Periodic Trends 2

Question 1

Physical Properties of Alkali Metals

What are the physical properties of Alkali Metals down the Group?

Answer 1

1. The atomic radii of Group I metals increases down the group.
2. The density of alkali metals increases down the group.
3. They are relatively soft and can be cut easily with a knife.
4. They have low melting and boiling points compared with other metals.
5. They are good conductors of heat and electricity.

Question 2

Physical Properties of Alkali Metals

What is the change in effective nuclear charge of Group I metals down the Group?

Answer 2

The effective nuclear charge decreases. From Lithium to Francium, as the number of protons increases, the nuclear charge also increases. However, as each of the Group I metals has an increase in 1 core electron shell, shielding effect also increases.
However, as increase in shielding effect is more than that of effective nuclear charge, this results in the nucleus exerting a weaker hold on its valence electrons.

Question 3

Physical Properties of Alkali Metals

Why do the density of alkali metals increase down the Group?

Answer 3

Down the Group, atomic mass increases faster than the atomic volume.

Question 4

Physical Properties of Alkali Metals

Why do the melting and boiling points of alkali metals decrease down the Group?

Answer 4

The size of the metal cations increases down the Group. As size increases, the distance between the cation and the sea of delocalised electrons increases, hence electrostatic forces of attraction between cation and sea of delocalised electrons decreases. Hence metallic bonding weakens down the Group, and less energy is required to break the weaker metallic bonds.

Question 5

Chemical Properties of Alkali Metals

What are the chemical properties of Alkali Metals

Answer 5

1. Group I metals are the most reactive metals in the Periodic Table.
2. They have similar chemical properties because they have similar electronic configurations.
3. Group I metals react rapidly with air and water. Hence, they are kept under oil.
4. They react rapidly with water to form alkalis.

Question 6

Chemical Properties of Alkali Metals

Explain why the reactivity of Group I metals increases down the Group.

Answer 6

Down the Group, the atomic radius of Group I metals increases due to more occupied electron shells. As atomic radii increases, 1st ionisation energy increases going down the Group, as the valence electrons become progressively further from the nucleus, and thus experiences more shielding between the nucleus and other electrons. Thus, the electrostatic forces of attraction between the nucleus and the valence electrons becomes weaker. Less energy is required to remove the valence electrons going down the Group.

Question 7

Reactions of Alkali Metals with Water

How do alkali metals react with water?

Answer 7

All the alkali metals react with cold water to form alkali salts and hydrogen gas.

alkali metal + water –> salt + Hydrogen gas

Question 8

Reactions of Alkali Metals with Water

How does Lithium react with water?

Answer 8

2Li(s) + 2H2O(l)–> 2LiOH(aq) + H2(g)

Lithium reacts quickly with cold water. Lithium floats on water, no flame is seen, and no efferversence is produced.

Question 9

Reactions of Alkali Metals with Water

How does sodium react with water?

Answer 9

2Na(s) + 2H2O(l) –> 2Na(OH)(aq) + H2(g)

Reacts very quickly with cold water. Na melts and burns with a yellow flame. The molten sodium darts around the surface of the water. Effervescence is produced.

Question 10

Reactions of Alkali Metals with Water

How does Potassium react with water?

Answer 10

2K(s) + 2H2O(l)–> 2K(OH) (aq) + H2(g)

Reacts violently with cold water. K melts and burns with a lilac flame and explodes. Effervescence is produced.

Question 11

Reactions of Alkali Metals with Oxygen

How do alkali metals react with oxygne?

Answer 11

All Group I metals react when ignited in air.

The reactivity of Group I metals with oxygen increases down the Group, a consequence of the increase in reducing strength of the metals.
(Reducing strength refers to how easily a metal can lose electrons)

The general equation for this reaction is

4M(s) + O2(g) –> 2M2O(s)

Question 12

Reactions of Alkali Metals with Oxygen

How does Lithium react with oxygen?

Answer 12

Vigorous Reaction

Question 13

Reactions of Alkali Metals with Oxygen

How does sodium react with oxygen?

Answer 13

Explosive reaction, very dangerous

Question 14

Reactions of Alkali Metals with Oxygen

How does potassium react with oxygen?

Answer 14

Very Explosive Reaction, extremely dangerous

Question 15

Group 17 Elements – Halogens

What molecules do halogen atoms form?

Answer 15

They form diatomic molecules, meaning F2, Cl2, I2, Br2, etc.

Question 16

Physical Properties of Halogens

Describe the physical properties of all halogens.

Answer 16

1. All halogens have low melting and boiling points.
2. All halogens are colored and are non-conductors of electricity.
3. Going down the Group, color intensity increases and melting and boiling points increases.

Question 17

Physical Properties of Halogens

Describe the color of Fluorine in gaseous state and in non-polar solvent.

