5.1.1 Trends in ionization energy and chemical reactivity

In the previous section, we introduced the general trends of Group 2 elements. Now, we will take a closer look at one of the most important trends: the decrease in ionization energy down the group and how this directly leads to an increase in chemical reactivity.

Definition

• Ionization Energy: The minimum amount of energy required to remove one mole of electrons from the isolated gaseous atom or ion.
• First Ionization Energy: The energy required to remove the first electron from a gaseous atom.
• Second Ionization Energy: The energy required to remove the second electron from a gaseous ion (with a +1 charge).

Simple Explanation: Ionization energy is a measure of how tightly an atom holds onto its electrons. A high ionization energy means the electron is held tightly; a low ionization energy means it is held loosely and can be removed easily.

Core Concept

The core concept is that as you move down Group 2, the atoms become larger, and the outermost electrons are further from the nucleus and are shielded by more inner electron shells. This increased distance and shielding reduce the attraction between the nucleus and the valence electrons. As a result, it becomes easier to remove the outer two electrons, meaning the ionization energy decreases down the group. The easier it is to lose electrons, the more reactive the metal is.

Detailed Explanation

Trends in Ionization Energy

The textbook provides a graph (Figure 5.1) that shows both the first and second ionization energies of Group 2 elements. The key observations are:

• Both the first and second ionization energies decrease going down the group.
• The decreasing order of the 1st ionization energy of Group 2 elements is:

• Increased Shielding Effect: As you go down the group, more inner electron shells are added. These inner electrons shield the outer electrons from the full attractive force of the positive nucleus.
• Larger Distance: The outermost electrons are in higher principal energy levels, which are further from the nucleus. The electrostatic attraction between the nucleus and the valence electrons is weaker at a greater distance.
• Result: The outer electrons are held less tightly and are easier to remove.

Trends in Chemical Reactivity

The graph (Figure 5.1) shows that going down the group, it becomes easier to remove the outer two electrons of the metals. As a result of this, the elements become more reactive going down the group, as it gets easier for the atoms to lose two electrons and become 2+ ions.

Evidence for Increased Reactivity Down the Group:

• Bubbles of hydrogen gas are given off much faster as you go down the group, indicating that the reactions become more vigorous.
• Example:
• This reaction becomes more vigorous from Mg to Ca to Sr to Ba.

2. 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. Barium reacts with oxygen to form barium oxide: 2Ba_{(s)} + O_{2(g)} → 2BaO_{(s)}

• Beryllium does not react with water (due to a protective oxide layer).
• Magnesium does not react with cold water but reacts with steam to form magnesium oxide and hydrogen gas.
• Calcium reacts more readily with water to form calcium hydroxide and hydrogen gas.
• Strontium and barium react even more vigorously with water.
• This demonstrates the increasing reactivity down the group.

Summary of the Relationship

The decrease in ionization energy down the group is the fundamental reason for the increase in chemical reactivity. The easier it is for a metal to lose its valence electrons, the more readily it can form positive ions and participate in chemical reactions.

Intermediate Level

The trend in ionization energy is explained by the atomic structure. As you go down Group 2, the atomic radius increases due to the addition of electron shells. The outermost electrons are at a higher principal quantum number, so they are further from the nucleus. Additionally, the inner electrons shield the outer electrons from the full nuclear charge, reducing the effective nuclear charge. Both these factors make the outer electrons less tightly bound, so the ionization energy decreases down the group. The reactivity of the metals depends on their ability to lose electrons to form positive ions. Since the ionization energy decreases down the group, the metals become more reactive. This is why barium is more reactive than magnesium, as seen in their reactions with water and oxygen.

Important Points

• - Ionization Energy: Energy required to remove an electron.
• - Trend: First and second ionization energies decrease down Group 2.
• - Reason: Increased atomic size and shielding effect.
• - Reactivity: Increases down the group.
• - Reason for reactivity: Easier to lose valence electrons.
• - Evidence: Reaction with HCl becomes more vigorous; reaction with oxygen becomes more vigorous; reaction with water becomes more vigorous.
• - Barium: So reactive that it must be stored in oil.

Remember

• - Ionization energy decreases down Group 2.
• - Reactivity increases down Group 2.
• - Be is the least reactive, Ba is the most reactive.
• - Trend: Be > Mg > Ca > Sr > Ba for ionization energy.
• - Reason: Increased atomic size and shielding make it easier to remove the outer electrons.
• - Ba is stored in oil because it is very reactive with oxygen.
• - Reactivity is determined by the ease of losing electrons.

