Atomic Structure

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  • Created by: rdowd40
  • Created on: 05-02-19 14:28

The Atom

Proton

Relative mass: 1

Relative charge: +1

Neutron

Relative mass: 1

Relative charge: 0

Electron

Relative mass: 1/2000

Relative charge: -1

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The Atom

Mass number: total number of protons and neutrons

Atomic number: number of protons

Istopes:

  • atoms with the same number of protons but different number of neutrons
  • same configuration of electrons, so the same chemical properties
  • slightly different physical properties because this tends to depend more on mass

 

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Relative mass

Relative atomic mass, Ar, is the average mass of an atom of an element

Ar = isotopic masses x percentages

                 total percentage

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Mass Spectrometer

Mass spectrometer can give information about relative atomic mass of an elemen and the relatve abundance of its isotopes, or the relative molevular mass of a molecule if you use it to analyse a compound.

TOF:

1.  Ionisation

The sample needs to be ionised before it eneters the mass spectrometer.

  • Electrospray ionisation - sample is dissolved in a solvent and pushed through a small nozzle at high pressure. A high voltage is applied to, causing each particle to gain an H+ ion. The solvent is then removed, leaving a gas made up of positive ions.
  • Electron impact ionisation - sample is vaporised and an 'electron gun' is used to fire high energy electrons at it. This knocks one electron off each particle, so they become +1 ions.
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Mass Spectrometer

2. Acceleration

The positive ions are accelerated by an electric field. The electric field gives the same kinetic energy to all the ions. The lighter ions experience a greater acceleration - they're given as much energy as the heavier ions, but they're lighter, so they acclerate more.

3. Ion drift

Next, the ions enter a region with no electric field. They drift through it at the same speed as they left the electric field. So the lighter ions will be drifting at higher speeds.

4. Detection

Because lighter ions travel through the drift region at higher speeds, they reach the detector in less time than heavier ions. The detector detects the current created when the ions hit it and records how long they took to pass through the spectrometer. This data is then used to calculate the mass or the charge values needed to produce a mass spectrum.

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Ionisation Energies

The first ionisation energy is the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions.

Factors affecting ionisation energy:

1. Nuclear charge

The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons.

2. Distance from nucleus

Attraction falls off very rapidly with distance. An electron close to the nucleus will be much more strongly attracted than one further away. 

3. Shielding

As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction to the nucleus.  

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Ionisation Energies

The second ionisation energy is the energy needed to remove an electron from each ion in 1 mole of 1+ ions. 

The second ionisation is affected by the same factors as the first ionisation energy. 

Second ionisation energies are greater than first ionisation energies because the electron is being removed from a positive ion, and not an atom, which will require more energy. 

The electron configuration of the atom will also play a role in how much larger the second ionisation energy is than the first. 

E.g. The first electron removed from a Li atom comes from the second shell (2s1). The second electron removed comes from the first shell (1s1).

So the second electron to be removed is closer to the nucleus and experiences a stronger nuclear attraction than the first electron to be removed. The second electron will also not experience shielding from any inner electron shells, unlike the first electron. This means that the second ionisation energy of lithium is higher than the first. 

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Ionisation energy trends

Group 2:

First ionisation energy decreases down group 2. 

This is because as you move down the group each element has one more shell than the one before, which increases shielding as well as putting more distance between the outer electron shell and the nucleus. 

Period 3: 

The general trend across this period is that the ionisation energies increase as you move from left to right. This is because the number of protons is increasing, which causes a stronger nuclear attraction. 

HOWEVER, there are small drops between groups 2 and 3, and 5 and 6. 

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Ionisation energy trends

Group 2 and 3:

Aluminium has a lower first ionisation energy than magnesium. 

Aluminium's outer electron is in a 3p orbital rather than a 3s. The 3p orbital has a slightly higher energy than the 3s orbital, so the electron is, on average, to be found further from the nucleus. 

The 3p orbital has additional shielding provided by the 3s electrons. 

These two factors override the effect of the increased nuclear charge. 

Group 5 and 6:

Sulphur has a lower first ionisation energy than phosphorus. 

The shielding is identical in the phosphorus and sulphur atoms, and the electron is being removed from the identical orbital. 

In phosphorus' case, the electron is being removed from a singly-occupied orbital. But in sulphur, the electron is being removed from an orbital containing two electrons. 

The repulsion between two electrons in the orbital means that electrons are easier to remove from shared orbitals. 

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Ionisation energy trends

Group 2 and 3: Aluminium has a lower first ionisation energy than magnesium. 

Aluminium's outer electron is in a 3p orbital rather than a 3s. The 3p orbital has a slightly higher energy than the 3s orbital, so the electron is, on average, to be found further from the nucleus. 

The 3p orbital has additional shielding provided by the 3s electrons. 

These two factors override the effect of the increased nuclear charge. 

Group 5 and 6: Sulphur has a lower first ionisation energy than phosphorus. 

The shielding is identical in the phosphorus and sulphur atoms, and the electron is being removed from the identical orbital. 

In phosphorus' case, the electron is being removed from a singly-occupied orbital. But in sulphur, the electron is being removed from an orbital containing two electrons.

The repulsion between two electrons in the orbital means that electrons are easier to remove from shared orbitals. 

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Ionisation energies and shell structure

Within each shell, successive ionisation energies increase. This is because electrons being removed from an increasingly positive ion - there's less repulsion amongst remaining electrons, so they're held more strongly by the nucleus. 

Big jumps in ionisation energies happen when a new shell is broken into - an electron is being removed from a shell close to the nucleus. 

To identify the group of an element when given a list of ionisation energy values. Look for the biggest jump with tells us that it has moved from group 1 to 8 so count backwards. 

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