5.1.2 Trends in atomic radius

In the previous section, we discussed how the ionization energy of Group 2 elements decreases down the group, making them more reactive. This trend is closely linked to another important property: the atomic radius. As we move down Group 2, the atoms become larger, which directly affects how strongly the nucleus holds onto the valence electrons.

Definition

• Atomic Radius: A measure of the size of an atom. It is typically defined as half the distance between the nuclei of two identical atoms bonded together.
• Trend in Group 2: The atomic radii of Group 2 elements increase going down the group.

Core Concept

The core concept is that as you go down a group in the periodic table, new electron shells are added. Each new period corresponds to a higher principal energy level (n=1, 2, 3, etc.). Because the outermost electrons are in a shell further from the nucleus, the size of the atom increases. Additionally, the inner electrons shield the outer electrons from the full attractive force of the nucleus. This increased distance and shielding cause the atomic radius to increase down the group.

Detailed Explanation

The Trend in Atomic Radius

The textbook states that the atoms of Group 2 elements get larger going down the group. This is because of the increase in extra shell and shielding effect; the attraction between the nucleus and valence electrons decreases. Hence, the size of the atom increases.

The table (Table 5.1) and the bar chart (Figure 5.2) in the textbook clearly show this trend:

As you can see, the atomic radius increases from beryllium (0.122 nm) to barium (0.217 nm).

Why Does the Atomic Radius Increase Down the Group?

There are two main reasons for this trend:

• Each element down the group has its valence electrons in a higher principal energy level (n).
• Beryllium's valence electrons are in the n=2 shell.
• Magnesium's are in the n=3 shell.
• Calcium's are in the n=4 shell, and so on.
• Because the electron shells are added successively, the outermost electrons are located further from the nucleus.

• The inner electrons (core electrons) repel the outer electrons and partially block the attractive force of the positive nucleus.
• As you go down the group, the number of inner electrons increases, so the shielding effect becomes stronger.
• The valence electrons experience a lower effective nuclear charge (Z_eff), meaning they are held less tightly.

Combined Effect: The increased distance from the nucleus (due to extra shells) and the increased shielding (due to more inner electrons) both contribute to the increase in atomic radius.

Relationship to Other Properties

The increase in atomic radius down the group explains several other trends:

• Decrease in Ionization Energy: Because the valence electrons are further from the nucleus and are more shielded, it is easier to remove them. This is why ionization energy decreases down the group.
• Increase in Reactivity: With lower ionization energy, the metals lose their valence electrons more easily, making them more reactive.
• Decrease in Melting Point: The larger atomic radius means the valence electrons are further from the nucleus, so the metallic bond is weaker. This leads to a lower melting point.
• Increase in Density: Although the atomic radius increases, the atomic mass increases more significantly, leading to a general increase in density.

Intermediate Level

The atomic radius of Group 2 elements increases down the group because of the addition of extra electron shells and the increased shielding effect. Each new element in the group has its valence electrons in a higher principal quantum level, which means they are further from the nucleus. Additionally, the inner electrons shield the outer electrons from the full nuclear charge, reducing the attraction between the nucleus and the valence electrons. This allows the atom to expand. The increase in atomic radius is responsible for the decrease in ionization energy and the increase in reactivity down the group.

Important Points

• - Atomic Radius: Measure of the size of an atom.
• - Trend: Atomic radius increases down Group 2.
• - Reason: Addition of extra electron shells and increased shielding effect.
• - Result: Valence electrons are further from the nucleus and less tightly held.
• - Consequence: Ionization energy decreases, and reactivity increases down the group.
• - Data: Be (0.122 nm) < Mg (0.160 nm) < Ca (0.197 nm) < Sr (0.215 nm) < Ba (0.217 nm).

Remember

• - Atomic radius increases down Group 2.
• - Reason: More electron shells and increased shielding.
• - Smaller atoms at the top (Be), larger atoms at the bottom (Ba).
• - Larger atoms have lower ionization energy and higher reactivity.
• - The trend is due to the addition of principal energy levels.
• - Shielding effect increases down the group.

Common Mistakes

• - Thinking that atomic radius decreases down the group. It increases because of the addition of electron shells.
• - Forgetting the role of shielding. The shielding effect of inner electrons is a key reason why the outer electrons are less tightly held.
• - Confusing atomic radius with ionic radius. The ionic radius of M²⁺ ions also increases down the group, but the values are different from the atomic radii.
• - Assuming that the trend is only due to the addition of shells. While that is the main reason, the shielding effect also plays a significant role.
• - Not connecting the atomic radius trend to other properties. The increase in atomic radius is the underlying reason for the decrease in ionization energy and the increase in reactivity.

