Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

The first ionization potential will be maximum for (a) uranium (b) iron (c) hydrogen (d) lithium

Step-by-Step Solution

Answer

The first ionization potential is maximum for hydrogen (c).

Step 1: Understanding Ionization Potential

Ionization potential is the energy required to remove an electron from a neutral atom in its gaseous state. It is influenced by atomic size, nuclear charge, and electron configuration.

Step 2: Analyzing Each Option

Hydrogen has one electron and a small atomic size with a relatively high ionization potential. Iron, lithium, and uranium are larger atoms with more electrons. Their increasing atomic sizes generally lower the ionization potential compared to hydrogen.

Step 3: Comparing Ionization Potentials

Among the elements listed, hydrogen will have the highest first ionization potential because it has only one electron tightly bound to its nucleus, while the other elements have additional electron shells increasing their atomic sizes.

Key Concepts

Atomic Size

Atomic size refers to the size of an atom, typically determined by the radius of its electron cloud. In simple terms, it is the distance from the nucleus to the outermost electron. This distance can vary among elements and has a direct impact on an element's chemical and physical properties.

For instance:

• Smaller atomic size typically means electrons are closer to the nucleus, leading to stronger attraction forces.
• Larger atomic size often results in weaker attraction, as the electrons are further from the nucleus and more shielded by inner electrons.

The atomic size influences how tightly electrons are held. Hence, a smaller atomic size generally corresponds to a higher ionization potential, as electrons are held more tightly. In the context of our original problem, hydrogen—having the smallest atomic size among the options—has a higher ionization potential due to its closely held electron.

Nuclear Charge

Nuclear charge is the total charge of the protons in an atom's nucleus. This charge plays a crucial role in the attraction between the nucleus and the electrons. The greater the nuclear charge, the stronger the pull on electrons.

Key points include:

• Nuclear charge affects how easily an electron can be removed from an atom. A higher charge means stronger attraction, making it more difficult to ionize the atom.
• In multi-electron atoms, the effective nuclear charge is less than the total nuclear charge due to electron shielding.

In elements like hydrogen, which has only one proton, the full nuclear charge affects its sole electron. This results in a high ionization potential compared to elements with multiple electron shells, where effective nuclear charge is reduced by shielding. Thus, even though uranium has a much larger nuclear charge in total, its ionization potential isn't the highest because of the shielding effect.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom's electron shells and subshells. It is a roadmap to understanding an atom's chemical behavior and reactivity.

Important aspects include:

• Atoms with a stable electron configuration, such as noble gases, will have higher ionization potentials.
• The presence of a single electron in the outer shell (as in hydrogen) increases the ionization potential as that electron is tightly bound.

In our context, hydrogen has a simple electron configuration of 1s¹, meaning its one electron is in the closest shell to the nucleus. This results in a significant attraction, explaining its high ionization potential. On the other hand, elements like iron and uranium have electrons in higher energy levels and greater degrees of electron repulsion, lowering their ionization potential.