T01/024 : ATOMIC STRUCTURE AND SUB ATOMIC PARTICLES

Question 1

Who are the main scientists involved in early atomic theory? (7)

Answer 1

Democritus, John Dalton, J.J Thomson, Ernest Rutherford, Niels, Bohr, Robert Millikan and Irwin Schrodinger.

Question 2

Describe the development of the different models of the atomic structure from 1803 to 1926

Answer 2

Democritus - theorized the existence of atoms.

Dalton - proposes atom as smallest unit of an element

JJ Thomson - Plum pudding Model (e reside in a sphere of uniform positive charge) using cathode ray

Ernest Rutherford - atoms contain a positive nucleus that contains almost all the mass of an atom & alpha and beta particles

Bohr - e are located on circular, fixed orbits around the nucleus

Lastly, e reside in spaces called orbitals around the nucleus.

Question 3

On what basis does the cathode ray tube experiment performed by JJ Thomson work?

Answer 3

The interaction between electromagnetic fields and charged particles

Question 4

When a charged particle enters a region where there is a uniform electric
field (and no magnetic field) it will follow a parabolic path. What factors affect the exact dimensions of the parabola?

Answer 4

• mass of the particle
• charge of the particle
• speed when it enters the field
• strength of the electric field

Question 5

How can the speed of a charged particle be determined?

Answer 5

It is determined indirectly by perfectly balancing the electric force against the magnetic force so that the charged particle goes in a straight line.

Question 6

Describe the cathode ray experiment.

Answer 6

1) Cathode ray tube is a sealed glass tube from which most of the air has been evacuated.
2) The tubes are called cathode ray tubes because the particle beam or “cathode ray” originates at the cathode.
3) By heating part of the cathode as an electric filament (marked F on the
diagram) like a light bulb, the negatively charged particles can be freed from the metallic structure of the cathode.
- A voltage applied between the cathode and anode accelerates these negatively charged particles away from the cathode and toward the anode.
4) The ray can be detected by painting a material known as phosphors onto the far end of the tube beyond the anode. The phosphors spark, or emit light, when impacted by the cathode ray.
5) When two oppositely-charged electric plates are placed around the cathode ray. The cathode ray is deflected away from the negatively charged electric plate and towards the positively-charged plate. This
indicates that the cathode ray is composed of negatively-charged particles.
6) When two magnets are placed on either side of the tube, the magnetic
field also deflects the cathode ray- the negatively charged particles are
attracted towards the north pole.
7) The results of these experiments helped Thomson determine the mass-tocharge ratio of the cathode ray particles. The mass of each particle was
much smaller than that of any known atom.

Question 7

Why is most of the air in a cathode ray tube evacuated?

Answer 7

Most of the air in a cathode ray tube (CRT) is evacuated to create a vacuum. This is important because it allows the electrons to travel freely without colliding with air molecules. If there were air inside, the electrons would scatter and lose energy, making the tube less efficient at producing images. The vacuum helps ensure a clear and bright picture.

Question 8

What results helped Thomson determine the mass-to-charge ratio of the cathode ray particles?

Answer 8

1. When an electric charge is established and applied across the cathode ray the beam deflects away from the negative plate. This indicates the cathode ray is composed of negatively-charged particles.
2. When a magnetic force is applied across the cathode ray it deflects . The negatively charged particles are attracted towards the north pole.

Question 9

Describe the right hand rule

Answer 9

Fleming’s Right Hand Rule states that if we arrange our thumb, forefinger and middle finger of the right-hand perpendicular to each other, then the thumb points towards the direction of the motion of the conductor relative to the magnetic field, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the induced current.

Question 10

How was it concluded that all matter contains negatively charged particles?

Answer 10

When different metals are used as electrode materials, the properties
of the cathode rays remained constant no matter what cathode material they originated from.

Question 11

Describe the plum pudding atomic model by JJ Thomson

Answer 11

• From the cathode ray experiment, JJ concluded that all matter contains negative particles and positively charged particles in a neutral atom to balance the negative ones.
• Based on this he proposed the plum pudding model which states that electrons are embedded in a uniform sphere that contained the positive charge and most of the mass of the atom.

Question 12

What is the formula for calculating the strength of the electric field applied across the two metal plates?

Answer 12

E = V/d

Where V represents the applied voltage and d is the distance
between the deflector plates (eg. d=0.80 cm).

Question 13

What is the formula for calculating magnetic strength, B?

Answer 13

B (4.17 * 10^-3 Tesla/Amp) * I

he magnetic field strength, B (measured in
tesla = N/ampere m), at the center of the coils
is determined by the amount of current, I (in
amperes), flowing through the coils.

Question 14

In what directions do the electric and magnetic force cause the negatively charged particles to be deflected?

Answer 14

The plates are oriented so that the negatively charged
particles experience a downward electrical force.
The coils are oriented so that the negatively charged particles experience an
upward magnetic force as they move through the apparatus.

Question 15

Derive the expression for the charge to mass ratio : q/m=(2y)v^2/x^2E

Answer 15

• At any time, the size of the electric force is given by strength of the electric field times the charge of the particle:
  1. F = qE
• and the size of the magnetic force is given by charge of the particle times strength of the magnetic force and speed of the charged particle.
  2. F=qvB
• When there is no electric field or magnetic field, the beam will travel virtually undeflected through the apparatus.
• The beam can be made to go straight by adjusting the deflector plate voltage and coil current so that the downward electric force is exactly balanced by the upward magnetic force.