Quick Revision

The atomic radius of Group 2 elements increases down the group. This is because new electron shells are added, placing the valence electrons further from the nucleus, and the shielding effect of inner electrons increases. As a result, the valence electrons are held less tightly, which leads to a decrease in ionization energy and an increase in reactivity down the group. The order of increasing atomic radius is Be < Mg < Ca < Sr < Ba.

60-Second Revision

The atomic radius of Group 2 elements increases down the group due to the addition of extra electron shells and increased shielding. This makes the atoms larger and the valence electrons easier to remove, leading to decreased ionization energy and increased reactivity. The order is Be < Mg < Ca < Sr < Ba.

Frequently Asked Questions (FAQs)

Q1. Explain the trend in atomic radius of Group 2 elements. Answer: The atomic radius of Group 2 elements increases down the group. This is because each successive element has an additional electron shell, so the outermost electrons are in a higher principal energy level and are further from the nucleus. Additionally, the inner electrons shield the outer electrons from the full attractive force of the nucleus, reducing the effective nuclear charge. Both factors contribute to the increase in atomic size.

Q2. Why does the atomic radius increase from magnesium to calcium? Answer: Calcium has one more electron shell than magnesium. The valence electrons of calcium are in the fourth shell (n=4), while those of magnesium are in the third shell (n=3). This additional shell places the valence electrons further from the nucleus, increasing the atomic radius. The increased shielding from the additional inner electrons also contributes to this increase.

Q3. How does the trend in atomic radius relate to the trend in ionization energy? Answer: The increase in atomic radius down the group means that the valence electrons are further from the nucleus and are more shielded. This reduces the attraction between the nucleus and the valence electrons, making it easier to remove them. Therefore, as the atomic radius increases, the ionization energy decreases.

Q4. What is the shielding effect, and how does it influence atomic radius? Answer: The shielding effect is the reduction in the effective nuclear charge experienced by an outer electron due to the repulsion from inner electrons. As you go down Group 2, the number of inner electrons increases, increasing the shielding effect. This allows the outer electrons to be less tightly held, contributing to a larger atomic radius.

Q5. Which Group 2 element has the largest atomic radius? Answer: Barium (Ba) has the largest atomic radius among the Group 2 elements listed in the textbook (0.217 nm). Radium (Ra) would be even larger, but it is not included in the table.

• Q6. Which Group 2 element has the smallest atomic radius?
• Answer: Beryllium (Be) has the smallest atomic radius among the Group 2 elements (0.122 nm).

Q7. Why do the atoms get larger as you go down a group in the periodic table? Answer: As you go down a group, you are adding electrons to new, higher-energy shells that are further from the nucleus. At the same time, the number of inner electrons (which shield the outer electrons from the nucleus) increases. Both factors cause the atomic radius to increase.

Q8. How does the atomic radius trend explain the increase in reactivity down Group 2? Answer: A larger atomic radius means the valence electrons are further from the nucleus and less tightly held. This makes it easier for the atoms to lose their two valence electrons, forming M²⁺ ions. The easier it is to lose electrons, the more reactive the metal is. Therefore, the increase in atomic radius down the group is directly linked to the increase in reactivity.

Q9. What is the relationship between atomic radius and metallic bond strength? Answer: As the atomic radius increases, the valence electrons are further from the nucleus, so the electrostatic attraction between the nucleus and the delocalized electrons (the metallic bond) becomes weaker. This is why the melting point of Group 2 metals decreases down the group.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in atomic radius down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains constant.
• D. It first increases then decreases.
• Correct Answer: A
• Explanation: The atomic radius increases down the group due to the addition of extra electron shells and increased shielding effect.

Q2. Why does the atomic radius increase down Group 2? A. Because the number of protons decreases. B. Because the addition of electron shells and increased shielding effect. C. Because the atomic mass decreases. D. Because the density increases. Correct Answer: B Explanation: The addition of extra electron shells places the valence electrons further from the nucleus, and the increased shielding effect reduces the effective nuclear charge.

• Q3. Which of the following is the correct order of increasing atomic radius?
• 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: Atomic radius increases down the group, so the correct order is Be < Mg < Ca < Sr < Ba.

• Q4. Which Group 2 element has the smallest atomic radius?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: A
• Explanation: Beryllium is at the top of the group and has the fewest electron shells, so it has the smallest atomic radius.

• Q5. Which Group 2 element has the largest atomic radius?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Barium is at the bottom of the group and has the most electron shells, so it has the largest atomic radius.

