periodicity Flashcards
Describe and explain the trend in atomic radius
across period 3
As you go across the period, atomic radius decreases:
● Number of protons in the nucleus/ nuclear charge increases.
● Number of electrons in the outer shell increases.
● Shielding remains the same.
● Nuclear attraction between the electrons and the nucleus
increases so electron shells are drawn closer to the nucleus,
decreasing the atomic radius.
Describe and explain the trend in ionic radius across
period 3
● From Na+ to Mg2+ to Al3+:
Ionic radius decreases because the number of electrons decreases so
there is greater attraction between outer shell electrons and the
nucleus meaning the electrons are drawn inwards.
● From P3- to S2- to Cl-:
Ionic radius increases because the number of electrons increases
which weakens the nuclear attraction meaning the electrons are not
drawn inwards as strongly.
Describe the trend in melting point
across period 3
● Melting point increases from sodium to silicon.
● There is a sharp decrease in melting point
between silicon and phosphorus.
● There is a slight increase in melting point between
phosphorus and sulfur.
● Melting point then decreases from sulfur to argon.
Why does melting point increase from sodium to
silicon?
Na → Mg → Al → Si
● Na, Mg and Al are all giant metallic structures.
● As you go from Na to Mg to Al, number of protons and electrons
increases. Atomic radius decreases.
● This leads to greater electrostatic attraction between nuclei and
electrons which requires more energy to overcome and melt the metal.
● Silicon has a giant covalent lattice structure which has strong
covalent bonds between atoms which require a lot of energy to overcome.
Why is there is a sharp decrease in melting point
between silicon and phosphorus?
● Silicon has a giant covalent lattice structure whereas
phosphorus has a simple covalent structure.
● The strong covalent bonds between the silicon atoms
require a lot of energy to overcome.
● The weak London forces between P4 molecules
require little energy to overcome.
Why is there is a slight increase in melting point between
phosphorus and sulfur?
● Sulfur has more atoms per molecule than phosphorus so
sulfur molecules contain more protons and electrons.
● As a result, the London forces between molecules are
stronger so more energy is required to overcome these
forces during melting.
● The increase is only small because sulfur is still a simple
molecular compound.
Why does melting point decreases from sulfur to argon?
● S8, Cl2 and Ar are all simple covalent substances.
● From S8 to Cl2 to Ar, the molecules are getting
smaller.
● This means that there are weaker intermolecular
(London) forces between molecules.
● As a result, less energy is required to overcome
these forces and melt the substance.
Describe and explain how electrical conductivity
varies across period 3
● Conductivity increases from sodium to magnesium to
aluminium because metallic bonding means that they
contain delocalised electrons that are free to move.
● Silicon is a semiconductor.
● Elements from phosphorus to argon are
non-conductors because they are simple molecular
substances (no delocalised electrons/ mobile charges).
How does ionisation energy vary across period 3?
In general, ionisation energy increases across a period
because:
● Nuclear charge and atomic radius increase, shielding
remains the same.
● Nuclear attraction increases.
● As a result, more energy is required to remove an
electron so ionisation energy increases.
What are the properties of ceramics?
● Strong
● High melting point
● Electrically insulating
Give some examples of ceramics
● Magnesium oxide (ionic)
● Aluminium oxide (ionic)
● Silicon dioxide (covalent)
How are the properties of ceramics based on their
structure?
● Strength: The ionic/ covalent bonds in ceramics are very
strong (giant structures).
● High melting points: Lots of energy is required to overcome
these strong ionic or covalent bonds to melt the substance.
● Electrically insulating: non-conductors. Covalent compounds
have no mobile electrons and when ionic compounds are
solid, the ions are fixed in a giant ionic lattice.
Describe how sodium reacts with oxygen (include an
equation)
2Na + ½O2 → Na2O
Sodium burns in oxygen with an orange
flame to produce sodium oxide, a white
solid.
Describe how magnesium reacts with oxygen
include an equation
Mg + ½O2 → MgO
Magnesium burns in oxygen with an
intense white flame to form magnesium
oxide, a white solid.
Describe how aluminium reacts with oxygen (include
an equation)
4Al + 3O2 → 2Al2O3 Aluminium will burn in oxygen if powdered. Sprinkling this powder into a bunsen gives white sparkles and forms aluminium oxide, a white solid.
Describe how phosphorus reacts with oxygen
include an equation
P4 + 5O2 → P4O10
White phosphorus catches fire
spontaneously in air (burns with a white
flame). In excess oxygen, phosphorus (V)
oxide forms.
