5.1 General Trends of Physical Properties and Chemical Reactivity of Group 2 Elements

Group 2 of the periodic table contains a family of elements known as the alkaline earth metals. These elements share similar chemical properties and show clear trends in their physical properties and reactivity as you move down the group. Understanding these trends is key to explaining their behavior in chemical reactions.

Definition

• Group 2 Elements: The elements in Group 2 of the periodic table are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
• Alkaline Earth Metals: The name comes from the fact that their oxides and hydroxides are water-soluble (alkaline) in nature and their oxides are found in the earth's crust.

Core Concept

The core concept is that all Group 2 elements have two electrons in their outermost shell (general electronic configuration ns²). They readily lose these two electrons to form ions with a +2 charge (M²⁺). As you move down the group, the atoms become larger, the outermost electrons are further from the nucleus and are shielded by more inner shells. This makes it easier to remove the outer electrons, leading to an increase in reactivity down the group.

Detailed Explanation

Electronic Configuration

The elements in Group 2 have two electrons in their outermost shell. These two outer electrons occupy the s subshell. Their general electronic configuration is ns². The electronic configurations of the first three elements in Group 2 are:

• Beryllium (Be): 1s² 2s²
• Magnesium (Mg): 1s² 2s² 2p⁶ 3s²
• Calcium (Ca): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

General Trends of Physical Properties

The textbook describes several key trends in the physical properties of Group 2 elements.

1. Trends in Ionization Energy and Chemical Reactivity

• Ionization Energy: The energy required to remove one mole of electrons from an isolated gaseous atom or ion. Both the first and second ionization energies decrease going down the group.
• Reason: The increased shielding effect and the larger distance between the outermost electrons and the nucleus make it easier to remove the outer two electrons.
• Chemical Reactivity: As a result of decreasing ionization energy, the elements become more reactive going down the group. It gets easier for the atoms to lose two electrons and become 2+ ions.

Evidence for Reactivity Trend:

• Reaction with dilute hydrochloric acid: Bubbles of hydrogen gas are given off much faster as you go down the group, indicating that the reactions become more vigorous.
• Reaction with oxygen: The metals get more reactive with oxygen down the group. Barium is so reactive that it must be stored in oil to prevent it from reacting with oxygen in the air.

2. Trends in Atomic Radius

• The atomic radii of Group 2 elements increase going down the group.
• Reason: The addition of an extra electron shell and the increase in shielding effect cause the attraction between the nucleus and the valence electrons to decrease. Hence, the size of the atom increases.

Table 5.1: Physical properties of alkaline earth metals

3. Trends in Melting Point

• The melting point of the elements decreases going down the group.
• Reason: As the outer electrons get further away from the nucleus, the attraction between the nucleus and the delocalized electrons decreases. This means the strength of the metallic bond decreases, causing a decrease in melting point.

4. Trends in Density

- The density of the elements generally increases going down the group, although there is a slight decrease from Be to Mg. This is due to the increase in atomic mass and the way the atoms are packed in the solid state.

Intermediate Level

The trends in Group 2 properties are a direct result of the increasing atomic size and shielding effect down the group. As the atomic radius increases, the first and second ionization energies decrease because the outer electrons are held less tightly. This increased ease of losing electrons explains the increase in reactivity down the group. The decrease in melting point is due to the weakening of metallic bonding as the atomic size increases. The increase in density is primarily due to the increase in atomic mass, despite the increase in atomic volume.

Important Points

• - Group 2 Elements: Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra).
• - Electronic Configuration: ns² (two valence electrons).
• - Ionization Energy: Decreases down the group.
• - Atomic Radius: Increases down the group.
• - Reactivity: Increases down the group.
• - Melting Point: Decreases down the group.
• - Density: Generally increases down the group.
• - Reason for trends: Increasing atomic size and shielding effect down the group.

Remember

• - Group 2 = 2 valence electrons.
• - Atomic radius increases down the group.
• - Ionization energy decreases down the group.
• - Reactivity increases down the group.
• - Melting point decreases down the group.
• - Density generally increases down the group.
• - Ba is more reactive than Mg.
• - Ba is stored in oil to prevent reaction with air.

Common Mistakes

• - Thinking that reactivity decreases down the group. It increases because the outer electrons are easier to lose.
• - Confusing atomic radius and ionic radius trends. Both increase down the group for Group 2 elements.
• - Assuming that all physical properties follow the same trend. Melting point decreases, but density generally increases (with a small dip from Be to Mg).
• - Forgetting the reason for the trends. The key reason is the increase in atomic size and shielding effect, which decreases the effective nuclear charge on the valence electrons.
• - Thinking that Group 2 metals are less reactive than Group 1 metals. Group 2 metals are less reactive than Group 1 metals in the same period because they have higher ionization energies (due to their smaller size and greater nuclear charge).

