Week 2 - Structure of the Atom

Question 1

The Atom

Answer 1

• Everything is made of atoms

Contains
o Nucleus
 Protons (+ charge)
 Neutrons (no charge)

o Electrons orbiting the nucleus (Bohr model of atom)
 -ve charge
 1/1840 mass of a proton (neutron)

Question 2

Ernest Rutherford

Answer 2

Shot alpha particles at gold foil
o Discovered some passed through, while some scattered back at very sharp angles
o Therefore, hypothesising that there was something large within the atom (nucleus)

Question 3

Ernest Rutherford: Model Problems

Answer 3

• Explained some observations
• Orbiting charged particles on a circular trajectory should radiate electromagnetic energy (lose energy)
  o Should therefore, spiral towards the nucleus
  o However, this is not what was observed

Question 4

Neils Bohr

Answer 4

• Uses basic concepts of the Rutherford model

- Postulated new rules for orbiting electrons

Question 5

Neils Bohr Model of the Atom

Answer 5

• Electrons orbit the nucleus at a fixed radii (i.e. in defined shells)
• Electrons do not radiate energy but their energy is fused and determined by shell number (and charge in nucleus)
• Electrons can make transitions from one level to another
  o And emit or absorb the energy difference

Question 6

Uniqueness of Atomic Elements

Answer 6

• Each atomic element has a unique number of protons, neutrons and electrons
• Unique set of electron orbital radii and energies
• Electron making transition between energy levels will emit or absorb electromagnetic radiation with a unique wavelength and energy
• Atomic signature

Question 7

Characterising Atoms

Answer 7

Atomic Number
- Number of protons in the nucleus

Mass Number
- Total number of protons and neutrons in the nucleus

Question 8

Isotopes

Answer 8

o Same number of protons

o Different number of neutrons

Question 9

Electrons

Answer 9

• In a neutral atom the number of electron equals the number of protons
• Atomic number = proton + electron
• Electrons exist in discrete orbitals or shells and energy levels
• As atomic number increases (number of electrons increase)
• Inner electron shells fill first
• Each shell can hold a maximum number of electrons

Question 10

Electron Shells

Answer 10

K Shell has the greatest energy

Closest to +ve nucleus

Question 11

Electron Transitions

Answer 11

If an electron is forcibly removed from an inner shell
o Electron from outer shell will make the transition to inner shell

o Electromagnetic wave is them emitted
 Visible light
 Ultraviolet
 X-rays

o Wavelength of EM wave depends on difference between energy levels

Question 12

Electron Binding Energy: Definition

Answer 12

• Amount of energy required to completely remove the electron from the atom

Question 13

Electron Binding Energy

Answer 13

• After removal of electron –> atom becomes ionised (+ve charge)
• The process of removing an electron is called ionisation
  o Electron must be bound to the atom initially
  o Must be free from the atom after the process

Question 14

Transition of Electron to Another Shell

Answer 14

• Excitation
• Fluorescence
  o If energy is not high enough to cause ionisation, electron just changes levels
  o Emission is determined by energy levels

Question 15

What Determines Electron Binding Energy?

Answer 15

• The number of protons in the nucleus
  o e.g., more protons = more +ve charge (electrons will be more tightly bound)
• The proximity of an electron to the nucleus (the orbit or shell it sits in)
• An electron in a particular shell in a particular atom will always have the same binding energy
• Binding energy of electron depends on the atomic number and shell
  o Tungsten (atomic number = 74)
   K-Shell binding energy is always 69.5 keV
   L-Shell binding energy is always 10.2 keV

Question 16

Atomic Balance and Creation of Ions

Answer 16

• Atoms can lose electrons
  o Becomes +ve charged (cation)
• Atom can gain electrons
  o Becomes -ve charged (anion)
• Atoms are generally neutrally balanced
  o Protons = Electrons

Question 17

Isotopes

Answer 17

• Nuclei which contain the same number of protons but different number of neutrons are known as isotopes

Question 18

Electrostatic Repulsive Force

Answer 18

• Protons repel each other –> electrostatic repulsion due to magnetic field

Question 19

Strong Nucleus Force

Answer 19

• Gravitational force between nucleon masses is too weak to overcome the electrostatic repulsive force
• Attractive force between nucleons (proton-proton, neutron-neutron, neutron-proton)
• Has a very short range (10^-15) (100 times larger than electrostatic repulsive force)
• At longer distance – insignificant
• Effectively only acts between neighbouring nucleons
• Allows nuclei to be stable

Question 20

Stable-Unstable Nuclei

Answer 20

• Strong force is able to balance out the repulsive force
• As number of protons increase –> more neutrons are required to counterbalance the repulsive force between protons (maintain stability)
• Balancing of repulsive and attractive forces is no longer possible by adding more neutrons
  o Limited Range of Strong Force (10^-15m)

Question 21

Bismuth

Answer 21

• For Z > 83 a nucleus is unstable
• Nuclei spontaneously break apart or rearrange their internal structure
  o Known as Radioactivity