Describe how sulfur reacts with oxygen (include an
equation)
S + O2 → SO2
Sulfur burns in air on gentle heating with a pale blue flame. This
produces colourless SO2 gas.
To convert SO2 to SO3:
⇌
● 2SO2 + O2 2SO3
℃
● 400 - 450 , 1-2 atm, V2O5 catalyst.
Describe sodium reacts with chlorine (include an
equation)
Na + ½Cl2 → NaCl
Sodium burns in chlorine with a bright
orange flame to produce sodium
chloride, a white solid.
Describe how magnesium reacts with chlorine
include an equation
Mg + Cl2 → MgCl2
Magnesium burns in chlorine with an
intense white flame to form magnesium
chloride, a white solid.
Describe how aluminium reacts with chlorine
include an equation
2Al + 3Cl2 → 2AlCl3
Dry chlorine is passed over aluminium foil to form aluminium
chloride, a very pale yellow solid.
⇌
2AlCl3 Al2Cl6
℃
At around 180 - 190 (dependent upon pressure), AlCl3 is
converted to Al2Cl6 which then vaporises.
Describe how silicon reacts with chlorine (include an
equation)
Si + 2Cl2 → SiCl4
If chlorine is passed over powdered silicon and
heated, it reacts to form silicon tetrachloride, a
colourless liquid, which then vaporises (can be
condensed further along the apparatus).
Describe how phosphorus reacts with chlorine
include an equation
P4 + 10Cl2 → 4PCl5
White phosphorus burns spontaneously
in excess chlorine to form PCl5, an
off-white/ almost yellow solid.
Describe how sodium reacts with water (include an
equation)
2Na + 2H2O → 2NaOH + H2
A very exothermic reaction forms
hydrogen gas and a colourless solution
of sodium hydroxide.
Describe how magnesium reacts with cold water
include an equation
Mg + 2H2O → Mg(OH)2 + H2
Magnesium hydroxide forms on the outside of
the metal strip. A few bubbles of hydrogen
float to the surface of container. The reaction
generally stops after this.
Describe how magnesium reacts with
steam (include an equation)
Mg + H2O → MgO + H2
Magnesium burns with its typical white
flame.
How do period 3 oxides (from sodium to sulfur) vary
in oxidation number?
Na2O, MgO, Al2O3, P4O10, SO2, SO3.
+1 +2 +3 +5 +4 +6
The general trend is that oxidation
number increase (apart from SO2) across period 3 oxides.
How do period 3 chlorides (from sodium to
phosphorus) vary in oxidation number?
NaCl, MgCl , Al Cl , SiCl , PCl
+1 +2 2 +3 2 6 +4 4 +5 5
From sodium to phosphorus, the
oxidation number increases.
Why does the oxidation number of period 3 oxides
and chlorides vary?
● Each element in period 3 has a different number of
electrons in its outer shell.
● Hence each element needs to gain/lose/share a different
number of electrons to have a full outer shell and form the
oxide/chloride.
● This leads to each element having a different oxidation
state.
Describe how sodium oxide reacts with water
(include an equation)
Na2O + H2O → 2NaOH
● Exothermic.
● Forms a highly alkaline solution.
Describe how magnesium oxide reacts with water
include an equation
MgO + H2O → Mg(OH)2
● Forms a slightly alkaline solution.
● Most of the Mg(OH)2 that is made is insoluble
and hence doesn’t dissolve in solution to
increase the pH.
Describe how phosphorus(V) oxide reacts with water
(include an equation)
P4O10 + 6H2O → 4H3PO4
● Forms an acidic solution.
● Violent reaction
Describe how sulfur dioxide reacts with
water (include an equation)
SO2 + H2O → H2SO3
● Forms an acidic solution
Describe how sulfur trioxide reacts with water
(include an equation)
SO3 + H2O → H2SO4
● Forms an acidic solution
● Violent reaction
How does sodium oxide react with hydrochloric
acid?
Na2O is a strong base. It reacts with an
acid to form a salt and water:
Na2O + 2HCl → 2NaCl + H2O
How does magnesium oxide react with hydrochloric
acid?
MgO is a weaker base than Na2O though. It
reacts with warm dilute HCl to form a salt and
water:
MgO + 2HCl → MgCl2 + H2O
What does amphoteric mean?
An amphoteric compound is able to act
as both an acid and a base.
How is aluminium oxide amphoteric?