Common Mistakes

• - Confusing ionization energy and reactivity trends. Ionization energy decreases, while reactivity increases down the group.
• - Thinking that a higher ionization energy means a more reactive metal. It's the opposite. A lower ionization energy means it's easier to remove electrons, making the metal more reactive.
• - Forgetting the role of shielding. The shielding effect of inner electrons is a key factor in reducing the ionization energy down the group.
• - Assuming all Group 2 metals react with water in the same way. Beryllium does not react with water, magnesium reacts only with steam, while calcium and the metals below it react more readily with cold water.
• - Not realizing the importance of the reaction with oxygen. The reaction with oxygen is a clear demonstration of increasing reactivity down the group, especially for barium.

Quick Revision

The first and second ionization energies of Group 2 elements decrease going down the group due to the increasing atomic radius and shielding effect. This makes it easier for the atoms to lose their two valence electrons and form M²⁺ ions. As a result, the chemical reactivity of the elements increases down the group. This trend is evident in their reactions with dilute hydrochloric acid (which become more vigorous), with oxygen (barium is so reactive it must be stored in oil), and with water (reactivity increases from magnesium to barium).

60-Second Revision

Down Group 2, ionization energy decreases because the atoms get larger and the outer electrons are shielded from the nucleus. This makes it easier to remove the valence electrons, so reactivity increases down the group. For example, barium is much more reactive than magnesium. The trend in ionization energy is Be > Mg > Ca > Sr > Ba.

Frequently Asked Questions (FAQs)

Q1. Why does the ionization energy of Group 2 elements decrease down the group? Answer: The ionization energy decreases down the group because the atomic size increases and the number of inner electron shells increases. The outer electrons are further from the nucleus and are shielded by the inner electrons, which reduces the effective nuclear charge they experience. This makes the outer electrons less tightly bound and easier to remove.

Q2. How does the decrease in ionization energy relate to the increase in chemical reactivity? Answer: The chemical reactivity of a metal depends on its ability to lose electrons and form positive ions. Since the ionization energy decreases down the group, it becomes easier for the metals to lose their outer electrons. This makes them more reactive. The easier it is to lose electrons, the more vigorously the metal will react with other substances.

• Q3. Provide an example of a reaction that shows the increase in reactivity down Group 2.
• Answer: The reaction with dilute hydrochloric acid is a good example. When Group 2 metals react with dilute HCl, hydrogen gas is produced:
• M_{(s)} + 2HCl_{(aq)} \rightarrow MCl_{2(aq)} + H_{2(g)}
• The rate at which bubbles of hydrogen gas are given off increases from magnesium to calcium to strontium to barium, indicating an increase in reactivity.

Q4. Why is barium stored in oil? Answer: Barium is stored in oil because it is a very reactive metal that reacts vigorously with oxygen in the air. The oil acts as a protective barrier, preventing the barium from coming into contact with oxygen and moisture.

Q5. Compare the reactivity of beryllium and barium. Answer: Barium is much more reactive than beryllium. Beryllium is at the top of the group and has a higher ionization energy, so it is less reactive. Barium is at the bottom of the group and has a lower ionization energy, so it is more reactive. Barium reacts with water and oxygen very readily, while beryllium does not react with water and is resistant to oxygen due to a protective oxide layer.

Q6. What is the trend in the first and second ionization energies of Group 2 elements? Answer: Both the first and second ionization energies of Group 2 elements decrease going down the group. The decreasing order of the first ionization energy is: Be > Mg > Ca > Sr > Ba.

Q7. Explain why the ionization energy of magnesium is higher than that of calcium. Answer: Magnesium has a smaller atomic radius than calcium. The valence electrons in magnesium are closer to the nucleus and are less shielded by inner shells. Therefore, the attraction between the nucleus and the valence electrons is stronger, making it harder to remove an electron. Hence, the ionization energy of magnesium is higher than that of calcium.

• The reaction with dilute HCl becomes more vigorous down the group.
• The reaction with oxygen becomes more vigorous down the group.
• The reaction with water becomes more vigorous down the group (from Mg to Ba).
• The fact that barium must be stored in oil to prevent reaction with air.

Q9. Why does the shielding effect increase down Group 2? Answer: The shielding effect increases down the group because more inner electron shells are added. Each shell contains electrons that repel the outer electrons and partially cancel the attractive force of the positive nucleus. This reduces the effective nuclear charge experienced by the valence electrons.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in first ionization energy down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains constant.
• D. It first increases then decreases.
• Correct Answer: B
• Explanation: The first ionization energy decreases down the group because the atomic size increases and the outer electrons are more shielded from the nucleus.

• Q2. What is the trend in chemical reactivity down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains constant.
• D. It is unpredictable.
• Correct Answer: A
• Explanation: Reactivity increases down the group because the ionization energy decreases, making it easier for the metals to lose their valence electrons.

• Q3. Which of the following shows the correct order of first ionization energy for 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: Ionization energy decreases down the group, so the correct order is Be > Mg > Ca > Sr > Ba.

