Problem 55 - Chemistry The Molecular Nature of Matter

Question

Astatine, a member of the halogen family, forms a compound with hydrogen. Predict its chemical formula.

Step-by-Step Solution

Answer

The predicted chemical formula for the compound formed between astatine and hydrogen is HAt.

Step 1: Identify Group of Astatine

Astatine is in the halogen family, which is the 17th group in the periodic table. Members of this group typically have seven valence electrons.

Step 2: Determine Valence Electrons for Hydrogen

Hydrogen has one valence electron and typically forms bonds in which it shares one electron.

Step 3: Predict the Chemical Formula

Since astatine is a halogen, it will most likely form a diatomic molecule with hydrogen, just like other halogens (e.g., chlorine in HCl, bromine in HBr). Thus, we predict that astatine will form the compound hydrogen astatide with the formula HAt.

Key Concepts

Halogen Family

When discussing the halogen family, we refer to a group of elements located in the 17th group of the periodic table. Elements such as fluorine, chlorine, bromine, iodine, and astatine all belong to this family. What characterizes these elements is their similar chemical behavior, primarily due to each of them having seven valence electrons. This makes them highly reactive, as they tend to gain one additional electron to achieve a full outer shell, creating a more stable electron configuration akin to noble gases.

Halogens are known for forming salts when they react with metals and also create various compounds with nonmetals, such as hydrogen. The nature of their reactivity is essential in predicting the types of chemical compounds they can form. For instance, when halogens react with hydrogen, they typically form acids in their aqueous form, such as hydrochloric acid (HCl) from chlorine.

Valence Electrons

The concept of valence electrons is central to understanding chemical bonding and molecule formation. Valence electrons are the electrons in the outermost shell of an atom that can participate in the formation of chemical bonds. For most atoms, having a full valence shell equates to stability, which is often achieved by following the octet rule—having eight electrons in the valence shell.

Hydrogen, however, is an exception, as it aims for a duet, having only two electrons. It has a single valence electron and, when bonding, will typically share this electron to achieve stability. In the case of the halogen family, their seven valence electrons make them just one electron short of the full octet which they can obtain by forming compounds with elements like hydrogen, which can donate or share that one needed electron.

Diatomic Molecule

A diatomic molecule is a molecule composed of only two atoms, which can be the same or different elements. Many nonmetal elements naturally occur as diatomic molecules. Oxygen (O₂), nitrogen (N₂), and all members of the halogen family in their elemental form are diatomic: F₂, Cl₂, Br₂, I₂, and At₂.

These molecules are diatomic because, by pairing up, they can fill their valence shell and reach a stable electron configuration. When halogens form compounds with hydrogen, they also form diatomic molecules, such as HF, HCl, HBr, HI, and in our exercise, HAt. The pairing of hydrogen and a halogen atom results in the sharing of their valence electrons, creating a strong bond that leads to the formation of a stable diatomic molecule.