, or divided into smaller particles

5. Thomson’s Model of the Atom
JJ Thomson – England
Studied electric currents in gas discharge tubes
charge on electrons all have charge of -1
He found the mass of electrons
Suggested the “raisin bun” model of the atom (positive bun in sea of negative electrons)
Positive blob
electrons
Thomson’s Model of the atom

6. Three distinct types of radiation
Using his famous ‘gold foil’ experiment, Rutherford determined that:
The atom is mostly ‘space’
The atom has a dense, positive nucleus
Inside the nucleus a positively charged particle called a proton and particle without a charge called a neutron
Ernest Rutherford
New Zealand

7. Rutherford’s scattering experiment   

8. Rutherford’s Atom

9. Niels Bohr and the Bohr Model of the Atom
Neils Bohr –
He studied gaseous atoms, which were made to glow by passing an electric current through them.
Bohr proposed that electrons travel in specific “energy levels” or “shells” When electrons drop from one energy level to another, they emit different colours of light!
He suggested that chemical properties of elements are determined by electrons in their outer energy levels.

10. Bohr Model

11. Spectra explained Hydrogen Spectrum Helium Spectrum

12. The Evolution of the Atomic Model
Thomson’s atom
Rutherford’s atom
Bohr’s atom
Schrödinger’s atom
“Standard Model” of Atom
(Current Model)
Chadwick’s atom

13. Inside the Atom An atom is the smallest particle of an element that
retains the properties of the element.
All atoms are made up of three kinds of particles called subatomic particles. These particles are:
electrons
protons
neutrons
Take the Section 1.3 Quiz
See pages

14. Mass: protons and neutrons have about 1800 time’s more mass than electrons.
Electric Charge:
Protons have a positive charge and electrons have a negative charge.
Each proton counts as +1 and each electron counts as -1
Together the charges add up to zero, making the atom neutral

15. The Nucleus: tiny region in centre of the atom.
It would take nuclei lined up to stretch across an atom.
The nucleus always has a positive charge because of it’s protons.
Protons and neutrons are held in the nucleus and cannot normally leave.

16. Electrons: Electrons occupy energy shells or levels around the nucleus.
If a hockey puck was the nucleus, the whole atom would include the ice rink, the seats and the walls of a hockey arena.
Each electron occupies one whole level at a time.

17. Visible Effects of Electron Energy Shells
Example: Neon light
When electricity is added to neon gas, electrons of neon atoms gain energy and jump to higher energy levels
Electrons can now fall back down to lower energy levels, releasing energy as light of a specific colour
Figure 2.30: Flame tests work by placing a sample of a compound containing a metal element in a flame

18. Discussion Questions
Compare and contrast models of the atom.
In your own words, describe Bohr’s contribution to atomic theory.

19. Concept 3: An atom is made up of electrons, neutrons, and protons.
Atom: the smallest particle of an element that retains the properties of that element
All matter is made up of atoms
Atoms are made up of subatomic particles
Figure 2.31: Model of the atom.

20. The Atom: Nucleus Nucleus Tiny region at the centre of the atom
Most hydrogen atoms: contain one proton
All other atoms: contain both protons and neutrons
Number of protons determines charge of the nucleus and the identity of an atom
Figure 2.31: Model of the atom.

21. The Atom: Electron Energy Shell
Region that electrons occupy accounts for over 99.99% of an atom’s volume
Electrons occupy specific regions (energy levels) that surround the nucleus
Electrons are like a spread-out cloud of negative charge that exists in the whole region at once
Figure 2.31: Model of the atom.

22. The Atom: Electric Charge
Comes in two types: positive and negative
Protons: positive charge (1+ each)
Electrons: negative charge (1– each)
Neutrons: no charge
Positive charge of protons in the nucleus attractions electrons
Overall charge of an atom: uncharged/neutral (equal numbers of protons and electrons)

23. The Atom: Size The Size of an Atom Atoms are incredibly small
Suppose you enlarged everything on Earth so that an atom would become as big as a large apple
An apple would be as big as Earth

24. The Atom: The Size of the Nucleus Compared with an Atom
If a nucleus were the size of a hockey puck sitting at centre ice, the whole atom would include:
Entire rink
Seats
Building
Surrounding streets
Walkways/parking lot

25. The Atom: The Nuclear Force
Nuclear Force (Strong Force)
Acts within nucleus to hold protons and neutrons together
Very strong across very short distances
Strong enough to counteract the repulsion between protons, keeping nucleus from flying apart
Figure 2.31: Model of the atom.

26. The Atom: Subatomic Particles
Name
Symbol
Electric Charge
Relative Mass
Location in the Atom
proton p+ 1+
1836
nucleus
neutron n0
1837
electron e– 1– 1
surrounding the nucleus

27. Discussion Questions What are the three subatomic particles?
Compare and contrast the electron and the proton.

28. Discussion Questions
Use an analogy to describe the size or composition of the atom.
What does the existence of a nuclear force explain?

29. Concept 4: Atomic theory continues to develop.
The atom is made up of smaller particles called subatomic particles
Some subatomic particles are made up of even smaller particles
Figure 2.32: Protons are made up of smaller elementary particles

30. Quarks
Quarks are elementary particles (cannot be split apart into smaller particles)
Six different “flavours” based on properties such as mass and charge: up, down, strange, charm, top, and bottom
Figure 2.32: Protons are made up of smaller elementary particles

31. Quarks
Protons and neutrons are composite particles (made up of quarks)
Also contain elementary particles called gluons, which act as a glue that binds quarks together
Experience the nuclear force (strong force), which is also involved in binding quarks together
Figure 2.32: Protons and neutrons are made up of smaller elementary particles

32. Leptons Electrons are elementary particles called leptons
Come in six “flavours”: electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino
Do not experience the nuclear force (strong force)

33. Table 2.4: Characteristics of Leptons
Description
electron
The electron is the lepton found in atoms.
Compared to the electron, muon and tau particles have the same charge (1–) but a much greater mass.
muon
tau
electron neutrinos
Neutrinos are very difficult to detect. They have no charge and are nearly massless.
Trillions of them pass through our bodies each second.
Neutrinos are produced by high-energy processes such as nuclear reactions in the Sun.
muon neutrinos
tau neutrinos

34. Research Continues: TRIUMF Cyclotron
TRIUMF cyclotron in Vancouver
Built to research particles that make up matter
Particle accelerator that produces a high-speed beam of protons
The proton beam collides with various materials
Detectors provide data about the products of the collisions
Figure 2.33: The TRIUMF cyclotron.

35. Discussion Questions Describe the structure of a proton.
Compare neutrinos and electrons.

36. Summary: How can we investigate and explain the composition of atoms?
Dalton developed an early atomic theory.
Many scientists contributed to the further development of atomic theory.
An atom is made up of electrons, neutrons, and protons.
Atomic theory continues to develop.