3.F(electric)=F(magnetic)
qE=qvB
E=vB … v=E/B
* When the magnetic field (𝐵) is off, and only an electric field (𝐸) is applied, the particle beam follows a parabolic path.
* This is due to the force exerted by the electric field.
* The setup involves adjusting the voltage across deflector plates to make the beam hit a specific pin.
Key factors:
* Vertical Deflection (𝑦): How far the beam moves vertically.
Horizontal Distance (𝑥): The distance the beam travels horizontally before hitting the pin.
The electric force (𝐹electric=𝑞𝐸) causes acceleration
4.(𝑎=𝑞𝐸/𝑚),
derived using Newton’s second law.
The vertical displacement 𝑦 is related to acceleration 𝑎 and time 𝑡 as:
5.𝑦=1/2𝑎𝑡^2
The vertical displacement 𝑦 can be measured since it equals half the distance between the plates when the beam hits the pin.
The time 𝑡 for the particle to reach the pin can be tricky to measure directly, so instead, we use the horizontal distance
6.𝑥 = 𝑣⋅𝑡 or 𝑡 = 𝑥/𝑣

Here,
𝑣
v is the horizontal speed of the particle.
Substitute
𝑡=𝑥/𝑣
into the equation for
𝑦=1/2 ⋅ 𝑞𝐸/𝑚 ⋅ (𝑥/𝑣)^2

Rearranging gives an expression for
𝑞/𝑚 =(2𝑦)𝑣^2/(𝑥^2​E)
To calculate , ensure all quantities are in the correct units:
Distance: meters (𝑚)
Speed: meters per second (𝑚/𝑠)
Electric Field (𝐸): volts per meter (𝑉/𝑚)

Question 16

What are the standard mass and charge of an electron?(standard unit)

Answer 16

e=1.602×10 ^−19C
m=9.108×10 ^−31kg

Question 17

Distinguish the 3 types of radioactive particles(alpha, beta and gamma)

Answer 17

alpha particles(helium nuclei) - readily absorbed by material, consisted of particles with a positive charge and massive compared to electrons.

Beta particles - same mass to charge ratio as Thomson’s e, are known to be high-speed electrons

Gamma rays - similar to the low-energy form of radiation called x-rays but higher in energy

Question 18

Distinguish the radioactive particles in regards to:
(a) how they are deflected by an electric field
(b)degree to which they penetrate matter

Answer 18

(a)
- alpha and beta particles are deflected in opposite directions
- alpha particles are deflected to a much smaller extent because of their higher mass-to-charge ratio.
- gamma rays have no charge so they are not deflected
(b)
- alpha particles have the least penetrating power and are stopped by a sheet of paper
- beta particles have a higher penetrating power so they pass through a thin sheet of metal but are absorbed by thick glass or lead foil
- gamma rays readily penetrate matter; thick blocks of lead are needed to penetrate them

Question 19

some alpah particles are deflected at very large angles

Describe the gold foil experiment carried out by Ernest Rutherford and its contribution to the nuclear atomic model proposed by Rutherford.

Answer 19

• Rutherford bombarded a thin beam of alpha particles (from radium inside a lead box with a small pinhole in it) at a very thin sheet of pure gold.
• The gold foil was surrounded by a detector screen that would flash when hit with alpha particles
• The scientists expected that all of the alpha particles would pass through the gold foil with only a slight deflection or none at all because by then, the accepted atomic model was one in which an atom’s mass and charge are uniformly distributed
  throughout the atom.
• While most of the alpha particles were undeflected, a very small percentage bounced off the gold foil at very large angles even redirected back towards the source.
• Rutherford’s conclusions included:
  1. Majority of the alpha particles passed through undeflected meaning most of the atom was empty space
  2. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom, meaning that most of positive chanrge and mass of the atom must have been concentrated in a very small space in the atom’s interior which he called the nucleus.(tiny, dense, central core)
  3. In the nuclear atom, the protons and neutrons, which comprise nearly all of the
  mass of the atom, are located in the nucleus at the center of the atom. The
  electrons are distributed around the nucleus and occupy most of the volume
  of the atom. The nucleus is very small compared to the rest of the atom.

Question 20

What is a mass spectrometer and its functions?

Answer 20

A mass spectrometer is an analytical tool used to measure the mass-to-charge ratio (m/z) of molecules to determine molecular weight, identify unknown compounds, quantify known compounds, and study molecular structures and properties.

Question 21

What are the components of a mass spectrometer?

Answer 21

• ionization source
• mass analyzer
• Detector

Question 22

How does a mass spectrometer work?

Answer 22

• Molecules are converted to gas-phase ions so that they can be moved about and manipulated by external electric and magnetic fields.
• The ions are accelerated towards a highly
  negatively charged plate with a hole in it.
• They are all given the same kinetic energy,
  KE = ½mv2
• The ion picks up an electron from the detector to
  complete the circuit

Question 23

How do you calculate mass of an ion in mass spectrometry?

Answer 23

Mass of one ion (kg) = (relative isotopic mass x 10-3)/
6.022 x 1023

Question 24

avogadro’s constant…

How do you compare isotopes in mass spectrometry?