Quick Revision

Group 2 elements, the alkaline earth metals, have two electrons in their outer shell (ns²). As you move down the group, the atomic radius increases and the ionization energy decreases. This makes the elements more reactive down the group, as it becomes easier to lose the two outer electrons to form M²⁺ ions. The melting point decreases down the group due to the weakening of metallic bonds. The density generally increases. These trends are explained by the increase in atomic size and shielding effect down the group.

60-Second Revision

Group 2 elements (alkaline earth metals) have two valence electrons. Down the group, atomic radius increases, ionization energy decreases, and reactivity increases because the outer electrons are easier to lose. Melting point decreases due to weaker metallic bonding. Density generally increases. These trends are due to increasing atomic size and shielding.

Frequently Asked Questions (FAQs)

• Atomic radius: Increases down the group.
• Ionization energy: Decreases down the group.
• Chemical reactivity: Increases down the group.
• Melting point: Decreases down the group.
• Density: Generally increases down the group.

Q2. Why does the reactivity of Group 2 elements increase down the group? Answer: The reactivity increases down the group because the ionization energy decreases. As the atoms get larger, the outer electrons are further from the nucleus and are shielded by more inner shells. This makes it easier to remove the two valence electrons and form M²⁺ ions, which is the key step in their reactions.

Q3. Why does the melting point of Group 2 elements decrease down the group? Answer: The melting point decreases because the strength of the metallic bond decreases. As the atomic radius increases, the distance between the nucleus and the delocalized electrons increases, weakening the electrostatic attraction. This makes it easier to break the metallic lattice, resulting in a lower melting point.

Q4. Explain the trend in atomic radius of Group 2 elements. Answer: The atomic radius increases down the group because of the addition of extra electron shells. Each new element in the group has its valence electrons in a higher principal energy level, which is further from the nucleus. Additionally, the shielding effect of the inner electrons reduces the effective nuclear charge experienced by the valence electrons, allowing the atom to expand.

Q5. Why is barium stored in oil? Answer: Barium is stored in oil because it is a very reactive metal, especially towards oxygen in the air. It would quickly oxidize and react with moisture if left exposed. The oil acts as a barrier, preventing contact with air and moisture.

Q6. Compare the reactivity of Group 2 elements with Group 1 elements. Answer: Group 2 elements are less reactive than Group 1 elements in the same period. This is because Group 2 elements have higher ionization energies (due to their smaller size and higher nuclear charge) and thus have a lower tendency to lose electrons. For example, sodium reacts more vigorously with water than magnesium.

Q7. What is the trend in ionization energy of Group 2 elements? Answer: The first and second ionization energies decrease down the group. This is because the atomic size increases, and the shielding effect of inner electrons increases. These factors make it easier to remove the valence electrons as you go down the group.

Q8. Why does the density of Group 2 elements generally increase down the group? Answer: The density generally increases down the group because the atomic mass increases more rapidly than the atomic volume. Although the atomic radius increases, the increase in mass is more significant, leading to a higher mass-to-volume ratio (density).

Q9. What is the general electronic configuration of Group 2 elements? Answer: The general electronic configuration is ns², meaning they have two electrons in their outermost s subshell. For example, Be is 1s² 2s², Mg is 1s² 2s² 2p⁶ 3s², and Ca is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².

Multiple Choice Questions (MCQs)

• Q1. The oxidation state shown by alkaline earth metals is:
• A. +2
• B. -2
• C. +1, +2
• D. -1, -2
• Correct Answer: A
• Explanation: Group 2 elements have two valence electrons, which they lose to form ions with a +2 charge.

• Q2. Which one of the following is the most soluble in water?
• A. Mg(OH)_2
• B. Ca(OH)_2
• C. Sr(OH)_2
• D. Ba(OH)_2
• Correct Answer: D
• Explanation: The solubility of Group 2 hydroxides increases down the group, so Ba(OH)_2 is the most soluble.

Q3. Which of the following alkaline earth metal hydroxides is amphoteric in character? A. Be(OH)_2 B. Sr(OH)_2 C. Ca(OH)_2 D. Ba(OH)_2 Correct Answer: A Explanation: Be(OH)_2 is amphoteric, meaning it can react with both acids and bases. This is due to the small size and high polarizing power of the Be^{2+} ion.

Q4. Of the metals Be, Mg, Ca, and Sr of Group 2A in the periodic table, the least ionic chloride would be formed by: A. Be B. Ca C. Mg D. Sr Correct Answer: A Explanation: Beryllium chloride is the least ionic (most covalent) because the Be^{2+} ion is very small and has a high polarizing power, which distorts the electron cloud of the chloride ion, leading to covalent character.

