Question

How many electrons are in the valence shells of (a) Be in \(\mathrm{BeCl}_{2},\) (b) \(\mathrm{B}\) in \(\mathrm{BCl}_{3}\), and (c) \(\mathrm{H}\) in \(\mathrm{H}_{2} \mathrm{O}\) ?

Short answer

Be in \(\mathrm{BeCl}_{2}\) has 2 valence electrons, B in \(\mathrm{BCl}_{3}\) has 3 valence electrons, and each H in \(\mathrm{H}_{2}O\) has 1 valence electron, making it 2 for both hydrogen atoms combined.

Step-by-step solution

Determine Valence Electrons for Be

Beryllium (Be) is in group 2 (IIA) of the periodic table, which means it has 2 valence electrons. Therefore, in \(\mathrm{BeCl}_{2}\), Be has 2 valence electrons.

Determine Valence Electrons for B

Boron (B) is in group 13 (IIIA) of the periodic table and has 3 valence electrons. In \(\mathrm{BCl}_{3}\), B retains these 3 valence electrons.

Determine Valence Electrons for H

Hydrogen (H) has only 1 electron, which is also its valence electron. In \(\mathrm{H}_{2}O\), each H atom has 1 valence electron. Since there are two hydrogen atoms, the total number of valence electrons for the hydrogens in the molecule is 2.

Key concepts

Understanding the Periodic Table Groups

The periodic table is a systematic way of organizing the chemical elements. Each column of the periodic table is known as a 'group', and these groups are important because they help us to understand the element's properties, including its valence electrons. Elements in the same group typically have the same number of valence electrons, which are the electrons in the outermost shell that are involved in chemical bonding.

For example, beryllium (Be), which is the exercise's focus, is in group 2, also known as the alkaline earth metals. This group is characterized by having two valence electrons. That's why for Be in BeCl₂, as indicated in the step-by-step solution, Be has 2 valence electrons. Understanding this group-based pattern makes it easier to predict how an element might react chemically and bonds form in compounds like BeCl₂.

Molecular Composition and Valence Electrons

Molecular composition refers to the number and type of atoms that constitute a molecule and how these atoms are bonded together. This concept is intricately linked with valence electrons, as these electrons are key to understanding how atoms will combine to form molecules.

Take water (H₂O), for instance. Hydrogen (H) atoms each have one valence electron, which bonds with the valence electrons of oxygen (O) to form H₂O. In the textbook exercise, it is illustrated that each hydrogen atom in water maintains its single valence electron, leading to a total of two valence electrons from hydrogen in the molecule. Recognizing the molecular composition is essential for grasping various chemical properties, including reactivity and bonding patterns.

Electron Configuration in Chemical Compounds

Electron configuration, the distribution of electrons among the orbitals of an atom, is fundamental for understanding chemical behavior. In a given element, electrons fill subshells (s, p, d, f) in order of increasing energy. For example, boron (B) has an electron configuration that ends in 2s²2p¹, reflecting its three valence electrons, as group 13 elements typically have.

In compounds like BCl₃, the valence electrons of boron are involved in forming covalent bonds with chlorine (Cl) atoms. Despite the complexities of molecular shapes and electron-sharing, the core idea is that valence electrons dictate how atoms come together to form molecules. Understanding how these electrons are arranged in an atom or ion can offer insight into its chemical behavior, such as the number and types of bonds it can form.