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

• 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 $$n=1$$