# 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 $$n=1$$

# 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