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The Atom

- Everything is made of atoms

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)


Ernest Rutherford

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)


Ernest Rutherford: Model Problems

- 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


Neils Bohr

- Uses basic concepts of the Rutherford model
- Postulated new rules for orbiting electrons


Neils Bohr Model of the Atom

- 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


Uniqueness of Atomic Elements

- 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


Characterising Atoms

Atomic Number
- Number of protons in the nucleus

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



o Same number of protons
o Different number of neutrons



- 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


Electron Shells

K Shell has the greatest energy
Closest to +ve nucleus


Electron Transitions

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


Electron Binding Energy: Definition

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


Electron Binding Energy

- 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


Transition of Electron to Another Shell

- Excitation

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


What Determines Electron Binding Energy?

- 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


Atomic Balance and Creation of Ions

- 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



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


Electrostatic Repulsive Force

- Protons repel each other --> electrostatic repulsion due to magnetic field


Strong Nucleus Force

- 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


Stable-Unstable Nuclei

- 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)



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