1. S3 “Atomic Structure”

2. Defining the Atom
OBJECTIVES:
Describe Democritus’s ideas about atoms.

3. Defining the Atom
OBJECTIVES:
Explain Dalton’s atomic theory.

4. Defining the Atom
The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”)
He believed that atoms were indivisible and indestructible
His ideas did agree with later scientific theory,and was not based on the scientific method – but just philosophy

5. Dalton’s Atomic Theory (experiment based!)
All elements are composed of tiny indivisible particles called atoms
Atoms of the same element are identical. Atoms of any one element are different from those of any other element.
John Dalton
(1766 – 1844)
Atoms of different elements combine in simple whole-number ratios to form chemical compounds

6. Sizing up the Atom
Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element
If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long
Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

7. Structure of the Nuclear Atom
OBJECTIVES:
Identify three types of subatomic particles.

8. Structure of the Nuclear Atom
OBJECTIVES:
Describe the structure of atoms, according to the Rutherford atomic model.

9. Structure of the Nuclear Atom
One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles:
Electrons, protons, and neutrons are examples of these fundamental particles
There are many other types of particles, but we will study these three

10. Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

11. Modern Cathode Ray Tubes
Television
Computer Monitor
Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

12. Mass of the Electron
Mass of the electron is
9.11 x g
The oil drop apparatus
1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

13. Conclusions from the Study of the Electron:
Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
Electrons have so little mass that atoms must contain other particles that account for most of the mass

14. Conclusions from the Study of the Electron:
Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

15. Mass (g) / Relative mass
Subatomic Particles
Particle
Charge
Mass (g) / Relative mass
Location
Electron
(e-) -1
9.11 x 10-28
1/1840
(negligible)
Electron cloud
Proton (p+) +1
1.67 x 10-24 1
Nucleus
Neutron
(n)

16. Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

17. the “plum pudding” model of JJ Thomson

18. Ernest Rutherford’s Gold Foil Experiment - 1911
Rutherford was a student of Thomson and decided to see if there was more evidence for the 'plum pudding /blueberry muffin' theory.
His idea: to bombard the atom with dense alpha particles
alpha particle = He atom with no electrons
- this was like shooting bullets at a blueberry muffin

19. Ernest Rutherford’s Gold Foil Experiment - 1911
this was like shooting bullets at a blueberry muffin…. So what did Rutherford expect?

20. Surprising result of Rutherford’s Gold Foil Experiment - 1911

21. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
VERY FEW were greatly deflected
“Like gun shells bouncing off of tissue paper!”

22. Rutherford’s Findings and deductions
Most of the particles passed right through
an atom was much more than just empty space and scattered electrons
A few particles were deflected,
very few were greatly deflected
Since alpha particles are relatively heavy, positively charged particles, the fact that the occasional particle would be deflected by either a small or large extent, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the nucleus).

23. Rutherford’s Findings and deductions

24. An illustration that compares the plum pudding model of an atom to what Rutherford observed in his experiment.
The top structure shows how the alpha particles would have passed through the gold foil atoms if the plum pudding model was correct in its assumptions.
The bottom structure shows what Rutherford and his collogues observed and is the true depiction of an atom's structure.

25. The nucleus as described by Rutherford:
The nucleus is small
The nucleus is dense
The nucleus is positively charged