Q4. Why does the ionization energy decrease down Group 2? A. Because the number of protons decreases. B. Because the atomic size increases and shielding effect increases. C. Because the number of neutrons decreases. D. Because the density increases. Correct Answer: B Explanation: The increase in atomic size and the shielding effect of inner electrons make it easier to remove the outer electrons, decreasing the ionization energy.

• Q5. Which Group 2 metal 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 in oil to prevent it from reacting with oxygen in the air.

• Q6. When Group 2 metals react with dilute hydrochloric acid, which gas is produced?
• A. Oxygen
• B. Chlorine
• C. Hydrogen
• D. Carbon dioxide
• Correct Answer: C
• Explanation: Group 2 metals react with dilute HCl to produce hydrogen gas:
• M + 2HCl \rightarrow MCl_2 + H_2

• Q7. Which Group 2 element is the least reactive?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: A
• Explanation: Beryllium is at the top of the group and has the highest ionization energy, making it the least reactive.

• Q8. As you go down Group 2, the ionization energy decreases. This is because:
• A. The atoms get smaller.
• B. The outer electrons are closer to the nucleus.
• C. The outer electrons are further from the nucleus and shielded.
• D. The nuclear charge decreases.
• Correct Answer: C
• Explanation: The increased distance and shielding of the outer electrons from the nucleus make them easier to remove.

• Q9. Which of the following statements is true about the reactivity of Group 2 elements?
• A. Reactivity decreases down the group.
• B. Reactivity increases down the group.
• C. Reactivity is independent of the group position.
• D. Reactivity increases from Ba to Be.
• Correct Answer: B
• Explanation: Reactivity increases down the group because the ionization energy decreases, making it easier to lose electrons.

• Q10. What is the trend in the second ionization energy of Group 2 elements?
• A. It increases down the group.
• B. It decreases down the group.
• C. It remains constant.
• D. It is not affected by the group position.
• Correct Answer: B
• Explanation: Like the first ionization energy, the second ionization energy also decreases down the group due to increased atomic size and shielding.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in first ionization energy down Group 2?
• A. Increases
• B. Decreases
• C. Remains constant
• D. First increases then decreases
• Correct Answer: B
• Explanation: First ionization energy decreases down the group due to increasing atomic size and shielding effect.

• Q2. Why does the reactivity of Group 2 elements increase down the group?
• A. Because their atomic mass increases.
• B. Because their ionization energy decreases.
• C. Because their density increases.
• D. Because their melting point decreases.
• Correct Answer: B
• Explanation: Reactivity increases because the ionization energy decreases, making it easier for the metals to lose their valence electrons.

• Q3. Which of the following is the correct order of ionization energy?
• 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: Ionization energy decreases down the group, so Be has the highest and Ba has the lowest.

• Q4. Which Group 2 element is stored in oil?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Barium is so reactive that it must be stored in oil to prevent it from reacting with oxygen in the air.

• Q5. What gas is produced when Group 2 metals react with dilute HCl?
• A. Oxygen
• B. Chlorine
• C. Hydrogen
• D. Carbon dioxide
• Correct Answer: C
• Explanation: Group 2 metals react with dilute HCl to produce hydrogen gas.

• Q6. Which Group 2 element is the most reactive?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Barium is at the bottom of the group and has the lowest ionization energy, making it the most reactive.

• Q7. Why does the ionization energy decrease down Group 2?
• A. The atoms get smaller.
• B. The outer electrons are closer to the nucleus.
• C. The outer electrons are further from the nucleus and shielded.
• D. The nuclear charge decreases.
• Correct Answer: C
• Explanation: The increased distance and shielding of the outer electrons make them easier to remove.

• Q8. Which of the following reactions becomes more vigorous down Group 2?
• A. Reaction with water
• B. Reaction with dilute HCl
• C. Reaction with oxygen
• D. All of the above
• Correct Answer: D
• Explanation: The reactivity of Group 2 metals increases down the group, so their reactions with water, dilute HCl, and oxygen all become more vigorous.

• Q9. What is the trend in the second ionization energy of Group 2 elements?
• A. Increases down the group.
• B. Decreases down the group.
• C. Remains constant.
• D. Is not affected by the group position.
• Correct Answer: B
• Explanation: Like the first ionization energy, the second ionization energy also decreases down the group.

• Q10. Which of the following statements is correct about the reactivity of Group 2 elements?
• A. Reactivity decreases down the group.
• B. Reactivity increases down the group.
• C. Reactivity is independent of the group position.
• D. Reactivity increases from Ba to Be.
• Correct Answer: B
• Explanation: Reactivity increases down the group because the ionization energy decreases, making it easier to lose electrons.