1. Atom structure

Question 1

What is the plum pudding model?

Answer 1

Historical scientific model of an atom - negatively charged plums embedded in positively charged pudding - proposed by J. J. Thomson in 1904

Question 2

What is atomic mass?

Answer 2

Atomic mass = protons + neutrons
(each 1 unit)
ex.: 1H - one proton => mass = 1

Question 3

Explain mass defect

Answer 3

Atomic mass of every atom (except 1H) is less than sum of proton + neutron + electron

Because -> mass converted into E when nucleus is formed (following E=mc^2)

Mass defect measures binding energy - E that would be released if all protons, neutrons and e were brought together
E = mc^2

Question 4

How to calculate binding E with E=mc^2?

Answer 4

1. Calculate observed mass (multiplying by one amu mass in kg from data card)
2. Calculate expected mass (addind up prot, neutr, e mass from data card)
3. Mass expected - mass observed
4. Mass difference into E=mc^2 with speed of light from data card -> binding E per atom
5. Multiply E by Avogadro’s # -> binding E per mole - how much E for 1 mol of nucleus formation

Question 5

How was the electron discovered?

Answer 5

Discovered by J. J. Thomson in 1897 by searching cathode rays (cathode ray tube) for the carrier of electrical properties in matter

Question 6

How was the nucleus discovered?

Answer 6

Rutherford, Marsden and Geiger discovered dense atomic nucleus by bombarding thin gold sheet with alpha particles emitted by radium (Ra) -> Niehl Bohr proposed the atom model replacing the plum pudding model

Question 7

How were neutrons discovered?

Answer 7

Chadwick proved the existence for neutrons in 1932 by calculating mass in collisions - fouund mass but no charge -> neutral particle (received Nobel Prize)