• Q5. The order of solubility of sulphates of alkaline earth metals in water is:
• A. Be > Mg > Ca > Sr > Ba
• B. Mg > Be > Ba > Ca > Sr
• C. Be > Ca > Mg > Ba > Sr
• D. Mg > Ca > Ba > Be > Sr
• Correct Answer: A
• Explanation: The solubility of Group 2 sulphates decreases down the group. The correct order is Be > Mg > Ca > Sr > Ba.

Q6. The solubilities of carbonates decrease down the magnesium group due to a decrease in: A. Hydration energies of cations B. Inter-ionic attraction C. Entropy of solution formation D. Lattice energies of solids Correct Answer: A Explanation: The solubility of Group 2 carbonates decreases down the group because the hydration energy of the cations decreases more than the lattice energy, making the dissolution process less favorable.

Q7. In which of the following is the hydration energy higher than the lattice energy? A. MgSO_4 B. SrSO_4 C. RaSO_4 D. BaSO_4 Correct Answer: A Explanation: For MgSO_4, the hydration energy of the small Mg^{2+} ion is high enough to overcome the lattice energy, making it soluble. For the heavier sulphates, the lattice energy dominates, making them less soluble.

Q8. Which of the following alkaline earth metal sulphates has hydration enthalpy higher than the lattice enthalpy? A. CaSO_4 B. BaSO_4 C. BeSO_4 D. SrSO_4 Correct Answer: C Explanation: BeSO_4 is the most soluble sulphate because the very high hydration enthalpy of the small Be^{2+} ion is greater than the lattice enthalpy.

• Q9. Which gas is released when CaCO_3 reacts with dilute HCl?
• A. H_2
• B. O_2
• C. CO_2
• D. Cl_2
• Correct Answer: C
• Explanation: Calcium carbonate reacts with dilute hydrochloric acid to produce calcium chloride, water, and carbon dioxide gas:
• CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2

• Q10. Brick red is the characteristic flame colour of:
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: C
• Explanation: Calcium compounds give a brick-red colour in a flame test. Strontium gives a scarlet/red colour, and barium gives an apple-green colour.

Multiple Choice Questions (MCQs)

• Q1. Which of the following elements is a Group 2 element?
• A. Sodium
• B. Magnesium
• C. Aluminium
• D. Potassium
• Correct Answer: B
• Explanation: Group 2 elements include beryllium, magnesium, calcium, strontium, barium, and radium. Magnesium is a Group 2 element.

• Q2. What is the general electronic configuration of Group 2 elements?
• A. ns¹
• B. ns²
• C. ns² np¹
• D. ns² np⁶
• Correct Answer: B
• Explanation: Group 2 elements have two electrons in their outermost s subshell, giving the general configuration ns².

• Q3. As you go down Group 2, the ionization energy:
• A. Increases
• B. Decreases
• C. Remains the same
• D. First increases then decreases
• Correct Answer: B
• Explanation: Ionization energy decreases down the group because the outer electrons are further from the nucleus and are shielded by more inner shells.

• Q4. As you go down Group 2, the atomic radius:
• A. Increases
• B. Decreases
• C. Remains the same
• D. First increases then decreases
• Correct Answer: A
• Explanation: Atomic radius increases down the group due to the addition of extra electron shells and increased shielding effect.

• Q5. As you go down Group 2, the reactivity of the elements:
• A. Increases
• B. Decreases
• C. Remains the same
• D. Is unpredictable
• Correct Answer: A
• Explanation: Reactivity increases down the group because it becomes easier to lose the two outer electrons due to decreasing ionization energy.

• Q6. As you go down Group 2, the melting point:
• A. Increases
• B. Decreases
• C. Remains the same
• D. First increases then decreases
• Correct Answer: B
• Explanation: Melting point decreases down the group because the metallic bond strength decreases as the atomic radius increases.

• Q7. Which Group 2 element is stored in oil to prevent it from reacting with air?
• A. Magnesium
• B. Calcium
• C. Strontium
• D. Barium
• Correct Answer: D
• Explanation: Barium is so reactive that it must be stored under oil to prevent it from reacting with oxygen in the air.

• Q8. Which Group 2 element reacts with water most readily?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Reactivity increases down the group, so barium reacts most readily with water among the Group 2 elements.

• Q9. Which of the following is the correct order of increasing reactivity of Group 2 elements?
• A. Be < Mg < Ca < Sr < Ba
• B. Ba < Sr < Ca < Mg < Be
• C. Mg < Be < Ca < Sr < Ba
• D. Ca < Mg < Be < Sr < Ba
• Correct Answer: A
• Explanation: Reactivity increases down the group, so the order is Be < Mg < Ca < Sr < Ba.

Q10. The decrease in melting point down Group 2 is due to: A. Increase in atomic mass B. Decrease in metallic bond strength C. Increase in nuclear charge D. Decrease in density Correct Answer: B Explanation: The melting point decreases because the strength of the metallic bond decreases as the atomic radius increases, making it easier to break the lattice.