• Q6. The increase in atomic radius down the group is primarily due to:
• A. Increase in nuclear charge
• B. Addition of electron shells
• C. Increase in the number of neutrons
• D. Decrease in the number of protons
• Correct Answer: B
• Explanation: The addition of new electron shells places the valence electrons further from the nucleus, increasing the atomic radius.

Q7. How does the shielding effect influence atomic radius? A. It increases the effective nuclear charge. B. It decreases the effective nuclear charge, allowing the atom to expand. C. It has no effect on atomic radius. D. It decreases the number of electrons. Correct Answer: B Explanation: The shielding effect of inner electrons reduces the effective nuclear charge experienced by the valence electrons, allowing them to be further from the nucleus, thereby increasing the atomic radius.

• Q8. What happens to the metallic bond strength as the atomic radius increases down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains the same.
• D. It becomes zero.
• Correct Answer: B
• Explanation: As the atomic radius increases, the valence electrons are further from the nucleus, weakening the metallic bond strength.

• Q9. The increase in atomic radius down Group 2 is accompanied by a decrease in:
• A. Reactivity
• B. Ionization energy
• C. Atomic mass
• D. Number of electron shells
• Correct Answer: B
• Explanation: As the atomic radius increases, the valence electrons are further from the nucleus and less tightly held, so ionization energy decreases.

• Q10. Which of the following properties increases down Group 2?
• A. Ionization energy
• B. Electronegativity
• C. Atomic radius
• D. Melting point
• Correct Answer: C
• Explanation: The atomic radius increases down the group, while ionization energy, electronegativity, and melting point generally decrease.

Multiple Choice Questions (MCQs)

• Q1. What is the trend in atomic radius down Group 2?
• A. Increases
• B. Decreases
• C. Remains constant
• D. First increases then decreases
• Correct Answer: A
• Explanation: Atomic radius increases down the group due to the addition of electron shells and increased shielding.

Q2. Why does the atomic radius increase down Group 2? A. The number of protons decreases. B. The addition of electron shells and increased shielding effect. C. The atomic mass decreases. D. The density increases. Correct Answer: B Explanation: The addition of extra electron shells places the valence electrons further from the nucleus, and the increased shielding effect reduces the effective nuclear charge.

• Q3. Which of the following is the correct order of increasing atomic radius?
• 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: Atomic radius increases down the group, so the correct order is Be < Mg < Ca < Sr < Ba.

• Q4. Which Group 2 element has the smallest atomic radius?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: A
• Explanation: Beryllium is at the top of the group and has the fewest electron shells, so it has the smallest atomic radius.

• Q5. Which Group 2 element has the largest atomic radius?
• A. Beryllium
• B. Magnesium
• C. Calcium
• D. Barium
• Correct Answer: D
• Explanation: Barium is at the bottom of the group and has the most electron shells, so it has the largest atomic radius.

• Q6. The increase in atomic radius down the group is primarily due to:
• A. Increase in nuclear charge
• B. Addition of electron shells
• C. Increase in the number of neutrons
• D. Decrease in the number of protons
• Correct Answer: B
• Explanation: The addition of new electron shells places the valence electrons further from the nucleus, increasing the atomic radius.

Q7. How does the shielding effect influence atomic radius? A. It increases the effective nuclear charge. B. It decreases the effective nuclear charge, allowing the atom to expand. C. It has no effect on atomic radius. D. It decreases the number of electrons. Correct Answer: B Explanation: The shielding effect of inner electrons reduces the effective nuclear charge experienced by the valence electrons, allowing them to be further from the nucleus, thereby increasing the atomic radius.

• Q8. What happens to the metallic bond strength as the atomic radius increases down Group 2?
• A. It increases.
• B. It decreases.
• C. It remains the same.
• D. It becomes zero.
• Correct Answer: B
• Explanation: As the atomic radius increases, the valence electrons are further from the nucleus, weakening the metallic bond strength.

• Q9. The increase in atomic radius down Group 2 is accompanied by a decrease in:
• A. Reactivity
• B. Ionization energy
• C. Atomic mass
• D. Number of electron shells
• Correct Answer: B
• Explanation: As the atomic radius increases, the valence electrons are further from the nucleus and less tightly held, so ionization energy decreases.

• Q10. Which of the following properties increases down Group 2?
• A. Ionization energy
• B. Electronegativity
• C. Atomic radius
• D. Melting point
• Correct Answer: C
• Explanation: The atomic radius increases down the group, while ionization energy, electronegativity, and melting point generally decrease.