Physical and chemical trends of Period3 elementa and their element, oxide and chloride reactions

List the elements of Period 3

Na, Mg, Al, Si, P, S, Cl, Ar

How many electron shells do Period 3 elements have?

3

What happens to the number of protons across Period 3?

Increases left to right

What is nuclear charge?

The total positive charge of the protons in the nucleus

What is effective nuclear charge?

The nuclear charge actually felt by the valence electron; always less than nuclear charge because inner shells shield it

What is shielding?

Inner shells of electrons block some of the nuclear charge from the valence electrons

Why does effective nuclear charge increase across Period 3?

Nuclear charge increases but shielding stays the same

What is the trend in atomic radius across Period 3 and why?

Decreases; nuclear charge rises with no change in shielding, so attraction between nucleus and electrons increases

What is the trend in electronegativity across Period 3 and why?

Increases; higher nuclear charge and smaller atomic radius make it easier to attract a bonding pair of electrons

Define electronegativity

The tendency of an atom to attract a bonding pair of electrons

Define first ionisation energy

The energy required to remove the outermost electron from the valence shell of an atom

What is the general trend in first ionisation energy across Period 3 and why?

Generally increases; valence electrons are held more strongly because atomic radius decreases

Which two elements are anomalies in the first ionisation energy trend?

Al and S (both drop)

What happens to metallic and non-metallic nature across Period 3?

Metallic nature decreases and non-metallic nature increases from left to right

Define charge density

How concentrated an ion's charge is: its charge divided by its size. A small ion with a high charge has a high charge density and strongly pulls electrons toward itself

Why does a smaller, more highly charged cation have a higher charge density?

The same or greater charge is packed into a smaller volume, so the charge is more concentrated

What is the structure of Na, Mg and Al?

Giant metallic lattice

What is the structure of Si?

Giant covalent lattice

What is the structure of P, S, Cl and Ar?

Simple molecular

What type of bonding is in Na, Mg and Al?

Metallic bonding

What type of bonding is in Si?

Covalent bonding

What forces act between molecules of P, S, Cl and Ar?

Van der Waals forces

Why does melting point increase from Na to Al?

Ionic charge increases, ion size decreases and more valence electrons are delocalised, so attraction between cations and delocalised electrons gets stronger

Why does melting point increase as the molecule gets bigger in simple molecular non-metals?

Larger molecules have stronger van der Waals forces, so more energy is needed to break them

If van der waals forces increase as the size of the molecule increases, increasing melting point, why does S have a higher melting point than P?

Because Sulphur exists as S8 and phosphorus as P4. Due to S8 having more electrons than P4, there arestronger Van der waals forces even though P has larger molecules

Which Period 3 element has the highest melting point and why?

Silicon, because it has a giant covalent lattice with strong covalent bonds

Why are Na, Mg and Al good electrical conductors?

They have delocalised electrons that can move

Why does conductivity increase from Na to Al?

The number of delocalised electrons increases

How does Si conduct electricity?

It is a semiconductor and only conducts under certain conditions

Why are P, S, Cl and Ar non-conductors?

They have no mobile charged particles

What is the density trend from Na to Si and why?

Generally increases; atoms get heavier and are pulled closer together

What is the density trend from P to Ar and why?

Generally decreases; intermolecular forces are weaker so molecules are further apart

Describe the observations or the reaction of Na with oxygen

Burns with a yellow flame to form a mixture of oxide and peroxide

Equation for Na reacting with oxygen to form the oxide

4Na + O2 → 2Na2O

Equation for Na reacting with oxygen to form the peroxide

2Na + O2 → Na2O2

Describe the observations of the reaction of Mg with oxygen

Burns with a brilliant white light to form white solid MgO

Equation for Mg reacting with oxygen

2Mg + O2 → 2MgO

Describe the observations of the reaction of Al with oxygen

Reacts quickly to form a thin oxide layer that inhibits further reaction; finely divided Al burns with a white light

Equation for Al reacting with oxygen

4Al + 3O2 → 2Al2O3

Describe the observations reaction of Si with oxygen

Burns if heated strongly enough to produce silicon dioxide

Equation for Si reacting with oxygen

Si + O2 → SiO2

Describe the observations of the reaction of P with oxygen

Burns vigorously with a brilliant pinkish-white flame, giving white fumes of the oxide

Which form of phosphorus inflames spontaneously in air?

White phosphorus

Which form of phosphorus needs heating to react with oxygen?

Red phosphorus

Equation for P reacting with excess oxygen

P4 + 5O2 → P4O10

Equation for P reacting with limited oxygen

P4 + 3O2 → P4O6

How does oxygen supply affect the phosphorus oxide formed?

The richer the O2 supply, the more P4O10 is formed

Describe the observation of the reaction of S with oxygen

Burns with a brilliant blue flame

Equation for S reacting with oxygen

S + O2 → SO2

Describe observations of a reaction between Cl annd oxygen?

