1. Structure of the Atom

2. We know now that atoms have two regions – a nucleus and an electron cloud.
The nucleus contains protons and neutrons.
Protons, neutrons, and electrons are called subatomic particles
But HOW did scientists discover these things?

3. The discovery of subatomic particles began with electrons
Electrons were detected during experiments that investigated the relationship between electricity and matter.
Cathode ray tubes were used to conduct these experiments.

5. Traveling science lecturers in the mid nineteenth century delighted audiences by showing them the ancestor of the neon sign. They took a glass tube with wires embedded in opposite ends... put a high voltage across... pumped out most of the air... and the interior of the tube would glow in lovely patterns.
In 1859 a German physicist sucked out still more air with an improved pump and saw that where this light from the cathode reached the glass it produced a fluorescent glow. Evidently some kind of ray was emitted by the cathode and lighting up the glass.

7. These particles were eventually named electrons.
“I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter."
Thomson
These particles were eventually named electrons.

8. Experiments conducted by an American physicist during the early 20th century were used to measure the charge and mass of an electron.

9. Once it was known that atoms contained these small particles called electrons that were negatively charged, scientists were able to make other inferences about atomic structure.

10. The mass of the electrons was so small, that atoms had to contain other particles that contained most of the mass of the atom.
Because atoms are electrically neutral, other particles in the atom had to carry a positive charge to balance out the negative charge of the electrons.

11. Thomson’s atomic model
“plum pudding” model
Negative electrons are spread evenly throughout the positive charge of the rest of the atom
Like seeds spread throughout a watermelon
Or plums spread throughout a pudding
New experiments disproved this model

12. Rutherford’s Atomic Model
“planetary” model
A very small nucleus with a positive charge with electrons surrounding it like planets surround the sun.

13. Continued experimentation led to the discovery that the positively charged nucleus contained two types of subatomic particles
Protons
Positively charged
Neutrons
No charge

14. Protons The number of protons in a nucleus gives an atom its identity.
For example, if an atom has 8 protons, it is an oxygen atom
The positive charge of a proton is equal in magnitude to the negative charge of an electron.
Because all atoms are neutral, an atom must have the same number of protons as electrons.

15. Neutrons Have no charge – they are neutral
Their mass is very slightly larger than a proton.
All atoms other than the simplest type of hydrogen atom contain at least one neutron.
Different atoms of the same element can have different numbers of neutrons
They are called isotopes.

16. Relative electric charge
See Table 1 on p. 76
Particle
Symbol
Relative electric charge
Mass number
Relative mass (amu)
Actual mass (kg)
electron e- -1
9.109 x 10-31
proton p+ +1 1
1.673 x 10-27
neutron no
1.675 x 10-27

17. Electrostatic forces
Particles with the same charge repel each other
Particles with opposite charges attract each other
A nucleus has positively charged particles packed closely together, along with neutral particles. So you might expect that those particles would repel each other, making the nucleus unstable.

18. Nuclear Forces
However, when two protons, two neutrons, or a proton and a neutron are extremely close to each other, they are strongly attracted to each other.
These attractive forces hold the neutrons and protons together very tightly in the nucleus, making the nucleus very stable.

19. Nuclear forces are very strong – and they provide the energy associated with nuclear power or the detonation of an atomic weapon.

20. Other subatomic particles
Other subatomic particles besides electrons, protons, and neutrons have been identified.
These other particles have little effect on the chemical properties of matter so they aren’t covered in an introductory chemistry course.
They include quarks, muons, bosons, etc.