Revision notes for AQA AS Chemistry Unit 3 and 4- Bonding and Periodicity

Units 3 and 4 for January AQA syllabus exams

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BONDING
TYPES OF BONDING
Ionic bonding
An ionic bond is an attraction between oppositely charged ions, which are formed by the
transfer of electrons from one atom to another.
This electrostatic attraction holds the ions together and is very strong.
Covalent bonding
A covalent bond is a pair of electrons shared between two atoms.

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Dative covalent bond
A dative covalent bond is a pair of electrons shared between two atoms, one of which provides
both electrons to the bond.
Covalent bonding happens because the electrons are more stable when attracted to two nuclei than
when attracted to only one. Covalent bonds should not be regarded as shared electron pairs in a
fixed position; the electrons are in a state of constant motion and are best regarded more as charge
clouds.…read more

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Electronegativity has no units and is measured on a scale from 0.7 to 4.0. This is called the Pauling
scale.
H He
2.1
Li Be B C N O F Ne
1.0 1.5 2.0 2.5 3.0 3.5 4.0
Na Mg Al Si P S Cl Ar
0.9 1.2 1.5 1.8 2.1 2.5 3.0
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9 1.6 1.6 1.…read more

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Therefore the
another. Therefore particles move freely.
the particles can move
freely and randomly.
Spacing of Particles In a solid, the particles are Packed together but In a gas, all the
tightly packed together in with spaces in particles are in rapid
a lattice. As such solids between the clusters and random motion,
are not easily for particles to move and are much further
compressed. through. apart than a solid or a
liquid.…read more

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Good conductors in liquid state- this is because the delocalised
electrons can carry a current.
Strong, malleable, ductile- this is because there are no bonds holding
specific ions together so the metal ions can slide over each other.
Insoluble because of the strength of the metallic bond.
GIANT COVALENT Very high mpt, bpt- this is because it has four strong covalent bonds.
Poor conductors in solid state as all outer electrons are held in
Eg diamond localised bonds.…read more

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They are the reason all compounds
can be liquefied and solidified.
The strength of the Van der Waal's forces in between molecules depends on two factors:
The number of electrons in the molecule- the greater the number of electrons in a molecule
(or bigger the electron cloud), the stronger the van der Waal's forces between the
molecules.
Surface area of the molecules- the larger the surface area of a molecule, the more exposed
electron cloud and therefore stronger van der Waals.
2.…read more

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Periodicity is the regular recurrence of the properties of elements when they are arranged in
atomic number order as in the periodic table.
Atomic Radius
Going across Period 3:
The number of protons in the nucleus increases so the nuclear charge increases because there are
more electrons, but the increase in shielding is negligible because each extra electron enters the
same principal energy level therefore the force of attraction between the nucleus and the
electrons increases so the atomic radius decreases.…read more

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Electronegativity
Going across Period 3:
The nuclear charge increases and the atomic radius decreases because there are more electrons,
but the increase in shielding is negligible because each extra electron enters the same principal
energy level so electrons will be more strongly attracted to the nucleus.
Melting and Boiling Points
Melting
When a substance melts, some of the attractive forces holding the particles together are broken or
loosened so that the particles can move freely around each other but are still close together.…read more

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Explaining the trend of melting and boiling points from Na to Al
Sodium, magnesium and aluminum
Sodium, magnesium and aluminum are all metals. They have metallic bonding, in which positive
metal ions are attracted to delocalized electrons.
Going from sodium to aluminum:
The charge on the metal ions increases from +1 to +3 (with magnesium at +2) and the number of
delocalized electrons increases so the strength of the metallic bonding increases and the melting
points and boiling points increase.…read more

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Silicon is called a semiconductor because:
The four outer electrons in each atom are held strongly in covalent bonds and few electrons have
enough energy at room temperature to enter the higher energy levels so there are few delocalized
electrons and silicon is a poor conductor but at higher temperatures more electrons are
promoted to the higher energy levels so there are more delocalized electrons to move and carry
charge.…read more

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