Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

Assertion: The electron gain enthalpies have large negative values toward the upper right of the periodic table preceding the noble gases. Reason: The effective nuclear charge increases from left to right across a period and consequently it will be easier to add an electron to a smaller atom since the added electron on an average would be closer to the positively charged nucleus.

Step-by-Step Solution

Answer

The reason correctly explains the assertion, as increased effective nuclear charge across a period leads to more negative electron gain enthalpies.

Step 1: Understand the assertion

The assertion states that electron gain enthalpies are highly negative in the upper right portion of the periodic table, just before the noble gases. This means that atoms in this area of the periodic table release a lot of energy when they gain an electron, which typically suggests a high affinity for gaining an electron.

Step 2: Analyze the reason

The reason suggests that as you move left to right across a period in the periodic table, the effective nuclear charge increases. This means the nucleus has a stronger positive charge that can attract electrons more effectively. For smaller atoms in the upper right, this means an added electron is closer to the nucleus, experiencing more attraction.

Step 3: Link the assertion and reason

To connect the assertion and reason, consider how a higher effective nuclear charge results in a stronger attraction for additional electrons. This translates to a greater release of energy (more negative electron gain enthalpy) when an electron is added, especially in smaller atoms where the electron is closer to the nucleus. This supports the assertion that electron gain enthalpies are more negative in elements preceding noble gases.

Step 4: Validate the assertion-reason relationship

Since an increase in effective nuclear charge across a period enhances the ability of an atom to gain and hold an additional electron close to a nucleus, the reason provided is correct and does explain the assertion. This alignment confirms that the electron gain enthalpies become more negative in such atoms.

Key Concepts

Effective Nuclear Charge

When we talk about effective nuclear charge, we are delving into an important concept in chemistry. It refers to the net positive charge experienced by electrons in an atom. Essentially, it's the pull that electrons feel from the nucleus, factoring in the shielding effect of electrons between the nucleus and the outer shell. In simpler terms, as you move across a period in the periodic table, each element has an additional proton in the nucleus. This increase in nuclear charge attracts electrons more strongly.

• The outer electrons in an atom do not experience the full charge of the nucleus because the inner electrons shield them from some of this charge.
• This shielding results in the outer electrons feeling a lower effective nuclear charge compared to the total number of protons in the nucleus.

These dynamics make effective nuclear charge a key player in determining atomic properties, such as electron affinity and atomic size.

Periodic Table Trends

Periodic table trends help us understand patterns in the properties of elements. These trends are a result of systematic changes in the atomic structure as you move across periods or down groups. Some major trends include:

• Atomic Size: As you move from left to right across a period, atoms generally become smaller because of the increased effective nuclear charge, pulling electrons closer to the nucleus.
• Ionization Energy: This is the energy required to remove an electron. It generally increases across a period and decreases down a group, again due to the effective nuclear charge.
• Electron Affinity: Elements on the right of the table have more negative electron affinities, indicating they release more energy when gaining an electron.

These trends are valuable for predicting and explaining the behavior of elements during chemical reactions.

Electron Affinity

Electron affinity is a measure of how much an atom wants to gain an electron. It's expressed as the change in energy when an electron is added to a neutral atom in the gas phase.

• The more negative the electron affinity value, the more energy is released, and the higher the tendency of an atom to accept an electron.
• Nonmetals, especially those found on the upper right of the periodic table, generally have high (negative) electron affinities.

These atoms are more willing to gain electrons because it allows them to achieve a more stable electron configuration, often reaching the stable octet seen in noble gases.

Noble Gases

Noble gases occupy a special place in the periodic table. They are known for their stability and lack of reactivity under normal conditions.

• Their outermost electron shell is full, which means they do not easily gain or lose electrons.
• Because of their full valence shells, noble gases have little to no tendency to accept additional electrons, reflected in their electron affinities being near zero.

The noble gases serve as a reference point for many trends in the periodic table, helping to explain why other elements strive to achieve a similar electron configuration.

Periodic Table Periodicity

Periodic table periodicity is the recurring patterns that we observe in the properties of elements as we move across periods or down groups. Understanding this periodicity is crucial in predicting how elements will behave chemically.

• Properties such as electronegativity, atomic size, and ionization energy show clear periodic trends.
• For example, as you move from left to right across a period, electronegativity and ionization energy increase, while atomic radius decreases.
• This is due to the increased effective nuclear charge felt by electrons across a period.

Periodicity is not only a fascinating aspect of the periodic table, but it also serves as a helpful tool for scientists and students to understand chemical behavior at a glance.