Answer 17

Color in gaseous state: Pale Yellow

Color in non-polar solvent: Very pale yellow

Question 18

Physical Properties of Halogens

Describe the colors of Cl2 in gaseous state, in aqueous solutions and in non-polar solvents.

Answer 18

Color in gaseous state: Yellow-Green

Color in aqueous solutions: Colorless when dilute and pale yellow when concentrated.

Color in non-polar solvents: Very pale green

Question 19

Physical Properties of Halogens

Describe the color of Br in gaseous state, in liquid state, in aqueous solutions and in non-polar solvents.

Answer 19

Color in gaseous state: Reddish-Brown
Color in liquid state: Reddish-Brown
Color in aqueous solutions: Yellow when dilute and Orange when concentrated
Color in non-polar solvents: Reddish-Brown

Question 20

Physical Properties of Halogens

Describe the color of I2 in gaseous state, in liquid state, in solid state, in aqueous solution and in non-polar solvents.

Answer 20

Color in gaseous state: Violet
Color in liquid state: Violet-Black
Color in solid state: Black
Color in aqueous state: Pale Yellow (in water) and brown (I- (aq))
Color in non-polar solvent: Violet

Question 21

Physical Properties of Halogens

Explain why the melting and boiling points of halogens increases down the Group.

Answer 21

Group 17 elements exist as diatomic molecules with dispersion forces acting between molecules.

Going down the group, the size of the electron cloud increases and hence the polarisability of tbe halogen molecule increases. Stronger dispersion forces exist between the molecules and thus more energy is required to overcome the stronger intermolecular forces of attraction. Hence, volatility of halogens decreases down Group 17, and melting and boiling points increases correspondingly.

Definition of Volatility:
- A more volatile halogen molecule evaporates more easily: lower boiling point
- A less volatile halogen molecule evaporates less easily: higher boiling point

Question 22

Chemical Properties of Halogens

List the chemical properties of Halogens.

Answer 22

1. Halogens react vigorously with metals to form salts known as metal halides. 2Fe(s) + 3Cl 2 (g) –> 2FeCl 3 (s)
2. They react with non-metals to give covalent compounds. H 2 (g) + Cl 2 (g) –> 2HCl (g)
3. Halogens are powerful oxidising agents as they are easily reduced to form halide ions.
4. Reactivity of halogen elements decreases going down the Group.

Question 23

Chemical Properties of Halogens

Explain why the reactivity of halogens decreases going down the Group.

Answer 23

Group 17 elements form ions by gaining one electron. However, atomic radius increases as the number of principal quantum shells increases going down the Group. Hence the electrostatic forces of attraction between the nucleus and valence electrons become weaker as they are further away from the nucleus. Thus, the halogen atoms gain electrons less readily.

Question 24

Chemical Properties of Halogens

What is the displacement reaction of Halogens?

Answer 24

A displacement reaction is one where one more reactive halogen element replaces the position of another halogen element in a reaction.

E.g Chlorine displaces bromine, as Chlorine is more reactive.

Recall that reactivity of halogen atoms decreases down the group.

Cl2 (aq) + 2KBr (aq) –> 2KCl (aq) + Br2 (aq)

Question 25

# Chemical Properties of Halogens What are the requirements for a displacement reaction to occur?

Answer 25

A more reactive halogen at the top of Group 17 displaces a less reactive halogen at the bottom of Group 17, from its halide.

Question 26

# Uses of Halogens What are the uses of fluorine?

Answer 26

Fluorine is added to tap water and used to kill bacteria and prevent tooth decay

Question 27

# Uses of Halogens What are the uses of chlorine?

Answer 27

1. Added to swimming pool and tap water to kill microorganisms and bacteria. 2. Chloride salts in foods replace those lost through sweat and urine. 3. Used as bleaching agents and in preparation of drugs and insecticides.

Question 28

# Uses of Halogens What are the uses of iodine?

Answer 28

Small amounts of iodine are needed as food supplements as deficiency can cause goitre. Iodine is also present in seafood.

Question 29

# Noble Gases Explain why melting and boiling points of noble gases increase going down the Group.

Answer 29

Noble gases have low melting and boiling points. Going down the Group, the size of electron cloud of noble gas atoms increases, and hence the forces of attraction between noble gas atoms increases. More energy is required to overcome the stronger dispersion forces between melting or boiling.

Question 30

# Noble Gases Explain why the densities of noble gas atoms increases going down the Group.

Answer 30

All noble gases have low densities. When going down the Group, the atomic mass and atomic size of each element increases down the Group. Thus, the density of each element increases going down the Group.

Question 31

# Noble Gases Why are noble gases unreactive?

Answer 31

Noble Gases have a stable electronic configuration because their outermost electron shell is full. All noble gases have either a duplet or an octet electron arrangement, and thus, they are inert.

