Module 3 Section 1: The Periodic Table Flashcards
How were chemical categorised in the early 1800s
By their physical and chemical properties and by their relative atomic mass
What did Johann Döbereiner do
Attempted to group similar elements - called Döbereiner’s triads
Noticed chlorine, bromine and iodine had similar properties and that properties of bromine fell halfway between chlorine and iodine
This was the same for lithium, sodium and potassium
Called these triads
What did John Newlands do
Noticed that if he arranged the elements in order of mass, similar elements appeared at regular intervals
Every eighth element was similar
Called this the law of octaves
However, this broke down on the third row
What did Dmitri Mendeleev do
Arranged all known elements by atomic mass
Left gaps in the table where the next element didn’t seem to fit
This meant he could keep elements with similar chemical properties in the same group
Also predicted the properties of undiscovered elements that would go in the gaps
When elements were later discovered that had properties that fitted with Mendeleev’s predictions (germanium, scandium and gallium) it showed that he had got it correct
How are elements ordered in the present day periodic table
Elements are arranged by increasing atomic number ( proton number )
They are arranged into groups and periods
How are elements arranged in periods
All elements within a period have the same number of electron shells
This means that there are repeating trends in the physical and chemical properties of elements across each period
What is periodicity
The repeating trend in the properties of elements
How are elements arranged in groups
All elements within a group have the same number of electrons in their outer shell
This means they have similar chemical properties
Group number tells you the number of electrons in the outer shell
How to use the periodic table for electron configuration
The periodic table can be split into an s-block, d-block, p-block which can show which sub-shells all the electrons go into
What is the first ionisation energy
The first ionisation energy is the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms
What type of process is ionisation
Energy must be put in so it’s an endothermic process
Equation for first ionisation energy
O (g) -> O+ (g) + e-
1st ionisation energy = +1314 kJ mol-1
Important notes on ionisation energy
You must use the gas state symbol, (g), because ionisation energies are measured for gaseous atoms
Always refer to 1 mole of atoms, as stated by the definition, rather than to a single atom
The lower the ionisation energy, the easier it is to form an ion
What factors effect ionisation energies
Nuclear charge
Atomic radius
Electron shielding
How does nuclear charge effect ionisation energy
The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons
Increases ionisation energy
How does atomic radius affect ionisation energy
Attraction falls off very rapidly with distance
An electron close to the nucleus will be much more strongly attracted than one further away
Decreases ionisation energy
How does shielding affect ionisation energy
As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge
This lessening of the pull of the nucleus by inner shells of electrons blocking the attraction is called shielding
Decreases ionisation energy
What does a high ionisation energy mean
Means that there’s a strong attraction between the electron and the nucleus, so more energy is needed to overcome the attraction and remove the electron
What happens to ionisation energy as you go down a group
The ionisation energies generally fall
This means it gets easier to remove outer electrins
This is because:
Elements further down a group have extra electron shells compared to one’s above
The extra shells man that the atomic radius is larger, so the outer electrons are further away from the nucleus, which greatly reduces their attraction to the nucleus
The extra inner shells shield the outer electrons from the attraction of the nucleus
(The positive charge of the nucleus does increase as you go down a group (due to extra protons), but this effect is overridden by the effect of the extra shells)
Why does ionisation energy decreasing as you go down a group provide evidence that electron shells exist
A decrease in ionisation energy going down a group supports the Bohr model of the atom
What happens to ionisation energies as you move across a period
The ionisation energies increases
This means it gets harder to remove the outer electrons
This is because the nuclear charge increases and atomic radius decreases as the electrons are pulled closer to the nucleus
The extra electrons that elements gain across a period are added to the outer energy level so they don’t provide any extra shielding effect
Why does the first ionisation energy decrease between groups 2 and 3
