Atomic Structure

Corresponds to topic 1.2

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  • Created by: Mary
  • Created on: 03-03-13 17:49

Properties of Protons, Neutrons and Electrons

Electrons are found in shells circling the nucleus. It has a charge of -1 and a relative mass of 1 / 1820

Protons are found in the nucleus of an atom. It has a charge of +1 and a relative mass of 1.

Neutrons are also found in the nucleus. They have no charge and a relative mass of 1. 

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Atomic Number and Mass Number

Atomic number: this is the number of protons in an atom. It is the smaller of the two numbers beside an atom in the Periodic Table. It tells you how many protons are present.

Mass Number: this is the number of protons and neutrons in an atom's nucleus. It is the larger of the two numbers beside an atom in the Periodic Table. It tells you how many protons and neutrons there are. 

To find the no. of protons = atomic number

To find the no. of electrons (in a neutral molecule) = atomic number

To find the no. of neutrons = mass number - atomic number

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Relative Atomic Mass and Relative Molecular Mass

Relative atomic mass (RAM) is defined as the mass of one atom of an element compared to 1/12 the mass of a carbon-12 atom

Relative molecular mass (RMM) is defined as the mass of one molecule of a compound compared to 1/12 the mass of a carbon-12 atom

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Isotopes and Relative Isotopic Mass

Isotopes are atoms of the same element, which have the same atomic number but different mass numbers. 

This means that isotopes have the same number of protons in their nuclei, but different numbers of neutrons. 

Relative isotopic mass (RIM): the mass of an isotope of an element compared to 1/12 the mass of a carbon-12 atom

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Interpreting Mass Spectra

Calculating relative atomic masses

The x-axis of a mass spectra is labelled m/z, which tells you that the peaks represent the mass to charge ratio. Since the charge is usually +1, this can usually just be thought of as the mass. 

Determining Isotopic Abundances

The y-axis of a mass spectra tells you the percentage abundance of each isotope. Therefore to find the relative atomic mass of an atom you must multiply each isotope by its percentage abundance, add them all together and divide by 100. 

x-axis = RAM

y-axis = % abundance

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Deducing Relative Molecular Mass

In a mass spectrum, the ion with the largest m/z value is the molecular peak. This peak corresponds to the RMM of the molecule. 

From this we can work out the molecule's RMM and what atoms it comprises. 

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Electronic Configurations

Electronic configuration for elements up to krypton (Kr). Use the Periodic Table method I taught you. 

When you get to Period 4, remember the order goes 4s, 3d ...

Electrons in Boxes:

1. First write out the configuration in s, p, d- form

2. Fill the orbitals with electrons singly first, only then do electrons double up

3. Remember when electrons double up they have opposite spin, so one points up and the other points down. 

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Shape of Orbitals

The s-orbital is circular shaped. 

The p-orbital is dumb-bell shaped. 

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Ionisation Energy

1st Ionisation Energy is defined as the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions with a +1 charge. 

M(g) = M+(g) + e-

2nd Ionisation Energy is defined as the energy required to remove one mole of electrons from one mole of gaseous ions with a +1 charge to form one mole of ions with a +2 charge. 

M+(g) = M2+(g) + e-

Note the equations above are balanced in terms of charges. 

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Trends in Ionisation Energy

Down Groups: the ionisation energy generally decreases down a group, because less energy is required to remove one mole of gaseous ions from one mole of atoms. This is because as you go down groups the size of the atoms is getting bigger, therefore it is easier to remove an electron because the outer shell electrons are further away from the nucleus.

Across Period: the ionisation energy generally increases across a group because the number of protons is increasing, therefore the positive charge of the nucleus is increasing, drawing electrons closer to itself and requiring more energy to remove.However there are exceptions -  aluminium has a lower ionisation energy than magnesium. This is because magnesium has a filled 3s subshell, which is relatively stable, therefore it requires a lot of energy to remove an electron from this atom, in comparison with aluminium. Phosphorus' ionisation energy is also higher than predicted because it has a half filled 3p subshell, which is also stable, therefore it requires more energy to remove an electron from it, compared to sulfur. 

Down group = decreases

Across period = increases (with exceptions)

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Ionisation Energy Graphs

If we look at the graph of the ionisation energy for the first 20 elements we can see there is a dip at element 3 (Li), element 5 (B), element 11 (Na), element 13 (Al) and element 19 (K). If we look at where these dips are happening we can see why:

Between He and Li - corresponds to moving from a 1s shell to a 2s shell

Between Be and B - corresponds to moving from a 2s shell to a 2p shell

Between Ne and Na - corresponds to moving from a 2p shell to a 3s shell

Between Mg and Al - corresponds to moving from a 3s shell to a 3p shell

Between Ar and K - corresponds to moving from a 3p shell to a 4p shell

This provides evidence of s, p and d-subshells.

Also if you look at the successive ionisation energies of an element you can see that moving to a new subshell requires a huge amount of energy - hence there is a large energy gap.

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Spectrum of Atomic Hydrogen

If we look at the emission spectrum of hydrogen we can see that the spectral lines eventually get closer and closer until they reach a limit - this limit is called the "series limit".

These lines correspond to the different energy levels in the hydrogen atom. What this means is there are only a certain number of energy levels which electrons can inhabit and these eventually converge (come together).

This tells us that energy levels can only have certain energies (they are discrete). 

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Emission Spectra

Emission spectra result from excitation of the atom. This causes it to gain energy and rise from the ground state to a higher energy level. When this happens, it wants to move back down to the lower energy level, and when it does this it releases energy as radiation. 

The radiation corresponds to light of a certain wavelength, from which the energy can be calculated. 

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Convergence of Lines in an Emission Spectrum

This corresponds to the convergence of energy levels, which gives us the value for the ionisation energy

We can calculate the ionisation energy using the equation:

E = hf where h= Planck's constant and f= frequency

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Flame Tests

Li+ = red

Na+ = yellow

K+ = lilac

Ca2+ = brick red

Ba2+ = apple green

Cu2+ = blue green

How does the colour arise?

The colour arises from the ion being excited to a higher energy level, when it is heated in the flame. This causes it to rise from the ground state to a higher energy level. The ion returns to the lower energy level releasing some of the energy of radiation. This radiation corresponds to energy of a certain wavelength and is given out as light

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