Problem 16 - A Complete Resource Book in Chemistry for JEE Main

Question

As the nuclear charge increases from neon to calcium, the orbital energies (a) increase very slowly (b) increase very rapidly (c) fall (d) increase

Step-by-Step Solution

Answer

(c) fall

Step 1: Understanding the Concept

As the nuclear charge increases (meaning the number of protons in the nucleus increases), the attraction between the nucleus and the electrons also increases. This is because the positive charge of the nucleus pulls more strongly on the negative electrons.

Step 2: Analyzing Orbital Energy

Orbital energy levels represent the energy required to keep an electron bound to the nucleus. A stronger nuclear charge leads to a greater attraction of electrons, which typically results in lower energy levels for the orbitals since the electrons are held more tightly and closer to the nucleus.

Step 3: Conclusion Based on the Trend

Since increased nuclear charge leads to greater electron-nucleus attraction, the orbital energies will decrease, not increase. Hence, as we move from neon to calcium (increasing nuclear charge), the orbital energies fall.

Key Concepts

Orbital Energies

When we talk about orbital energies in atoms, we're referring to the energy levels associated with electrons being held in orbitals around the nucleus. As electrons occupy higher energy orbitals, they are further from the nucleus and have higher energy compared to electrons in lower orbitals.

• Higher orbital energy means electrons are less tightly bound to the nucleus and can be easily excited or removed.
• Lower orbital energy means stronger binding to the nucleus, resulting in greater stability of the electron within that orbital.

The increase of nuclear charge (protons) in an element as you move across the periodic table affects these energies. Specifically, it results in a stronger pull on the electrons, potentially lowering the energy of the orbitals by bringing electrons closer to the nucleus. Consequently, in elements with increased nuclear charge, the orbital energies may decrease, making them more stable.

Electron-Nucleus Attraction

The concept of electron-nucleus attraction is fundamental to understanding atomic structures. This attraction is governed by the electrostatic force between the positively charged nucleus and negatively charged electrons.

• Greater nuclear charge means more protons in the nucleus and stronger attractive force.
• As a result, electrons are drawn closer, reducing the atomic radius.

In terms of chemistry and physics, the attraction plays a crucial role in determining an element's chemical properties. It influences how easily an electron can be removed (ionization energy) or how tightly it is bound to an orbital. As the attraction increases with more nuclear charge, electrons reside closer to the nucleus, contributing to lower orbital energy levels.

Energy Levels

Energy levels, also known as electron shells, denote the position and groups of orbitals that electrons can inhabit around an atom's nucleus. These levels indicate potential energy states of electrons, and their distribution among different orbitals is based on energy minimization principles.

• Electrons in lower energy levels are closer to the nucleus and require more energy to be moved away.
• As energy levels increase, the potential energy and the distance of the electron from the nucleus also increase.

The arrangement in these levels is stepwise; electrons fill the lowest available energy levels first before they can occupy higher ones. When nuclear charge increases with more protons present, the energy levels effectively become lower for the electrons, as seen when transitioning from elements like neon to calcium. This reduction in energy level is due to the stronger pull from a more positively charged nucleus.