This is due to sub shell structure
The outer electron in group 3 elements is in a p orbital rather than s orbital
A p orbital has a slightly higher energy than an s orbital in the same shell, so the electrons is, on average, to be found further from the s electrons
These factors override the effect of the increased nuclear charge, resulting in the ionisation energy dropping slightly
Why does ionisation energy drop between groups 5 and 6
p orbital structure
In the group 5 elements, the electron is being removed from a singly-occupied orbital
In the group 6 elements, the electron is being removed from an orbital containing two electrons
The repulsion between two electrons in an orbital means that electrons are easier to remove from shared orbitals
How remove all electrons from an atom
This would be done using successive ionisation energies
Each time you remove an electron, there’s a successive ionisation energy
O (g) -> O+ (g) + e-: first ionisation energy
O+ (g) -> O2+ (g) + e-: second ionisation energy
Why does a graph showing successive ionisation energy show the ionisation energy as increasing
Within each shell, successive ionisation energies increase
This is because electrons are being removed from an increasingly positive ion
Also less repulsion amongst the remaining electrons
So the electrons are held more strongly by the nucleus
The big jumps in ionisation energy happen when a new shell is broken into
This is where an electron is being removed from a shell closer to the nucleus
How can successive ionisation graphs tell you what group the element is in
Count how many electrons are removed before the first big jump to find the group number
How can successive ionisation graphs tell you the electronic structure of an element
Working from right to left, count how many points there are before each big jump to find how many electrons are in each shell, starting with the first
What are giant covalent lattices
Huge networks of covalently bonded atoms
(Sometimes called macromolecular structures too)
Carbon atoms can form this type of structure because they can each form four strong, covalent bond
Examples of giant covalent lattices
Diamond
Graphite
Graphene
What are allotropes
Different forms of the same element in the same state
Structure of diamond
Each carbon atom is covalently bonded to four other carbon atoms
The atoms arrange themselves in a tetrahedral shape
It’s got a crystal lattice structure
Properties of diamond
Lots of strong covalent bonds
Very high melting point (sublimes at over 3800K)
Very hard - can be used in diamond tipped drills and saws
Vibrations travel easily through the lattice, so it’s a good thermal conductor
Can’t conduct electricity - all outer electrons are held in localised bonds
Won’t dissolve in any solvent
Structure of silicon
Silicon (same group as carbon)
Forms crystal lattice structure with similar properties to carbon
Each silicon can form 4 strong covalent bonds
Structure of graphite
Weak forces between layers of graphite are easily broken, so the sheets can slide over eachother, can be used in lubricants
The delocalised electrons in graphite aren’t attached to any particular carbon and are free to move along the sheets, so an electric current can flow
Layers are quite far apart compared to the length of the covalent bonds, so graphite is less dense than diamond and is used to make strong, lightweight sports equipment
Strong covalent bonds in the hexagon sheets, graphite has a very high melting point
Properties of graphite
Can be used as a lubricant
Conducts electricity
Relatively low density
High melting point (sublimes at over 3900K)
Graphite is insoluble in any solvent, covalent bonds in the sheets are too strong to break
What is graphene
A sheet of carbon atoms joined together in hexagons
Sheet is one atom thick, making it a 2D compound
Properties of graphene
The delocalised electrons are free to move along the sheet
Without layers, they can move quickly above and below the sheet, making graphene very electrically conductive
The delocalised electrons strengthen the covalent bonds between the carbon atom which makes graphene strong
Single layer of graphene is transparent and very light
High melting and boiling point
Insoluble due to strong covalent bonds
Uses of graphene
Can be used in high-speed electronics and aircraft technology
This is due to its high strength, low mass and good electrical conductivity
Flexibility and transparency also allows it to be used for touchscreens
Structure of metals
Giant metallic lattice structures
Electrons in outer shell of metal atoms are delocalised- electrons are free to move around the metal
This leaves a positively metal cation
Metal cations are electrostatically attracted to the delocalised negative electrons
They form a lattice of closely packed cations in a sea of delocalised electrons - this is metallic bonded