1.01 Atomic Structure

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  • Created on: 20-10-20 10:55

Fundamental Particles

.................................Relative Mass     |     Relative Charge 

Protons                               1                              +1

Neutrons                             1                               0

Electrons                            1/1840                         -1

Atomic number (Z) = number of protons 

Mass number (A) = number of protons + neutrons 

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Development of the Atomic Models

John Dalton (1803) described atoms as solid spheres, different elements had atoms of different sizes. 

J.J. Thompson (1897) discovered that atoms contained tiny, negatively charged particles (electron) = 'plum pudding model'.

Rutherford (1909) conducted alpha particle scattering experiement, if the plum pudding model was correct the particles would have been deflected, instead most of them passed straight through = empty space. Nuclear model - small, positively charged nucleus in centre surrounded by a cloud of negative electrons. 

However, scientists realised that electrons in a cloud would be attracted to the nucleus, causing the atom to collapse.

Bohr (1913) suggested that electrons were contained in shells at a fixed distance from the nucleus.

Scientists later discovered that not all electrons in a shell have the same energy = sub-shells

Chadwick (1932) provided evidence for neutral participles in the nucleus (neutrons). 

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Isotopes

Isotopes are the same element with the same number of protons but different number of neutrons. 

The chemical properties of isotopes are the same because they have the same electron configuration .

The relative atomic mass of an element is the average mass of the element compared to 1/12 the mass of a Carbon-12 atom. 

RFM= Sum of (isotope mass number x isotope abundance) / Sum of abundances  

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TOF Mass Spectrometer

1. Ionisation: 

Electron impact sample is vaporised and high energy electron fired at it from electron gun, knocks off one electron to form a 1+ ion

Electrospray ionisation sample is dissolved in volatile solvent and pushed through a small nozzle which is connected to positive terminal of high-voltage suppply. The particles gain a proton so become positively charged ions.

2. Acceleration: positive ions accelerated by electric field (towards negatively charged plate). All ions given same kinetic energy, so lighter ions travel faster (KE = 1/2 m v).

3. Ion drift: ions travel through flight tube (since lighter ions travel faster, they will reach the end of the flight tube first). 

4. Detection: time taken to reach the detector is used to find m/z value (since the ions normally have a charge of 1+, m/z gives mass). When the positive ions reaches the detector it picks up an electron which causes a current to flow, current is proportional to abundance. 

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Calculating TOF

When KE is unknown...                                       KE in J   t= seconds   d= metres   m= kg                         

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NB: to find mass=                 mass number     /

                            1000 x Avogadros constant (6.022×10²³)

When KE & length is known...                        therefore, to find d...      

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Electron Arrangement (Sub-shells)

Main (principal) energy levels are further split into sub-shells.

Sub-level    |   Max no. of electrons                                Main energy level (n)  |   Sub-shells        |   Max no. of electrons

s                                 2                                                                   1                     1s                                    2

p                                 6                                                                   2                     2s, 2p                              8

d                                10                                                                  3                     3s, 3p, 3d                       18

f                                 14                                                                  4                     4s, 4p, 4d, 4f                   32

There is an overlap between principal levels n=3 and n=4. The 4s sub-shell has a lower energy than the 3d sub-shell (= 4s filled first). Filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d...

NB: therefore, for neutral transition metal atoms the 4s sub-shell is lower in energy than the 3d sub-shell. However, for transition metal ions the 3d sub-shell falls below the 4s, so the 4s electrons are removed first. 

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Filling Order

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Exceptions: Cu has 1 electron in 4s sub-shell, and 10 in 3d subshell, and Cr has 1 electron in 4s sub-shell and 5 electrons in 3d sub-shell. 

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Electron Arrangement (Orbitals)

An atomic orbital is a volume of space in which there is a 95% chance of finding an electron. NB: it is not a fixed electron orbit around which an electron moves.

An atomic orbital can hold a maximum of two electrons. 

An atomic orbit can be represented by a box. 

Two electrons occupying the same orbit must be spinning in opposite directions:                                                                     Untitled picture Double-tap with two fingers to show the edit menu.
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     = lower energy arrangement                                                          

Where there is more than one orbital available in a sub-shell, the orbitals are first occupied singly by electrons (negative electrons repel, further apart = lower energy arrangement).

Singly occupied orbitals have parallel spins. 

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Ionisation Energy

The first ionisation energy is the energy require to remove one electron from each atom in a mole of gaseous atoms producing one mole of 1+ gaseous ions.

The second ionisation energy is the energy required to remove one electron from each ion in a mole of 1+ gaseous ions producing one mole of 2+ gaseous ions. 

Each subsequent removal of an electron requires a greater amount of energy due to an increased attractive force between the negative leaving electron and the ion of increasing positive charge.

Successive ionisation energy values: provide evidence for the existence of main electron energy shells in an atom. A relatively large 'jump' between two values indicates electron removal from a different shell, closer to the nucleus. 

Electrons are more easily removed from the outer shell of an atom because they are further from the nucleus (=weaker attractive force) and more shielded (by electrons in inner shells between nucleus and outer electron). 

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Trends in Ionisation energy

Decreases down a group: as each element down a group gains an electron shell, the inner shells will shield the outer electrons from the attraction of the nucleus. The extra shell also means outer electrons are further away from the the nucleus, reducing the nucleus's attraction. Therefore it gets easier to remove an outer electron = lower ionisation energy.

Increases across a period: the number of protons increases across the group, resulting in a stronger nuclear attraction, and the shielding remains similar. Therefore, it gets harder to remove an outer electron = higher ionisation energy. 

Discontinuities: 

  • Fall in first ionisation energies between group 2 and 3 e.g magnesium & aluminium: outer electron is in 3p sub-shell rather than 3s. The 3p sub-shell has higher energy so is further away from the nucleus, and is shielded by electrons in 3s sub-shell = less energy needed to remove electron.
  • Fall in first ionisation energies between group 5 and 6 e.g. phosphorus & sulfur: in sulfur the electron is being removed from an orbital containing two (spin paired) electrons. There is more repulsion between spin paired electrons so less energy needed to remove electron. 
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