Question

What is the Lewis symbol for each of the following atoms or ions? (a) \(\mathrm{K}\), (b) \(\mathrm{As}\), (c) \(\mathrm{Sn}^{2+}\), (d) \(\mathrm{N}^{3-}\).

Short answer

The Lewis symbols for the given atoms and ions are: (a) K: K• (b) As: \[\begin{array}{c|c} • & • \\ \hline As & ••• \\ \hline • & \end{array}\] (c) Sn²⁺: \[\begin{array}{c|c} & • \\ \hline Sn²⁺ & • \\ \hline & \end{array}\] (d) N³⁻: \[\begin{array}{c|c} • & ••••• \\ \hline N³⁻ & \\ \hline • & • \\ \end{array}\]

Step-by-step solution

Determine the number of valence electrons for each atom or ion

To find the number of valence electrons, we can refer to the periodic table. The group number corresponds to the number of valence electrons an element has. Note that for ions, we must add or subtract electrons according to the charge of the ion. (a) K (Potassium) is in Group 1, so it has 1 valence electron. (b) As (Arsenic) is in Group 15, so it has 5 valence electrons. (c) Sn^(2+) (Tin ion) is in Group 14, meaning it normally has 4 valence electrons. However, since it has a charge of +2, it has lost 2 electrons, leaving it with 2 valence electrons. (d) N^(3-) (Nitride ion) is in Group 15, meaning it normally has 5 valence electrons. Since it has a charge of -3, it has gained 3 electrons, so it has 8 valence electrons.

Draw the Lewis symbols for each atom or ion

To draw the Lewis symbols, we will first write the symbol for the atom or ion, and then place dots around the symbol to represent the valence electrons. There can be a maximum of 8 electrons (4 pairs) surrounding an atom or ion. Start with single dots, and then pair them up if necessary. (a) K has 1 valence electron, so its Lewis symbol is: K• (b) As has 5 valence electrons, so its Lewis symbol is: • As••• • (c) Sn^(2+) has 2 valence electrons, so its Lewis symbol is: Sn²⁺ • • (d) N^(3-) has 8 valence electrons (4 pairs), so its Lewis symbol is: N³⁻ • •••• • Now, we have found the Lewis symbols for each of the given atoms and ions.

Key concepts

Valence Electrons

Understanding valence electrons is fundamental to grasping many concepts in chemistry, especially when forming chemical bonds. Valence electrons are the electrons located in the outermost shell of an atom, and they play a pivotal role in an atom's ability to interact with other atoms.

Each element has a characteristic number of valence electrons that determines its chemical properties and reactivity. We determine the number of valence electrons by looking at an element's group number in the periodic table. For example, elements in Group 1 have 1 valence electron, while those in Group 2 have 2 valence electrons. This pattern is consistent until Group 18, whose elements possess a full set of 8 valence electrons in their outer shells.

In the case of ions, the number of valence electrons changes because ions are atoms that have gained or lost electrons. A positively charged ion, or cation, has lost electrons, thus having fewer valence electrons than its neutral counterpart. Conversely, a negatively charged ion, or anion, has gained electrons, increasing its valence electron count. It's important to adjust for these gains or losses to accurately represent the Lewis symbols for ions.

Here's how you identify the number of valence electrons:

Chemical Notation

Chemical notation is a standardized way of representing elements and their ions, as well as the number and arrangement of electrons around them. Key to this notation are the Lewis symbols, which provide a visual representation of the valence electrons as dots around the symbol of an element or ion.

The Lewis symbol is simple yet informative; it indicates how an atom might engage in chemical bonding. To construct a Lewis symbol, you write down the elemental symbol then strategically place dot(s) around it. Each dot signifies a valence electron. For neutral atoms, you merely follow the valence electron count. When it comes to ions, however, the charge needs to be considered as it affects the total count of valence electrons.

Following the 'octet rule', most atoms seek to have eight valence electrons to achieve a stable electronic configuration similar to noble gases. However, there are exceptions to this rule, and some elements are stable with fewer electrons (like hydrogen) or hold more in expanded valence shells (like phosphorus or sulfur).

For instance, the Lewis symbol for potassium (K) shows a single dot, representing its one valence electron. The Lewis symbol is clear, straightforward, and universally understood in the language of chemistry.

Periodic Table

The periodic table is not just a list of elements; it's a comprehensive chart that organizes elements according to their atomic numbers and properties, and it offers insights into the behavior of atoms during chemical reactions. One of the most crucial aspects of the periodic table when it comes to understanding Lewis symbols is that it categorizes elements into groups (also known as families) that have similar valence electron configurations.

Generally, elements within the same group have the same number of valence electrons, which is why they often exhibit similar chemical characteristics. There are 18 groups on the periodic table, and each can be identified by its topmost number or name. Group 1 contains alkali metals, Group 2 holds the alkaline earth metals, and Groups 3-12 are the transition metals. The Group number can help predict the number of valence electrons; for instance, Group 17 elements have seven valence electrons, pointing towards their reactivity and tendency to form salts.

Using the periodic table correctly allows students and chemists alike to determine not only the atomic structure but also potential chemical behavior, such as reactivity, bonding tendencies, and polarity. This is why the periodic table is a fundamental tool for anyone studying chemistry.