Question

Hydrogen- 1 can take the form of a molecule, an anion \(\left(\mathrm{H}^{-}\right)\), or a cation \(\left(\mathrm{H}^{+}\right)\). (a) How many protons, electrons, and neutrons are in each possible species? (b) Write the name and formula for the compound formed between hydrogen and a metal in Group 2 with 12 protons. (c) What is the general name of the aqueous compounds in which hydrogen is a cation?

Short answer

Answer: In a hydrogen molecule (H2), each hydrogen atom has 1 proton, 1 electron, and no neutrons. In a hydrogen anion (H-), there is 1 proton, 2 electrons, and no neutrons. In a hydrogen cation (H+), there is 1 proton, 0 electrons, and no neutrons. The compound formed between hydrogen and the Group 2 metal with 12 protons (magnesium) is magnesium hydride (MgH2). The general name for aqueous compounds with hydrogen as a cation is "acid."

Step-by-step solution

(a) Identifying protons, electrons, and neutrons in different hydrogen species

First, we need to examine each species of hydrogen given in the exercise: the molecule, anion, and cation. 1. Molecule: A hydrogen molecule is denoted as H2, which consists of two hydrogen atoms. A single hydrogen atom contains 1 proton, 1 electron, and no neutrons. 2. Anion: A hydrogen anion (H-) has gained an additional electron, making it negatively charged. Thus, it has 1 proton, 2 electrons, and no neutrons. 3. Cation: A hydrogen cation (H+) has lost an electron, making it positively charged. It has 1 proton, 0 electrons, and no neutrons.

(b) Finding the compound formed between hydrogen and a Group 2 metal

The metal in Group 2 with 12 protons is magnesium (Mg). When hydrogen forms a compound with a metal, it forms hydrides. In this case, the compound is a magnesium hydride. Since magnesium has a +2 charge, we need two hydrogen anions to balance the charge. Thus, the formula for the compound is MgH2, and the name is magnesium hydride.

(c) General name of aqueous compounds with hydrogen as a cation

When hydrogen is a cation in an aqueous compound, it is called a hydronium ion (H3O+). The general name for the aqueous compounds containing hydronium ions is "acid." Examples of such compounds include hydrochloric acid (HCl) and sulfuric acid (H2SO4), where hydrogen acts as the cation in the compound.

Key concepts

Protons and Electrons

Hydrogen, the simplest and most abundant element in the universe, is unique due to its atomic structure. A single hydrogen atom consists of just one proton and one electron. The proton resides in the nucleus, which is the central part of the atom. This gives hydrogen its identity and distinct characteristics.

Electrons, on the other hand, are located in the electron cloud surrounding the nucleus. For a neutral hydrogen atom, the number of protons equals the number of electrons, which is one. However, different species of hydrogen, such as anions and cations, can have different numbers of electrons.

• A hydrogen anion \((\mathrm{H}^-)\) gains an additional electron, resulting in a total of two electrons. This makes the anion negatively charged.
• A hydrogen cation \((\mathrm{H}^+)\) loses its single electron, having none left, which gives it a positive charge.

Understanding these changes in electron numbers is crucial in chemistry, as they significantly influence the molecule's chemical behavior and reactions.

Hydrogen Anion

The hydrogen anion, represented as \(\mathrm{H}^-\), is a negatively charged ion formed when a hydrogen atom gains an additional electron. This anion is important in many chemical reactions due to its negative charge and increased electron density.

The additional electron in a hydrogen anion creates an extra layer of complexity. It enhances the anion's ability to participate in covalent bonding, particularly with metals. This attribute is vital for forming compounds like hydrides.

• In these compounds, the \(\mathrm{H}^-\) ion commonly bonds with metals from various groups of the periodic table.
• Hydrogen anions serve as excellent reducing agents in chemical reactions.

Their high reactivity and unique electronic structure make them essential in industrial and laboratory chemical processes.

Group 2 Metals

Group 2 of the periodic table is known as the alkaline earth metals. They include elements like beryllium, magnesium, calcium, and others, and are characterized by having two electrons in their outermost energy level. These metals are relatively reactive, though not as much as alkali metals in Group 1.

When Group 2 metals interact with hydrogen anions, they form compounds known as hydrides. A prominent example is magnesium hydride (MgH2). Here, magnesium donates its two valence electrons to two hydrogen anions to create a stable ionic compound.

• Magnesium has an atomic number of 12, meaning it has 12 protons.
• In forming hydrides, these metals generally oxidize, losing their outer electrons to hydrogen.

This reaction with hydrogen is a great way to store and release energy chemically, showcasing the importance of these metals in industrial applications.

Hydrides

Hydrides are compounds that result when hydrogen binds with more electropositive elements, notably metals. In these compounds, hydrogen typically exists in the anion form, \(\mathrm{H}^-\). These compounds can vary widely in composition and structure but are categorized mainly as ionic, covalent, or metallic hydrides.

Common factors of hydrides include:

• Ionic hydrides often involve hydrogen bonding with Group 1 or Group 2 metals, like sodium hydride (NaH) or magnesium hydride (MgH2).
• Covalent hydrides, such as methane (CH4), feature hydrogen forming single covalent bonds with elements like carbon.
• Metallic hydrides, where hydrogen is interstitially incorporated into metal lattices, are crucial for hydrogen storage.

Understanding hydrides is imperative for fields like energy storage, where they hold potential for hydrogen fuel technologies.

Aqueous Compounds

Aqueous compounds are those dissolved in water, forming solutions. When hydrogen participates in forming these compounds as a cation, it commonly results in the existence of acidic solutions.

Acids are well known for containing hydronium ions \(\mathrm{H}_3\mathrm{O}^+\). Reactions with water allow hydrogen to acquire a positive charge, effectively resulting in a hydronium ion. Such activity underpins the behavior of:

• Common acids like hydrochloric acid (HCl) and sulfuric acid (H2SO4).
• The strength of acids, often related to the extent of dissociation in water.

These compounds are fundamental in various chemical processes, including metabolism, industrial processes, and manufacture of materials, making their study essential for chemistry students.

Hydronium Ion

The hydronium ion, \(\mathrm{H}_3\mathrm{O}^+\), occupies a significant role in acid-base chemistry. It forms when hydrogen ions, \(\mathrm{H}^+\), associate with water molecules. In aqueous solutions, hydrogen doesn't exist in isolation; it binds with water to form hydronium.

This ion is key in determining the acidity of solutions. It reflects the presence of excess hydrogen ions, which are indicative of acidic environments.

• Hydronium ions are produced in various acid reactions, playing a central role in defining solution pH levels.
• The concentration of hydronium ions directly correlates with a solution’s acidity.

Recognizing the behavior and formation of the hydronium ion is crucial in understanding complex chemical reactions and equilibriums. Its influence makes it vital for disciplines ranging from biochemistry to industrial chemistry.