Question 32

# Uses of Noble Gases What are the uses of Helium?

Answer 32

Helium is used to fill balloons and airships as it is light and non-flammable, cool superconducting magnets in MRI scanners, mix with oxygen for deep-sea diving, detect leaks, and provide an inert atmosphere for welding and electronics.

Question 33

# Uses of Noble Gases What is Neon used for?

Answer 33

Neon is used in advertising lights and television tubes.

Question 34

# Uses of Noble Gases What is Argon used in?

Answer 34

Argon is used to fill light bulbs.

Question 35

# Uses of Noble Gases What is Xenon used for?

Answer 35

Xenon is used for making electron tubes and stroboscopic lamps.

Question 36

# Uses of Noble Gases What is Radon used for?

Answer 36

Radon is used for the treatment of cancer.

Question 37

# Uses of Noble Gases What is Krypton used for?

Answer 37

Krypton is used for lasers to repair the retina of the eye and is used to fill photographic flash lamps.

Question 38

# Transition Elements What are transition elements?

Answer 38

Transition elements are d-block elements which form some compounds containing its ion with an incomplete d-subshell. Hence, Scandium and Zinc are not considered to be transition elements because Scandium forms the Sc3+ ion which has a 3d0 configuration, while Zinc forms the Zn2+ ion which has a 3d10 configuration.

Question 39

# Transition Elements Name the electronic configuration of a few transition elements.

Answer 39

1. Calcium: 1s2 2s2 2p6 3s2 3p6 4s2 2. Titanium: 1s2 2s2 2p6 3s2 3p6 3d2 4s2 3. Vanadium: 1s2 2s2 2p6 3s2 3p6 3d3 4s2

Question 40

# General and Physical Properties of Transition Metals Why are the melting and boiling points of transition metals higher than that s-block elements?

Answer 40

S-block metals have generally low melting and boiling points due to only 1 or 2 valence electrons (s orbitals) being involved in metallic bonding. Hence, less energy is reauired to overcome weaker metallic bonding. In transitional metals, both 3d and 4s electrons are involved in metallic bonding due to their proximity in energies, hence more energy is required to overcome stronger metallic bonding.

Question 41

# General and Physical Properties of Transition Metals Describe the trend in melting and boiling points of transition metals across the period.

Answer 41

From Sc to V, the melting and boiling points increase due to an increase in the number of unpaired electrons which are involved in metallic bonding. Mn has a lower boiling point as it has an electronic configuration of 3d5 4s2. Due to the apparent stability of the d5 configuration (a half filled d subshell, all five d orbitals have have one electron, hence this maximises electron distribution. ), the 3d electrons are less available for delocalisation, resulting in weaker metallic bonding. From Fe to Zn, the melting points and boiling points generally decreases as electrons begin to pair up in the 3d orbitals. The paired electrons do not participate fully in metallic bonding, and this decreases the strength of the metallic bond.

Question 42

# General and Physical Properties of Transition Metals Compare the densities of s-block metals and transition metals

Answer 42

S-block metals have low densities, as they are low in atomic masses, and have large atomic radius, hence, there is less efficient packing and less atoms per unit volume. On the other hand, transition metals are extremely high in density, due to their higher atomic masses. They also have smaller atomic radius and close packing of atoms, resulting in more atoms per unit volume.

Question 43

# General and Physical Properties of Transition Metals State the trend in change of density of transition metals across the period.

Answer 43

From Sc to Cu, there is an increase in density. This is due to the small decrease in atomic radius coupled with increasing atomic mass. The small atomic radius results in better and closer packing of atoms in the metallic lattice. Although the overall change in radius is small, the effect is magnified as the atomic volume decreases with the cube of the radius. Thus density increases as volume decreases and atomic mass increases.

Question 44

# Chemical Properties of Transition Metals State the difference between transition metals and s-block metals in terms of their oxidation states.

Answer 44

Transition metals can form ions and compounds with the metal having a variety of oxidation states. e.g Mn - Mn2+, Cr - Cr3+ While s-block metals form ions with fixed oxidation states.

Question 45

# Chemical Properties of Transition Metals State the difference between transition metals and s-block metals in terms of the catalytic properties that they exhibit.

Answer 45

Transition metals and their compounds display a variety of catalytic properties. E.g Iron, Fe, in the Haber Process and Vanadium Oxide in the Contact Process.

Question 46

# Chemical Properties of Transition Metals State a difference between transition metals and s-block metals in terms of the ions that they form.

Answer 46

Transition Metals have a strong tendency to form complex ions. While s-block metals do not readily form complexes.

Question 47

# Chemical Properties of Transition Metals State the difference between transition metals and s-block metals in terms of the color of their compounds formed.

Answer 47

Transition metals tend to form colored compounds. e.g Cr 3+ (aq): green, Cr 2 O 7 2– : orange, Fe 3+ (aq): yellow, MnO 4 – : purple S-block metals form white solids or colorless solutions.