Topic 3: Periodicity

Question 1

How do you find the effective nuclear charge of an element?

Answer 1

Atomic number - inner electronseg. Na11 - 10 = +1

Question 2

Define the atomic radius.

Answer 2

The distance from the nucleus to the outermost electrons of the Bohr atom.

Question 3

Describe the trends of atomic radii in the periodic table.

Answer 3

Atomic radii decreases across a period because the nuclear charge increases while the number of shielding electrons remains constant. This means that the attraction between the nucleus and the outer electrons increases so the atomic radii decreases.Atomic radii increases down a group because the number of occupied energy levels increases.

Question 4

Describe the trends of ionic radii in the periodic table.

Answer 4

Ionic radii decreases across a period from G1 to G14. These ions are isoelectronic. They have all lost electrons to gain a full valence shell. Across a period, a proton is successively added (increasing the nuclear charge) while the number of electrons stays the same. The added proton exerts a stronger pull on the electrons so the ionic radii decreases.Ionic radii decreases across a period from G14 to G17. These ions are isoelectronic. They have all gained electrons to fill their valence shell (with the exception of Si⁴⁺). Across a period, a proton is successively added (increasing the nuclear charge) while the number of electrons stays the same. The added proton exerts a stronger pull on the electrons so the ionic radii decreases. The discontinuity in G14 is because positive ions are smaller than negative ions because they have lost a energy level to gain a full valence shell. Ionic radii increases down a group as the number of occupied energy levels increases.

Question 5

Define the first ionisation energy.

Answer 5

The energy required to remove one mole of electrons from one mole of gaseous ions to produce one mole of gaseous charged ions.

Question 6

Describe the trends of ionization energy in the periodic table.

Answer 6

Ionization energy increases across a period. Across a period, a proton is successively added so the nuclear charge increases and the atomic radius decreases. This means there is a greater attraction between the nucleus and the outer electrons therefore it requires more energy to remove the outer electrons.Ionization energy decreases down a group. Down a group the number of occupied energy levels increases so the atomic radii increases, therefore there is a weaker attraction between the nucleus and the outer electrons. So it requires less energy to remove the outer electrons.

Question 7

Why are there exceptions to the general trend of ionization energy across a period?

Answer 7

Due to the orbital that the outer electron occupies. For example, electrons in p orbitals have a higher energy and are further away from the nucleus than electrons in the s orbital so they require less energy to remove.Also, double occupied orbitals. The electrons repel each other so it requires less energy to remove one than an electron in a half filled orbital.

Question 8

Define the first electron affinity.

Answer 8

The energy change when one mole of electrons is added to mole of of gaseous atoms to form one mole of gaseous -1 ions.

Question 9

Describe the trends in electron affinity in the periodic table.

Answer 9

In general, the greater the atomic radius and the greater the electron shielding, the less energy is released when an electron is added.

Question 10

Define electronegativity.

Answer 10

A measure of the attraction of an atom for a bonding pair of electrons.

Question 11

Describe the trends in electronegativity in the periodic table.

Answer 11

Electronegativity increases across a period because protons are successively added so the nuclear charge increases. There is an increased attraction between the nucleus and the bonded electrons.Electronegativity decreases down a group because the number of occupied energy levels increases so the bonding electrons are further away from the nucleus. Therefore there is a reduced attraction.

Question 12

Describe the trends in melting point in G1 and G17.

Answer 12

Melting point decreases down G1. The elements have a metallic structure held together by the attractive forces between the delocalised electrons and the positively charged ions. Down a group, the number of occupied energy levels increases so the atomic radii increases, decreasing the attraction. So less energy is needed to break the forces and trigger a change in state. Melting point increases down G17. The elements have a molecular structure held together by temporary dipoles. These increase with the number of electrons in the element.

Question 13

What does the melting point of different elements depend on?

Answer 13

The structure of the element and the type of bonding.In increasing melting point:Molecular covalent - metallic bonding - giant covalent network.

Question 14

Give the following information on the alkali metals.- Physical properties- Chemical properties- Storage- Reaction with water

Answer 14

Physical properties:- Good conductors of electricity and heat- Low density- Grey, shiny surface when freshly cut with a knife.Chemical properties:- Very reactive. Chemical reactivity of the alkali metals increases down the group because the valence electrons are further from the nucleus.Storage:- Stored in oil to prevent contact with air and water.Reaction with water:Alkali metal + water -> metal hydroxide + hydrogen

Question 15

Give the following information on the halogens.- Physical properties- Chemical properties- Displacement reactions

Answer 15

Physical properties:- Coloured.Chemical properties:- Very reactive non-metals. Chemical reactivity decreases down the group.Displacement reactions:The reactivity of the halogens can be determined by placing them in direct competition for an extra electron. The more reactive molecule will become ionic.

