Question

Write Lewis structures for the following: (a) \(\mathrm{Na}\) (b) \(\mathrm{Br}^{-}\) (c) \(\mathrm{O}^{2-}\)

Short answer

The Lewis structure for: (a) Na is a single dot around the symbol ‘Na’. (b) Br- is a symbol 'Br' with 8 dots with brackets around it and a '-' outside. (c) O^2- is a symbol 'O' with 8 dots, also enclosed in brackets with a '2-' outside.

Step-by-step solution

Sodium (Na)

Identify the total number of valence electrons for sodium. Sodium is in Group 1, so it has 1 valence electron. In its elemental form, it will be depicted as an 'Na' symbol with a single dot representing its one valence electron.

Bromide Ion (Br-)

Determine the valence electrons for bromine and consider the extra electron due to the negative charge. Bromine is in Group 17, so it has 7 valence electrons naturally. With an extra electron, it has a total of 8 valence electrons. Draw the symbol 'Br' and surround it with 8 dots indicating the electrons. Since it is an anion, place brackets around the structure and write the charge (-) outside the top right corner of the bracket.

Oxide Ion (O^2-)

Calculate the valence electrons for oxygen and account for the added electrons from the negative charge. Oxygen is in Group 16 and has 6 valence electrons. With an additional 2 electrons, it has a total of 8 valence electrons. Draw the symbol 'O' and surround it with 8 dots representing the electrons. As it is an anion, enclose the structure in brackets and write the charge (2-) outside the top right corner of the bracket.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom that are involved in chemical bonding. These electrons play a pivotal role in determining how an element will interact with others to form compounds. For example, in the case of sodium (Na), which is located in Group 1 of the periodic table, it possesses a single valence electron.

This solitary valence electron is easily lost or shared during chemical reactions, often leading to the formation of an ion, in this case, a cation with a positive charge. In contrast, atoms like bromine (Br) have seven valence electrons since they are in Group 17, making them one electron shy of a full octet, which is a stable configuration of eight valence electrons. Bromine atoms will typically gain or share an electron to achieve this stable state.

Chemical Bonding

Chemical bonding is the process where atoms combine by sharing or exchanging valence electrons to achieve greater stability. The nature of the chemical bond—ionic or covalent—is determined by the manner in which the atoms share or transfer electrons. In our exercise examples, sodium (Na) tends to lose its single valence electron to achieve a stable configuration, forming an ionic bond with another nonmetal atom that gains this electron.

Meanwhile, bromide (Br-) and oxide (O^2-) ions, form when bromine and oxygen gain one and two electrons respectively, achieving their stable octets. In an environment where these ions interact with appropriate cations, they can form ionic bonds. Additionally, atoms can share electrons to form covalent bonds, resulting in molecules where the shared electrons count toward the octet of both atoms involved in the bond.

Electron Dot Structures

Electron dot structures, also known as Lewis dot structures, provide a visual representation of the valence electrons in an atom, molecule, or ion. Each dot represents a valence electron, while each element is represented by its chemical symbol. These structures allow us to predict the types and number of bonds that a particular atom may form.

In the Lewis structures for the ions given in the exercise:

• Sodium (Na) is represented with a single dot around the symbol 'Na', indicating its solitary valence electron.
• Bromide (Br-) has eight dots around the 'Br' symbol in pairs, indicating a full octet achieved through the gain of one electron, with brackets indicating its ionic charge.
• Oxide (O^2-) is drawn with eight dots around the 'O' symbol in pairs, showing that it has achieved a full octet by gaining two electrons, and its charge is indicated similarly with brackets.

These structures are fundamental for students to understand the basics of chemical bonding and the properties of elements and ions.