Problem 14 - General Chemistry: Principles and Modern Applications

Question

How would you expect the sizes of the hydrogen ion, \(\mathrm{H}^{+},\) and the hydride ion, \(\mathrm{H}^{-},\) to compare with that of the H atom and the He atom? Explain.

Step-by-Step Solution

Answer

The hydrogen ion (\(H^{+}\)) is much smaller than the hydrogen atom because it has lost its sole electron, while a hydride ion (\(H^{-}\)) is larger than the hydrogen atom due to the additional electron, but smaller than the helium atom thanks to helium's larger positively charged nucleus.

Step 1: Understanding of Atomic Structure

An atom consists of a nucleus containing protons and neutrons, and electrons that move around the nucleus. The size of an atom is determined by the outermost electronic cloud. An atom becomes an ion when it loses or gains an electron.

Step 2: Compare the Sizes of Hydrogen Ion and Hydrogen Atom

The hydrogen ion (\(H^{+}\)) is formed when a hydrogen atom loses an electron. As such, \(H^{+}\) consists only of one proton without any electron, thereby making it practically point-sized.

Step 3: Compare Sizes of Hydride Ion and Helium Atom

The hydride ion (\(H^{-}\)) is formed when a hydrogen atom gains an electron. This means that it has two electrons orbiting one proton, similar to the helium atom. However, since the helium atom has two protons and two neutrons in its nucleus, it has a greater positive charge attracting the electrons, making it smaller than the hydride ion.

Step 4: Size Comparisons

From Step 2 and Step 3, we can conclude that \(H^{+}\) is smaller than the hydrogen atom, and the hydride ion (\(H^{-}\)) is larger than the hydrogen atom but smaller than the helium atom.

Key Concepts

Atomic structure

Atoms are the basic building blocks of matter. They are composed of a central nucleus and surrounding electrons. The nucleus, which is comprised of protons and neutrons, accounts for almost all of an atom's mass. Electrons, which are much lighter, orbit the nucleus at various energy levels, forming an "electron cloud". This electron cloud determines the size of the atom.

When an atom gains or loses electrons, it transforms into an ion. Gain of electrons leads to a negative ion, while loss results in a positive ion. The size of the ion compared to the original atom changes, because of the rearrangement of the electrons and their interaction with the nucleus.

• Protons are positively charged.
• Neutrons have no charge.
• Electrons are negatively charged.

Understanding these fundamentals helps illustrate how ions, like hydrogen ion (H⁺) and hydride ion (H^-), compare to their neutral counterparts.

Hydrogen ion

The hydrogen ion ( H⁺ ) is a common ion often found in nature. It is formed from a hydrogen atom, which typically has one proton and one electron, by losing the electron. This results in a positively charged ion with simply one proton.

Without an electron, the hydrogen ion is incredibly small, essentially reduced to just its proton. This factor makes the H⁺ ion practically point-sized. This tiny size is important for understanding reactions in both chemistry and biology, where the H⁺ ion plays a critical role.

Some key characteristics of hydrogen ions include:

• They are positively charged with no accompanying electrons.
• They are much smaller than the typical hydrogen atom.
• They influence acidity levels in solutions (common in acids).

Hydride ion

The hydride ion ( H^- ) represents the opposite situation to the hydrogen ion. It is formed when a hydrogen atom gains an extra electron. This means that a hydride ion has two electrons orbiting a single proton. The presence of one more electron than the hydrogen atom results in a negative charge.

Compared to a helium atom, which also has two electrons, the hydride ion is larger. This is because, while helium and the hydride ion have the same number of electrons, helium's nucleus contains two protons, creating a stronger attraction that pulls the electron cloud closer, making helium smaller.

Significant features of hydride ions include:

• They have a negative charge due to the extra electron.
• They are larger than a hydrogen atom but not as compact as a helium atom.
• They play a role in reduction reactions in chemistry.

Helium atom

Helium is notable for its complete and stable electron shell, which makes it chemically inert. A helium atom consists of a nucleus with two protons and two neutrons, surrounded by two electrons.

These two electrons fill the first energy level, which significantly influences the size and stability of the helium atom. The ability of the two protons to tightly hold the electron cloud makes helium compact in size.

Despite its small size, helium finds wide application due to its unique properties:

• Helium is non-reactive due to its filled outer electron shell.
• It has a compact atomic structure relative to ions with similar electron numbers.
• Helium is used in applications requiring non-reactive environments, such as in balloons and cooling systems for superconductors.

Understanding helium’s atomic structure helps explain its stability and how it compares to ions like the hydride ion structurally and size-wise.