Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

The statement that is not correct for periodic classification of elements is (a) the properties of elements are a periodic function of their atomic numbers. (b) non-metallic elements are less in number than metallic elements. (c) the first ionization energies of elements along a period do not vary in a regular manner with in crease in atomic number. (d) for transition elements, the d-subshells are filled with electrons monotonically with increase in atomic number.

Step-by-Step Solution

Answer

Statement (d) is incorrect since d-subshells aren't always filled monotonically.

Step 1: Understand the question

We need to determine which of the given statements is incorrect in the context of periodic classification of elements.

Step 2: Analyze statement (a)

Statement (a) is correct. The modern periodic law states that the properties of elements are a periodic function of their atomic numbers, meaning elements show repeating patterns in their properties when arranged by increasing atomic number.

Step 3: Analyze statement (b)

Statement (b) is correct. In the periodic table, metallic elements are indeed more numerous than non-metallic elements. Metals constitute most of the elements on the left and center of the periodic table.

Step 4: Analyze statement (c)

Statement (c) is correct. The first ionization energies of elements across a period generally increase, but there are exceptions due to electronic configurations, leading to some variations.

Step 5: Analyze statement (d)

Statement (d) is incorrect. For transition elements, the d-subshells are not always filled in a monotonically increasing manner because some elements experience electron reconfiguration due to stability provided by half-filled or fully filled subshells.

Key Concepts

Modern Periodic Law

The Modern Periodic Law is a fundamental concept in chemistry that has shaped how we understand elements and their properties. It states that the properties of elements are periodic functions of their atomic numbers. This means that when elements are arranged in order of increasing atomic number, their physical and chemical properties exhibit a repeating or periodic pattern. Before the advent of the modern periodic law, elements were classified based on atomic masses, which led to several inconsistencies. The modern approach corrects these inconsistencies by ensuring that elements with similar properties fall within the same vertical groups in the periodic table. Some key features to note are:

• Elements in the same group have similar valence electron configurations, which dictate their chemical properties.
• The periodic table is structured into periods and groups, with each period corresponding to the filling of a new electron shell.
• As a result of the periodicity, trends such as atomic radius, ionization energy, and electronegativity can be observed across periods and down groups.

Understanding the modern periodic law is crucial for predicting how different elements will interact in chemical reactions.

Transition Elements

Transition elements hold a special place in the periodic table, filling in the d-block, and are characterized by their partially filled d-subshells. These elements exhibit unique properties due to the d-electrons, which can participate in bonding, leading to a variety of oxidation states. These elements are found in groups 3 through 12 and include common metals such as iron, copper, and gold. There are a few key traits to highlight:

• They often form colored compounds, a result of d-d electron transitions.
• Many transition metals are excellent catalysts, facilitating chemical reactions without being consumed.
• The d-subshell allows for complex electron configurations, often resulting in unpredicted electron filling sequences.

An interesting aspect is that despite the periodic trend, some anomalies exist. Transition metals do not always follow the "monotonic" electron filling pattern due to stability preferences such as half-filled or fully filled subshells, making their study exciting and sometimes challenging.

Non-metallic vs Metallic Elements

In the periodic table, there is a clear distinction between non-metallic and metallic elements, both having distinct properties owing to their electron configurations. The periodic table is predominantly occupied by metals, but non-metals play critical roles in chemical processes and biological systems. Key differences include:

• Metallic elements, found mostly on the left and in the center of the periodic table, are generally good electrical and thermal conductors, malleable, ductile, and have a shiny appearance.
• Non-metallic elements, clustered on the right side, are typically poor conductors and may exist in solid, liquid, or gaseous states at room temperature.
• Metals tend to lose electrons easily to form cations, whereas non-metals tend to gain electrons to form anions in chemical reactions.

Understanding these differences aids in predicting the behavior of elements when they form compounds, which is essential in fields ranging from materials science to biochemistry.

Ionization Energy

Ionization energy is a critical concept that helps describe how tightly an atom holds onto its electrons. It refers to the energy required to remove an electron from a gaseous atom or ion. Generally, across a period, ionization energy increases as we move from left to right, due to increasing nuclear charge pulling electrons closer. Despite its general trend, there are notable exceptions, often due to electron configurations:

• Noble gases at the end of each period have very high ionization energies due to their stable, filled electron shells.
• Elements like oxygen have slightly lower first ionization energies than expected due to electron repulsion in paired p-orbitals.
• The first ionization energy typically decreases as we move down a group because of increasing atomic radius and electron shielding.

Understanding ionization energy helps predict and explain various chemical behaviors, including bonding tendencies and the reactivity of different elements.