The periodic table (elements in period 3)

Question 1

Atomic radius

Answer 1

the distance between the nucleus and the outermost electron of an atom

Question 2

Trends across the period

Answer 2

• atomic radii decreases
• ionic radius decreases
• general increase in m.p (increases from Na-Si and then decreases after)
• general decrease in electrical conductivity (increases from Na-Al) and then decreases for the other elements)
• period 3 chlorides and oxides become more covalent and their structure shifts from giant ionic to simple molecular + their reactions with water become more vigorous and as a result of this it becomes easier to hydrolyse the chlorides oxides

Question 3

Across the period, the atomic radii decrease, because:

Answer 3

• number of protons (nuclear charge) and the number of electrons increases by 1 every time you go an element to the right
• shielding effect is the same as the elements in a period all have the same number of shells
• as you go across the period the nucleus attracts the electrons more strongly, pulling them closer to the nucleus
• therefore, atomic radius (and the size of the atom) decreases

Question 4

Ionic radius

Answer 4

the distance between the nucleus and the outermost electron of an ion

Question 5

Across the period, the ionic radius decreases, because:

Answer 5

• metals produce + cations, non-metals produce - anions
• cations have lost their valence electrons, therefore smaller than parent atoms as there are less electrons and less shielding of the outer electrons
• anions are larger than original parent atoms as they have gained an electron which increases repulsion between the electrons while the nuclear charge remains the same

Na+ - Si 4+= the ions get smaller due to increase in nuclear charge attracting the outer electrons in the 2nd principal quantum shell

P3- - Cl- = ionic radius decreases as the nuclear charge increases + fewer electrons are gained

Question 6

Trend in m.p.

Answer 6

general increase in m.p. for the period 3 elements up to silicon (has the highest m.p.) and after silicon the m.p. decreases significantly

Question 7

Why melting point increases from Na-Al (refer to bonding and sea of electrons)

Answer 7

• Na, Mg, Al are metallic elements which form positive ions arranged in a giant lattice in which ions are held together by a sea of delocalised electrons around them
• Na will donate 1 electron into the sea, Mg 2 and Al 3 and as a result of this the metallic bonding in Al is stronger than in Na
• it is stronger, because the electrostatic forces between the 3+ ion and the larger number of negatively charged delocalised electrons is much larger

Question 8

Why Si has the highest melting point

Answer 8

due to its giant molecular structure in which each Si atom is held to its neighbouring Si atoms by strong covalent bonds

Question 9

Why the melting point decreases from P-Ar

Answer 9

• P, S, CI, Ai are non-metallic elements and exist as simple molecules
• the covalent bonds within the molecules (intramolecular forces) are strong, but between the molecules (intermolecular forces) are weak instantaneous dipole-induced dipole forces and it therefore doesn’t take much energy to break them

Question 10

Melting point from highest to lowest

Answer 10

S8 > P4 > CI2 > Ar
- sulfur has a larger electron density therefore it has a stronger instantaneous dipole-induced dipole forces

Question 11

Trend in electrical conductivity

Answer 11

• general trend is that it decreases significantly
• increases from Na-Al and then decreases for the other elements

Question 12

Na-Al

Answer 12

• there is an increase in the number of valence electrons that are donated to the sea of delocalised electrons
• because of this, in Al there are more electrons available to move around and conduct electricity which makes Al a better conductor than Na

Question 13

Why Si and P are not good conductors

Answer 13

• Si is a giant molecular structure and therefore has no delocalised electrons that can move freely within the structure
• classed as a semi-metal
• P and S cant conduct electricity due to the lack of delocalised electrons

Question 14

Period 3 elements with oxygen

Answer 14

metal/non-metal + oxygen = metal/non-metal oxide
*all produce a solid except for sulfur (g)

Question 15

With chlorine

Answer 15

metal + chlorine = metal chloride (vigorous)
non-metal + chlorine = non-metal chloride (slow + forms a liquid product)

Phosphorus forms a mixture of chlorides
* P4 + 6CI2 = 4PCI3
* P4 + 10CI2 = 4PCI5

Question 16

metal oxides (Mg and Na) + water (form alkaline solutions and become hydroxide ions)

Answer 16

metal + water = metal hydroxide + hydrogen

Question 17

Sodium with water

Answer 17

2Na (s) + 2H2O (l) = 2NaOH (aq) + H2 (g)
- vigorous with cold water
- forms strong alkaline solution (pH 14)

Question 18

Magnesium with water

Answer 18

Mg (s) + 2H2O (l) = Mg(OH)2 (aq) + H2 (g)
- slow with cold water

Mg (s) + H2O (g) = MgO (s) + H2 (g)
- vigorous with steam
- forms weak alkaline solution (pH 11)

Question 19

Why is sodium more reactive than magnesium ?

Answer 19

as the amount of energy required to remove an electron from sodium is lower, therefore more energy is needed for magnesium to react

Question 20

non-metal oxide (P and S) + water= form acidic solutions when added to water

Answer 20

P4O10 (s) + 6H2O (l) = 4H3PO4 (aq)

SO2 (g) + H2O (l) = H2SO3 (aq)
SO3 (g) + H2O (l) = H2SO4 (aq)

*the acids can dissociate in solution, forming H+ ions and negative ions

Question 21

Silicon dioxide

Answer 21

• forms giant covalent structure which has many strong covalent bonds that require lots of energy to break
• therefore silicon dioxide is insoluble in water
• it will react with a base to form a salt, therefore classed as an acid

Question 22

Aluminium oxide

Answer 22

• amphoteric
• behaves as an acid and a base
• insoluble in water

Question 23

Acid/base behaviour of period 3 oxides

Answer 23

metal oxide (basic) + acid = salt + water
non-metal oxide (acidic) + base = salt + water

Question 24

Al2O3 exception
(one reacts with sulfuric acid and the other with sodium hydroxide)

Answer 24

Al2O3 (s) + 3H2SO4 (aq) = AI2(SO4)3 (aq) + 3H2O (l)

Al2O3 (s) + 2NaOH (aq) + 3H2O (I) = 2NaAI(OH)4 (aq)

Question 25

SO2/SO3 exception
(both with sodium hydroxide)

Answer 25

SO2 (g) + 2NaOH (aq) = Na2SO3 (aq) + H2O (I)

SO3 (g) + 2NaOH (aq) = Na2SO4 (aq) + H2O (I)

Question 26

metal hydroxide + acid =

Answer 26

salt + water

Question 27

Al(OH)3 (amphoteric)

Answer 27

Al(OH)3 (s) + 3HCI (aq) = AICI3 (s) + 3H2O (I)
Al(OH)3 (s) + NaOH (aq) = NaAI(OH)4 (aq)

Question 28

Chlorides react with water giving off

Answer 28

white fumes of hydrogen gas