No reaction

Describe the reaction of Na with water

Vigorous, exothermic reaction producing a colourless solution of the hydroxide

Equation for Na reacting with water

2Na + 2H2O → 2NaOH + H2

Describe the reaction of Mg with cold water and with steam

Slow reaction with cold water giving Mg(OH)2; very exothermic reaction with steam giving MgO

Equation for Mg reacting with cold water

Mg + 2H2O(l) → Mg(OH)2 + H2

Equation for Mg reacting with steam

Mg + H2O(g) → MgO + H2

Describe the reaction of Al with water

Slow reaction with steam only to form the white oxide

Equation for Al reacting with steam

2Al + 3H2O(g) → Al2O3 + 3H2

Which Period 3 elements do not react with water?

Si, P, S and Ar

Describe the observations of the reaction of Cl2 with water

Dissolves to give a green solution containing HCl and chloric(I) acid (HOCl)

Equation for Cl2 reacting with water

Cl2 + H2O ⇌ HOCl + HCl

Describe the reaction of Na with chlorine

Burns with an orange flame to form a white solid

Equation for Na reacting with chlorine

2Na + Cl2 → 2NaCl

Describe the reaction of Mg with chlorine

Burns with an intense white flame to form a white solid

Equation for Mg reacting with chlorine

Mg + Cl2 → MgCl2

Describe the reaction of Al with chlorine

Burns to form a pale yellow solid which sublimes

Equation for Al reacting with chlorine

2Al + 3Cl2 → 2AlCl3

Describe the reaction of Si with chlorine

Forms a volatile, colourless liquid

Equation for Si reacting with chlorine

Si + 2Cl2 → SiCl4

Describe the reaction of P with chlorine

Forms a mixture of PCl3 (colourless fuming liquid) and PCl5 (off-white solid)

Equations for P reacting with chlorine

2P + 3Cl2 → 2PCl3 and PCl3 + Cl2 → PCl5

Describe the reaction of S with chlorine

Forms an orange, pungent liquid, disulphur dichloride

Equation for S reacting with chlorine

2S + Cl2 → S2Cl2

List the two chlorides formed by phosphorus

PCl3 and PCl5

What is the structure and bonding of NaCl and MgCl2?

Giant ionic lattice with ionic bonding

What is the structure and bonding of AlCl3?

Giant ionic lattice with strong covalent character at room temperature, simple molecular at higher temperatures

What is the structure and bonding of SiCl4, PCl3, PCl5, S2Cl2 and Cl2?

Simple molecular with covalent bonds within molecules and weak van der Waals forces between molecules

Why do Na and Mg form ionic chlorides?

They have low charge density and electronegativity so they lose valence electrons to the more electronegative chlorine

Why do the chlorides become more covalent across Period 3?

As the element gets more electronegative, electron sharing occurs rather than electron loss to chlorine

What is the trend in ionic and covalent character of the chlorides across Period 3?

Ionic character decreases and covalent character increases

What is the pH of NaCl solution?

7

Why does NaCl dissolve without hydrolysis?

Polar water molecules attract the ions and pull them out of the lattice; Na+ has too low a charge density to cause hydrolysis

What is the pH of MgCl2 solution?

About 6.5

Explain why MgCl2 solution is slightly acidic

Mg2+ has a higher charge density than Na+ and slightly pulls electrons from surrounding water, causing a small amount of hydrolysis releasing H+

Equation for hydrolysis of Mg2+ in water

[Mg(H2O)6]2+ ⇌ [Mg(H2O)5OH]+ + H+

Where does the equilibrium lie for hydrolysis of Mg2+?

Far to the left

What is the pH of AlCl3 solution?

About 3

Why is AlCl3 solution more acidic than MgCl2 solution?

Al3+ has a greater charge density than Mg2+, so it polarises water more and hydrolysis occurs to a greater degree

Equation for hydrolysis of Al3+ in water

[Al(H2O)6]3+ ⇌ [Al(H2O)5OH]2+ + H+

Where does the equilibrium lie for hydrolysis of Al3+?

Far to the right

What happens to the trend in acidity as cation charge density increases?

The solution becomes more acidic

Equation for SiCl4 reacting with water

SiCl4 + 4H2O → Si(OH)4 + 4HCl

What is observed when SiCl4 reacts with water?

White precipitate and white fumes of HCl

Equation for PCl3 reacting with water

PCl3 + 3H2O → H3PO3 + 3HCl

Equation for PCl5 reacting with water

PCl5 + 4H2O → H3PO4 + 5HCl

What is the pH of solutions of the non-metal chlorides (SiCl4, PCl3, PCl5, Cl2)?

About 2

What do non-metal chlorides produce when they react with water?

White fumes of HCl gas, which dissolve to form an acidic solution of HCl

Why do non-metal chlorides undergo hydrolysis?

The high electronegativity of the non-metal attracts the lone pairs on oxygen in water

How do non-metal chlorides accept electrons from water?

They use vacant 3d orbitals to accept a lone pair from oxygen, breaking a Cl bond and an H+ from water

When does hydrolysis of a non-metal chloride stop?

When all Cl atoms have been replaced with OH groups

List the oxides of sodium, magnesium and aluminium

Na2O, MgO, Al2O3