Chemistry, EL Revision Notes

EL1

Simple Model of the Atom

Atoms can be considered to consist of 3 sub-atomic particles

  • Proton, Mr 1, Charge of +1

  • Neutron, Mr 1, Neutral

  • Electron, Mr \(\frac{1}{2000}\), Charge of -1

  • Most of the atom is empty space

Nuclear Symbols

  • Atomic Number, Z

    • Number of protons

    • Lower number

    • Equal to charge on Nucleus

  • Mass Number, A

    • Number of protons and neutrons

    • Highest number

Isotopes

  • Atoms of same element with different mass numbers

  • Different number of neutrons

Relative Atomic Mass

  • Average of relative Isotopic Masses relative to Carbon-12

  • Taking abundance into account

  • Mass Spectrometry used to find it

Mass Spectrometry

  • Measures atomic/molecular mass of different particles, and relative abundances

  • Ionised to cations

  • Separated by mass to charge ratios

Nuclear Fusion

In a nuclear fusion reaction, two light atomic nuclei fuse together to form a single, heavier nuclei, releasing huge amounts of energy in the process of doing so

  • Impossible at normal temperature and pressure

  • Positive nuclei repel too strongly

  • Possible in stars, repulsion overcome

\[{^{1}_{1}H}+{^{2}_{1}H}={^{3}_{2}He}+\gamma\]

EL2

Spectroscopy

Under certain conditions, a substance will absorb or emit electromagnetic radiation in a characteristic way. Analysis of this can lead to identification of the substance.

Absorption Spectra

  • Glowing stars emit all frequencies between UV and IR

  • Chromosphere contains ions, atoms, and small molecules

  • These absorb radiations

  • Emitted light is missing specific frequencies

  • Black lines on coloured backround correspond to particles in chromosphere

Emission Spectra

  • Atoms/molecules/ions raised from ground state energy level when they absorb energy

  • Become excited

  • Lose energy by emitting EM radiation

  • Spectra of coloured lines on black backround

Continuous and Atomic Spectra

White light contains all visible wavelengths, and has a continuous spectra. Light from stars is not the same.

Atomic Spectra of Hydrogen Atoms

  • Balmer in visible light

  • Lyman in UV light

Spectra are the result of the interaction of light and matter

Bohr’s theory and Wave-Particle Duality (WPD)

Wave Theory of Light

  • Light is a form of EMR, so has wavelength and frequency

  • Moves at the speed of light in a vacuum

  • Different colours have different wavelengths

\[c=\lambda \times v\]

Particle Theory of Light

  • Light is a stream of packets of energy

  • Photons

\[E=h \times v\]

Bohr’s Theory

  • An excited atoms electrons will jump into higher energy levels

  • When they drop back, they emit EMR, giving an emission spectrum

When white light is passed through a cool sample of a gaseous element, black lines appear in the absorption spectrum. These correspond to the frequencies absorbed by the atoms in the sample. The intensities of these lines provide a measure of abundance.

  • Explains Spectra

  • Relies on quantisation of energy

  • Electrons exist in definite, discrete levels/shells

  • A photon must be emitted when the electron drops

Energy Levels and Quanta

An electron can only possess definite amounts of energy; QUANTA.

  • Higher energy levels are further from the nucleus

  • Ground state is \(n=1\)