Question

Name a Group \(1 \mathrm{~A}\) to \(8 \mathrm{~A}\) element that could be the central atom (X) in these compounds. (a) \(\mathrm{XCl}_{2}\) (b) \(\mathrm{XH}_{2}\) with two lone pairs of electrons (c) \(\mathrm{XF}_{4}\) with one lone pair of electrons (d) \(\mathrm{XF}_{4}\)

Short answer

a) Be, b) O, c) Br, d) Si.

Step-by-step solution

Identify Valence Electrons for XCl2

Compound (a) is given as \(\text{XCl}_2\). Chlorine (Cl) is a Group 7A element, each contributing one electron. Since there are two chlorine atoms, the central atom X should have two extra electrons to satisfy 8 valence electrons (octet rule). A Group 2A element like Beryllium (Be), with 2 valence electrons, would work since it forms 2 bonds.

Determine Lone Pairs in XH2

For compound (b), \(\text{XH}_2\) with two lone pairs, means the central atom X has a total of four electron pairs (two bonded with H, two as lone pairs). This geometry suggests the atom X is from Group 6A, such as Oxygen (O), which has 6 valence electrons.

Accounting Lone Pair in XF4

In compound (c), \(\text{XF}_4\) with one lone pair, X must share 4 electrons with F and have one lone pair, making a total of 10 valence electrons, which fits a Group 7A element like Bromine (Br).

Valence Electrons for XF4

For compound (d), \(\text{XF}_4\), X shares 4 bonds with F and has no extra lone pairs, indicating 8 electrons total. A Group 4A element, like Silicon (Si), would work as it has 4 valence electrons to form 4 covalent bonds.

Conclusion

The solutions involve determining the total valence electrons, bonding, and lone pairs needed for the chemical formula. Suitable central atoms from the periodic groups found in earlier steps are suggested.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in the formation of chemical bonds. The number of valence electrons determines how an element reacts chemically with other elements. Each column in the periodic table, known as a group, contains elements with the same number of valence electrons.For example: - Group 1A elements have 1 valence electron.- Group 2A elements have 2 valence electrons.- Group 7A elements have 7 valence electrons.These electrons are important because they are involved in bonding. They are either shared, donated, or accepted in chemical reactions. Understanding the number of valence electrons helps predict how an element might bond with others in a compound. For instance, in the case of \( ext{XCl}_2\), the central atom X needs enough valence electrons to complete the electron sharing with two chlorine atoms.

Chemical Bonding

Chemical bonding refers to the attraction between atoms that allows the formation of chemical substances containing two or more atoms. There are several types of chemical bonds, with covalent and ionic bonds being the most common.- **Covalent Bonds:** These bonds form when atoms share pairs of valence electrons. They can be single, double, or triple bonds depending on the number of shared electron pairs.- **Ionic Bonds:** These occur when one atom transfers valence electrons to another, resulting in a positive and a negative ion that attract each other.In the compound \( ext{XCl}_2\), covalent bonding occurs as the central atom (perhaps beryllium from Group 2A) shares its electrons with chlorine atoms. In \( ext{XF}_4\), silicon from Group 4A would form four covalent bonds with fluorine atoms due to its four valence electrons. Recognizing the type of bond helps understand the compound's structure and properties.

Lone Pairs of Electrons

Lone pairs of electrons are pairs of valence electrons that are not involved in bonding and remain on a single atom. These electron pairs can significantly affect the shape and reactivity of a molecule.- **Determining Lone Pairs:** By counting the valence electrons and considering the bonded pairs, lone pairs can be found by subtracting the bonded electrons from the total.- **Effects on Molecular Shape:** Lone pairs can influence the geometry of a molecule. For instance, lone pairs occupy space and can lead to angular or bent molecular shapes.In \( ext{XH}_2\), the central atom X might be oxygen, which can form two bonds with hydrogen while having two lone pairs. This arrangement is reminiscent of water's structure, resulting in a "bent" shape rather than a linear one. For \( ext{XF}_4\) with a lone pair, bromine could be the central atom. This creates a see-saw molecular shape due to the lone pair pushing away the bonded fluorine atoms. Understanding lone pairs is essential for predicting and explaining molecular geometries and reactivities.