Question 16

Give the colours of the following elements:FluorineChlorineBromineIodine

Answer 16

Fluorine: Pale yellow gasChlorine: Greenish-yellow gasBromine: Reddish-brown liquidIodine: Purple solid

Question 17

How can a halide be identified?

Answer 17

Halogens form insoluble salts with silver. Adding a solution containing the halide to a solution containing silver ions produces a precipitate which is useful in identifying the halide ion.

Question 18

Give the equations of the basic oxides.

Answer 18

Na₂O (s) + H₂O (l) -> 2NaOH (aq)MgO (s) + H₂O (l) -> Mg(OH)₂ (aq)pH will increase as an alkaline solution is formed.

Question 19

Give the equations of the acidic oxides.

Answer 19

SO₃ (g) + H₂O (l) -> H₂SO₄ (aq)P₄O₁₀ (s) + 6H₂O (l) -> 4H₃PO₄ (aq)3NO₂ (g) + H₂O (l) -> 2HNO₃ (aq) + NO (g)pH will decrease as an acidic solution is formed.

Question 20

Give the equation of the amphoteric oxide.

Answer 20

Behaves as a base with sulfuric acid.Al₂O₃ (s) + 3H₂SO₄ (aq) -> Al₂(SO₄)₃ (aq) + 3H₂O (l)Behaves as an acid with alkalis like sodium hydroxide.Al₂O₃ (s) + 3H₂O (l) + 2OH⁻ (aq) -> 2Al(OH)₄⁻ (aq)

Question 21

Define a transition metal element.

Answer 21

An element that has an incomplete d-sub level as either an atom or an ion.

Question 22

Why is Zinc not considered a transition metal element?

Answer 22

Because it does not have an incomplete d-sub level as an atom or an ion.

Question 23

Why do the first row d-block elements have similar properties?

Answer 23

Because there is a relatively small range in atomic radii. This is because the 4s sub shell is filled first. Moving across the period a proton and electron are successively added. The electron is added to the 3d sub shell which shields the outer electrons from any effect by the added proton (i.e. the effect of the increased nuclear charge is offset by the added electron).

Question 24

What are the physical properties of the transition metal elements?

Answer 24

Physical properties: (Due to strong metallic bonding)- Good conductor of heat and electricity- High melting point- Malleable- Ductile- High tensile strength- Show magnetic properties (magnetism increases with more unpaired d electrons)

Question 25

What are the chemical properties of the transition metal elements?

Answer 25

Chemical properties: (Due to the incomplete d-sub levels)- Form a variety of complex ions- Form coloured compounds- Act as catalysts- Form compounds with more than one oxidation number

Question 26

Why do the transition metal elements have a wide range of oxidation numbers in compounds?

Answer 26

Because the 3d¹⁰ and 4s² sub shells have similar energies, therefore it requires similar ionisation energies to remove their outer electrons. So many electrons can be removed leading to a large number of ion charges/oxidation numbers.

Question 27

What does a complex ion consist of?

Answer 27

A complex ion is a central metal ion bonded to ligands by coordinate covalent bonds.

Question 28

Define the coordination number.

Answer 28

The number of coordinate bonds from the ligands to the central metal ion.

Question 29

Give examples of transition elements as catalysts.

Answer 29

Transition elements can act as homogenous and heterogenous catalysts.As heterogenous catalysts:- Iron in the Haber process.- Nickel in the hydrogenation of margarine (alkenes to alkanes)- Palladium and Platinum in catalytic converters- Manganese dioxide in the decomposition of hydrogen peroxide- Vanadium oxide in the Contact processAs homogenous catalysts:- Iron 2+ in haemoglobin (transports oxygen around the body)- Cobalt 3+ in vitamin B12 (important for a healthy nervous system)

Question 30

Describe the Crystal Field Theory.

Answer 30

The Crystal field theory states that the colours observed of transition metals are caused by the splitting of d orbitals into two sets with different energies.

Question 31

Why are transition metal compounds coloured?

Answer 31

When a ligand approaches a metal ion there is a repulsion between the ligand’s lone pair of electrons and the 3d orbital electrons. The 3d orbital splits and two d orbitals move to a higher energy level. There is an unpaired electron at the higher energy level so an electron from the lower energy level jumps up. This is called a “d to d transition” and requires the absorption of energy. The energy absorbed by the electron corresponds to the wavelengths of visible light.

Question 32

Whaat does the colour of the transition metal compounds depend on?

Answer 32

The following factors affect the energy separation between the orbitals and hence the colour of the compound.- The identity of the central metal ion↑ENC ↑ΔE ↓λ- The charge density of the ligand (use the spectrochemical series in the data booklet to determine this)↑Ligand strength ↑ΔE ↓λ- The oxidation number of the central metal ion↑ON ↑ΔE ↓λ- Geometry