Atomic and nuclear structure

Question 1

What are elementary particles?

Answer 1

The particles that make up atoms

Question 2

What is the difference between the mass number of an atom and its atomic weight?

Answer 2

Mass number: number of protons and neutrons in the atom

Atomic weight: Weighted average of all naturally occurring isotopes of an element

Question 3

How does the mass number of an atom effect energy released from nuclear reactions?

Answer 3

MN is over 60: Fission gives energy

MN is under 60: Fusion gives energy

Question 4

What are fission/fusion reactions?

Answer 4

Fission: Nucleus splits into smaller nuclei

Fusion: Smaller nuclei combine to form a larger nucleus

Question 5

What are the most stable nuclei? (in terms of protons and neutrons)

Least stable?

Answer 5

The ones with an even number of protons and an even number of neutrons are most stable.

Those with odd number of protons are odd number of neutrons are least stable.

As the atomic number increases, there are more neutrons needed for the nuclei to be stable

Question 6

How can unstable nuclei become stable? (2)

Answer 6

• Fission to smaller nuclei (rare)

- Absorption/emission of small particles (more common, especially with radioactivity)

Question 7

What are nuclear reactions?

How can they be balanced?

Answer 7

Nuclear reactions are reactions in which changes in nuclear composition occur (ie, change in electrons, neutrons or protons).

The sum of the atomic number and mass number of the atoms on one side of the chemical equation, must equal the sums on the other side.

Question 8

What order are spontaneous radioactive decay reactions?

Answer 8

First order processes

The rate of decay is directly proportional to the amount of material present.

Δm/Δt = rate of decay = -km

(k is the decay constant)
k = -(Δm/m)/ΔT = fraction of the mass that decays with time

Question 9

If an electron moves from infinity into an energy level, is energy gained or released?

What happens when an electron moves to a higher orbital?

How can you calculate the frequency of light necessary to cause excitation when there is an energy shift?

Answer 9

Released, from infinity to 4, from 4 to 3, from 3 to 2 and from 2 to 1.

IE. When an electron moves to a higher orbital (eg. n=1 to n=2), energy is released and the frequency of light necessary to cause excitation is:

E2 -E1 = hf

E1: energy level 1
E2: energy level 2
h: Planck’s constant
f: frequency of light absorbed or emitted

Question 10

What is fluorescence?

Answer 10

An emission process that occurs after light absorption excites electrons to higher electronic and vibrational levels.

The electrons spontaneously lose excited vibrational energy to the electronic states. There are certain molecular types that possess this property (such as some amino acids - eg. tryptophan)

Question 11

Radioactive potassium (40-K) decays to argon gas (40-Ar). What kind of object is emitted in the decay of radioactive potassium?

Answer 11

A positron.

THe 40-K to 40-Ar radioactive decay does not change the atomic mass (40) of the nucleus, but does change its atomic number (19 to 18, as determined by reference to the periodic table in the test booklet). These conditions point to a positron as the emitted object in the radioactive decay.

Question 12

μ is an intrinsic property of the atomic and subatomic particles. Energy is emitted when an H nucleus relaxes.

Consider an H nucleus with μ pointing in a direction 180 degrees from a magnetic field. When the nucleus relaxes, what will happen:

A. The magnitude of μ will decrease to zero
B. The magnitude of the magnetic field will decrease to zero
C. The nucleus will emit a photon
D. The nucleus will absorb a photon

Answer 12

C. The nucleus will emit a photon.

This is the only answer compatible with energy emittance.

Question 13

If a photon is emitted or absorbed by an atom, then the photon has a level of energy equivalent to?

Answer 13

The difference between the energies of the atomic states

Question 14

Describe scattering and fluorescence (in regards to nuclear physics)

Answer 14

Scattering and fluorescence occur when an atom absorbs a photon, transitions to a higher energy state, and then releases one back.

Scattering: The photon emitted back is the same energy as the one absorbed

Fluorescence: The photon emitted back is a different frequency from the one absorbed

Question 15

True or false? The ground state is always the lowest energy state?

Answer 15

True! Any state above has more energy, but is negative, as transitioning states releases energy.

Question 16

Predict the type of radioactive decay for the following three atoms

A. An atom with many neutrons and protons

B. An atom with many neutrons, compared to protons.

C. An atom with many protons

Answer 16

A. Alpha decay (helium emittance)

B. Beta decay (electron emittance)

C. Positron emittance (a type of beta decay)

Question 17

A mass of thorium undergoes alpha decay for a period of time (lets say its half life). A scientist calculates the amount of thorium that has decayed and found that there is a bit of discrepancy, more thorium has appeared to decay than predicted. Why? How can this be quantified into a meaningful value?

Answer 17

The extra decay is decay into energy. There is a deep connection between mass and energy, where mass is converted into energy, and the amount of energy can be determined if we know the mass deficiency sufficiently well.

E = mc^2

Where m is the mass deficiency (the difference of the mass of products and the mass of reactants)

c = 3x10^8 m/s

Question 18

2 moles of polonium undergoes alpha decay at a half life of 8 hours. How many moles of alpha particle will be emitted in 16 hours? How many grams is this?

Answer 18

1. 16 hours is 2 half lives. 3/4 of the original nucleus will have decayed.
2. 3/4 x 2 ml = 3/2 mol decayed
3. 3/2 mol alpha particles is produced (He)
4. (3/2 mol)(4 g/mol) = 6 g He emitted in 16 hours.

Question 19

Why do anions have greater ionic radii than cations?

Answer 19

Anions are more electronegative and therefore propel electrons further away from the nucleus.

Cations are electron affinitive, pulling them in close to the nucleus, these have a greater atomic radius than ionic radius.

Question 20

Modern nuclear reactors typically have several boron rods that can be inserted varying distances into a fission nuclear reactor. How do these rods control the rate of reaction?

Answer 20

By absorbing neutrons.

Fission reactions produce many neutrons that can sustain and even grow the fission process. A modern reactor must have a means to remove some or all neutrons in order to control the rate of fission.

Question 21

Of all the shapes you can think of, which has the lowest surface area? What is the biggest clue for this in nature?

Answer 21

Speres!

Think of a drop of water. Because of surface tension water droplets assume a spherical shape to minimize surface area.

Water droplets are most spherical when cohesion forces between a substrate are lowest (eg. rain drop and a waxy leaf) because surface tension pulls them together like an elastic membrane.