Aluminium oxide (Al2O3) is amphoteric as it reacts with both acids and bases
How does phosphorus (V) oxide react with NaOH?
There are many different reactions that
can occur between phosphorus (V) oxide
and NaOH, an example is:
P4O10 + 12NaOH → 4Na3PO4 + 6H2O
How does sulfur dioxide react with NaOH?
● Sulfur dioxide is bubbled through sodium
hydroxide solution:
SO2 + 2NaOH → Na2SO3 + H2O
● If the sulfur dioxide is in excess:
Na2SO3 + H2O + SO2 → 2NaHSO3
What oxides don’t react with water?
● Aluminium oxide - insoluble in water
● Silicon dioxide - breaking up its giant
covalent lattice structure is too difficult.
Does silicon dioxide react with acids or bases?
Bases (e.g. sodium hydroxide)
Describe how NaCl reacts with water (include an
equation)
NaCl dissolves in water to form a neutral
solution (pH 7).
NaCl(s)+ → Na- (aq) + Cl (aq)
Describe how MgCl2 reacts with water (include an
equation)
MgCl2 dissolves in water to form a faintly acidic solution
(pH 6)
2+ -
MgCl2 + 6H2O → [Mg(H2O)6] + 2Cl
A small proportion of hydrogen ions are removed from
the hydrated magnesium ion, as it a weak acid:
2+ ⇌ + +
[Mg(H2O)6] + 2H2O [Mg(H2O)5(OH)] + H3O
Describe how AlCl3 reacts with water (include an
equation)
AlCl3 + 6H2O → [Al(H2O)6] + 3Cl
Hydrated aluminium ions are a stronger acid than
hydrated magnesium ions so the position of
equilibrium lies further to the right:
3+ ⇌ 2+ +
[Al(H2O)6] + H2O [Al(H2O)5(OH)] + H3O
Describe how SiCl4 reacts with water (include an
equation)
SiCl4 + 2H2O → SiO2 + 4HCl
Violent reaction, produces silicon dioxide and
fumes of hydrogen chloride gas.
Write an equation for the reaction between PCl5 and
cold water
PCl5 + H2O → POCl3 + 2HCl
How does PCl5 react with boiling water?
With water: PCl5 + H2O → POCl3 + 2HCl
If the water is boiling the POCl3 will continue to react:
POCl3 + 3H2O → H3PO4 + 3HCl
Overall boiling water equation:
PCl5 + 4H2O → H3PO4 + 5HCl
Describe how the bonding in group 3 oxides and
chlorides varied across the period
Sodium and magnesium form ionic bonds with
oxygen and chlorine. Aluminium forms covalent
bonds with oxygen and either covalent or ionic
bonds with chlorine. Other period 3 elements
form simple covalent compounds.
Why does the bonding in group 3 oxides
and chlorides vary across the period?
The difference in electronegativity between chlorine/
oxygen and the period 3 element decreases across the
period. There is sufficient difference in the
electronegativity of chlorine/ oxygen and sodium or
magnesium to form ions. After aluminium, the difference
in electronegativity is too small for ions to form.
How can physical properties be used to predict the
type of chemical bonding in group 3 oxides and
chlorides?
A high melting point indicates a giant
molecular structure. This could either be
ionic (like NaCl and MgO) or covalent
(like SiO2).
How can chemical properties be used to predict the
type of chemical bonding in group 3 oxides and
chlorides?
● Chlorides and water: ionic chlorides form a solution with a pH close to 7.
Covalent chlorides react to form an acidic solution and HCl gas.
● Oxides and water: covalent oxides form an acidic solution. Ionic oxides
may react to form an alkaline solution or they may not react.
● Acids and bases: ionic oxides are generally basic (react with acids).
Covalent oxides tend to be acidic (react with bases). Amphoteric oxides
such as aluminium oxide are usually ionic with some covalent character.
● Electrolysis: only molten ionic chlorides/ oxides undergo electrolysis.
What is meant by periodicity?
The recurring variations or trends in the
properties of elements in the periodic
table.
What group normally forms stable -1 ions?
Group 7:
F–, Cl–, Br– and I–
What group normally forms stable +1 ions?
Group 1:
Li+, Na+, K+ and Rb+
What group normally forms stable +2 ions?
Group 2:
Be2+, Mg2+, Ca2+ and Sr2+
What structure do group 4 elements
normally have?
Giant covalent structure
Which elements in the periodic table
form giant metallic structures?
The metals.
