chapter 10

Question 1

Mass and charge of subatomic particles:

Answer 1

Proton | Neutron | electron

Mass: 1.672 x 10^-27 (1 Da, 1 amu) | 1.674 x 10^-27 (1 Da, 1 amu) | 9.019 x 10^-31 kg (5.486 x 10^-4 Da/amu)

Charge: 1.602 x 10^-19 C (+1 e) | 0 C (0 e) | -1.602 x 10^-19 (-1 e)

Question 2

What is the strong nuclear force?

Answer 2

• an interaction between protons and neutrons that keeps protons together in the nucleus

Question 3

Since the strong nuclear force holds protons together despite the repulsive force among them that exists due to electromagnetisim, a certain amount of potential energy is rpeent in the nucleus of every atom. This potential energy is known as?

Answer 3

• the binding energy of a nucleus, and this energy is what makes nuclear power and nuclear bombs so powerful
  
  • to get a sense of this power, we need to take advantage of E = mc². This equation refers to a principle known as mass-energy equivalence.

Question 4

what is the mass defect?

Answer 4

• Mass defect is the difference between the predicted mass and the actual mass of an atom’s nucleus. The binding energy of a system can appear as extra mass, which accounts for this difference.

Question 5

• it is often inconvenient to use J to talk about energy on the subatmoc scale, so we use electron-volts (eV) which is defined as the energy equivalent to the work needed to move an electron through an electric potential difference of 1V.
  
  • its conversion is 1 eV = 1.602 x 10^-19 J

Question 6

electrons show what?

Answer 6

• wave-particle duality
  
  • electrons and light

Question 7

electrons are what kind of waves>

Answer 7

• probability waves
  
  • the areas across which a probability wave is dispersed for a given electron are known as orbitals

Question 8

what is the photoelectric effect?

Answer 8

• refers to a phenomenon in which a substance (usually a metal) emits electrons in response to a beam of photons being shined onto it
  
  • the basic idea behind this effect is that the energy of incident photons is absorbed by the material and excites electrons to the point that they are ejected from their atoms
  • frequency dependent (if the light shined on a substance is below a certain frequency (known as the threshold frequency), nothing will happen. However once the frequency of the light sourve surpasses the threshold frequency, not only does electron ejection happen, but it becomes proportional to the intensity of the light beam

Question 9

the energy of a photon is given by the following equations:

Answer 9

• E = hf
• E = hc/lamda

Question 10

the minimum amount of energy needed to expel an electron from an atom of a substance is known as the work function of that substance. Since that corresponds to the threshold frequency, we can define the work function as follows:

Answer 10

• E_{work function} = hf_threshold

Question 11

What happens if a metal is hit by a photon carrying more energy than the bare minimum necessary to eject the electron?

Answer 11

• the extra energy can go towards the kinetic energy of the electron, the maximum kinetic energy of which corresponds to the energy of the incident photon minus the work function (or the energy that was spent ejecting the electron):
  
  • KE_max = E_incident - E_{work function}
  • KE_max = hf_incident - hf_{work function}

Question 12

Hydrogen is often used as a model atom for absorption and emission, and the wavelength of light corresponding to the energy needed to transition between any two enery levels in a hydrogen atom is given by the Ryedberg equation:

Answer 12

• 1/lamda = R (1/n₁² - 1/n₂²)
  
  • in this equation, R is the Ryedberg constant (1.1 x 10⁷ m^-1)
  • n₁ and n₂ and the two orbital levels in question
    
    • energy (in the form of a photon) will be absorbed if an electron is being excited to a higher energy level
    • energy will be released if an electron is dropping back down to a lower energy level

Question 13

An absorption spectrum shows what?

An emission spectrum sjows what?

Answer 13

• shows you the range of visible light with black gaps corresponding to the wavelengths of light that are absorbed
• a black background on which the emitted wavelnegths of light appear

Question 14

the number of protons that an atom has is its?

Answer 14

• atomic number
  
  • this criterion essentially determines its identity

Question 15

the protons and neutrons of an atom are added together to make up its?

Answer 15

• atomic weight
  
  • isotopes is a change in neutrons

Question 16

what is nuclear fusion and nuclear fission?

Answer 16

• Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy.

Question 17

what is radioactive decay?

Answer 17

• refers to the spontaneous breakdown of certain isotopes, in which unstable nuclei eject mass (and therefore energy) or photons as radiation

Question 18

what occurs during alpha decay?

Answer 18

• an alpha particle is emitted
  
  • alpha particles are simply helium nuclei, wtih 2 protons, 2 neutrons and a +2 charge so they’re relatively massive and interact easily with matter

Question 19

what is beta decay?

Answer 19

• a beta particle is released
  
  • has 2 forms
    
    • beta minus- a neutron is converted into a proton in the nucleus, and an electron is ejected to maintain charge balance
    • beta plus- a proton is converted into a neutron and a positron is emitted to preserve charge
  • a beta^- particle is actually just an electron and a beta⁺ particle is a positron (e⁺), which is a particle with te same mass as an electron but with a positive charge
  • in both cases, a massless particle known as a neutrino is also ejected

Question 20

what is gamma decay?

Answer 20

• involves the emission of a gamma ray, which is a high energy photon, from an excited nucleus
  
  • gamma decay involves an excited nucleus, not an excited electron and gamma rays are well outside the range of visible light
  • gamma rays have no mass or charge (because they’re photons), so we do not have to deal with atomic numbers or weights changing
  • think of them as the manifestaion of the energy that is being lost from an excited nucleus

Question 21

what is electron capture?

Answer 21

• a nucleus “grabs” an electron, whcih changes a proton into a neutron.
  
  • the idea is that the positive and negative charges of the proton and electron, respectively, cancel out
  • the atomic weight stays the same but the atomic number decreases by one

Question 22

what does half-life refer to?

Answer 22

• the amount of time it takes for half of a radioactive sample to decay (t_1/2)

Question 23

mathematically, we can describe exponential decay as follows.

Answer 23

• If we want to calculate n, the remaining nuclei that have not yet decayed at a given time t, it turns out that all we need to do is to know the original amount of nuclei (n₀) and multiply it by a certain power of e:
  
  • n = n_oe^-(lamda)t
    
    • lamda is a quantitiy referred to as the decay constant
      
      • 1, 100%
      • 1/2, 50%
      • 1/4, 25%
      • 1/8, 12.5%
      • 1/16, 6.25%
      • 1/32, 3.125%

Question 24

since radioactive decat is exponential, it can be presented graphically as an exponential curve. Such graphs can also be transformed into?

Answer 24

• semi-log plots, in which the y-axis (the remaining number of nuclei) is expressed logirithmically, while the x-axis) here, time or the number of half-lives) remains unchanged. This transformation results in a linear graph