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  • 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 \(n=1\)

    EL3

    Shells of Electrons

    It is more appropriate to talk about shells than energy levels, due to complexity of atoms beyond hydrogen.
    Each shell has a maximum number of electrons that it can hold. For a higher value of \(n\), the shell is further from the nucleus and has greater energy.

    First Shell \(n=1\) 2 electrons
    Second/Third Shells \(n=2\)/\(n=3\) 8 electrons
    Fourth/Fifth Shells \(n=4\)/\(n=5\) 18 electrons
    Sixth/Seventh Shells \(n=6\)/\(n=7\) 32 electrons


    The lowest energy shells are filled first. Much of chemistry is decided by the outer shell electrons.

    Sub-Shells of electrons

    Sub-Shells are labelled s, p, d, f. These correspond to the shells:

    • \(n=1\) has an s sub-shell

    • \(n=2\) has s and p

    • \(n=3\) has s, p, and d

    • \(n=4\) has s, p, d, and f


    Sub-Shell Maximum number of electrons
    s 2
    p 6
    d 10
    f 14


    In atoms other than hydrogen, sub-shells within a shell have different energies. The shells of 3d and 4s have an overlap in energies.

    Atomic Orbitals

    • S sub-shells have one s-orbital

    • P sub-shells have three p-orbitals

    • D sub-shells have five d-orbitals

    • F sub-shells have seven f-orbitals

    In an isolated atom, orbitals within the same sub-shell have the same energy.

    • Each orbital can hold a max of 2 electrons

    • Must have opposite spin

    • Corresponds to clockwise or anti-clockwise

    The position of an electron is mapped with a probablilty function as it cannot be pinpointed exactly.

    Filling Atomic Orbitals

    The orbitals are filled to give the lowest energy arrangement possible. To do so, they are filled in order of increasing energy.

    • Orbitals will take one electron until all are full

    • 4-s fills before 3-d as it is lower energy

    • 4-s also empties first

    • Not required to write electron configs for Copper or Chromium.

    • Eg. Scandium: 1s22s22p63s23p63d14s2