1. Lecture 15 The Atom Ozgur Unal
NIS – CHEMISTRY
Lecture 15
The Atom
Ozgur Unal

2. The Atom Consider the carbon.
You crush it into dust and get very small particles.
If you keep dividing into smaller and smaller particles, eventually you will end up with the smallest carbon particle that cannot be divided into other particles without changing the properties of carbon.
The smallest particle of an element that retains the properties of the element is called an atom.
How small do you think atoms are?

3. Looking at Atoms www.princeton.edu/~kahnlab/STMImages.html
Atoms are quite small, so is it possible to SEE them?
An instrument called scanning tunneling microscope (STM) allows individual atoms to be seen.
Seeing atoms allows scientist to form shapes and patterns by using them.
This lead to a new field
called nanotechnology.

4. The Electron
Once the scientists were convinced that atoms exist, they asked more questions about its structure.
Were atoms uniform throughout?
Were they made up of even smaller particles?
These questions eventually lead to the discovery of electron.
Scientists in the meantime started to make connections between electric charge and matter.
Cathode ray tube is a device that
scientists used to study the
relationship between mass and
charge.

5. The Electron
English Physicist Sir William Crookes noticed a green flash of light (called cathode ray) within a cathode ray tube.
This accidental discovery lead to the following results by other scientists:
Cathode rays were a stream of charged particles.
The particles carried a negative charge (exact value not known).
Since changing the metal of the electrodes or varying the gas did not affect the cathode ray, researchers concluded that this negative charge existed in all forms of matter.
These negatively charged particles are now known as electrons.

6. The Electron
No one could determine the mass of a single cathode ray particle…
Until English physicist JJ Thomson who began a series of cathode ray tube experiments to determine the ratio of its charge to its mass.
He was able to determine this ratio.
This ratio was much less than that of a hydrogen atom (the lightest known atom).
This conclusion was shocking because it meant there were particles smaller than the atom (Dalton had been incorrect!)
Atoms were divisible into smaller subatomic particles.

7. The Charge of an Electron
After JJ Thomson, the next significant development about electrons came with American physicist Robert Millikan.
He was able to determine the charge of an electron using oil-drop experiment.
The charge of one electron is x 10-19C.
This charge is now equated to a single unit of negative charge.

8. The Mass of an Electron
After finding the charge, Millikan was able to calculate the mass of an electron, which is 9.1 x kg, or 1/1840 times the mass of one hydrogen atom.
The Plum Pudding Model:
Electrons are negatively charged and matter is neutral.
If the electrons are so small, what accounts for the rest of the mass in a typical atom?
JJ Thomson proposed a model of atom to answer these questions.
The model consisted of a spherically shaped atom composed of a uniformly distributed positive charge in which the individual electrons reside.
Check out Figure 4.9!

9. Lecture 16 The Nucleus Ozgur Unal
NIS – CHEMISTRY
Lecture 16
The Nucleus
Ozgur Unal

10. Dalton’s Atomic Theory
Atomic Models
Dalton’s Atomic Theory
J.J. Thomson’s Atomic Model

11. Rutherford’s Experiment
Like charges repel each other, unlike charges attract each other.
Alpha particles are positively charged.
Ernest Rutherford, in 1911, started experimenting to see if alpha particles would be deflected as they passed through thin gold foil.
According to plum-pudding
model of Thomson, alpha
particles should not be deflected
much.
This is because the positive
charge in an atom is uniformly
distributed within the atom.

12. Rutherford’s Experiment
The set up of the experiment:

13. Rutherford’s Experiment
According to the results of this experiment, most of the alpha particles passed through the foil undeflected, some particles were deflected at a small angle.
However, few alpha particles were deflected at large angles.
This was inconsistent with
Thomson’s atomic model.

14. Rutherford’s Atomic Model
Rutherfod concluded that Thomson’s atomic model was incorrect.
He calculated that an atom consisted mostly of empty space through which electrons move.
He also concluded that atom’s positive charge and almost all of its mass were concentrated in a tiny, dense region at the center, which he called nucleus.

15. Rutherford’s Atomic Model
Nucleus contains positively charged particles called protons.
It also contains neutral particles called neutrons.
The mass of a proton is nearly equal to the mass of a neutron.
The repulsive force between the
positive nucleus and positive alpha
particles causes the deflections.

16. Completing the Model of the Atom
All atoms are made up of electrons, protons and neutrons.
Atoms have dense nucleus, which consists of protons and neutrons.
Most of atom consists of fast moving electrons traveling through empty space surrounding the nucleus.
Most of atoms mass is
concentrated at the nucleus.
The nucleus occupies one
ten-thousandth of the total
volume of the atom.