Question

What is the Lewis symbol for each of the following atoms or ions: (a) \(\mathrm{Ca}\), (b) \(\mathrm{P}\), (c) \(\mathrm{Mg}^{2+}\), (d) \(S^{2-}\) ?

Short answer

The Lewis symbols for the given atoms and ions are: (a) \(\dot{\imath}\hspace{-2pt}\mathrm{Ca}\hspace{-2pt}\dot{\imath}\), (b) \(\dot{\imath}\hspace{-4pt}\mathrm{P}\hspace{-4pt}\ddot{\imath} \Longrightarrow \dot{\imath}\), (c) \(\mathrm{Mg}^{2+}\), and (d) \(\ddot{\imath}\hspace{-4pt}\mathrm{S}^{2-}\hspace{-4pt}\ddot{\imath} \Longrightarrow \ddot{\imath}\).

Step-by-step solution

Determine the number of valence electrons for each atom or ion

To determine the valence electrons, we have to look at the periodic table and find each element's group number. The group number usually corresponds to the number of valence electrons for elements in Groups 1 to 18. For ions, we have to either add or subtract the charge to find the number of valence electrons. (a) Ca (Calcium): Group 2 - has 2 valence electrons. (b) P (Phosphorus): Group 15 - has 5 valence electrons. (c) Mg²⁺ (Magnesium ion): Group 2 - has lost 2 of the 2 valence electrons due to the charge (+2), so it has 0 valence electrons. (d) S²⁻ (Sulfide ion): Group 16 - has gained 2 electrons due to the charge (-2), so it has 6+2=8 valence electrons.

Draw the Lewis symbols for each atom or ion

(a) Ca (Calcium): Since calcium has two valence electrons, we place two dots around its symbol. Lewis symbol for Ca: \(\dot{\imath}\hspace{-2pt}\mathrm{Ca}\hspace{-2pt}\dot{\imath}\) (b) P (Phosphorus): Phosphorus has five valence electrons, so we place five dots around its symbol. Lewis symbol for P: \(\dot{\imath}\hspace{-4pt}\mathrm{P}\hspace{-4pt}\ddot{\imath} \Longrightarrow \dot{\imath}\) (c) Mg²⁺ (Magnesium ion): The magnesium ion has lost its two valence electrons and has no dots around its symbol. We also indicate the charge by adding a superscript next to the symbol. Lewis symbol for Mg²⁺: \(\mathrm{Mg}^{2+}\) (d) S²⁻ (Sulfide ion): The sulfide ion has eight valence electrons, so all four positions around the symbol will have a pair of dots. We also indicate the charge by adding a superscript next to the symbol. Lewis symbol for S²⁻: \(\ddot{\imath}\hspace{-4pt}\mathrm{S}^{2-}\hspace{-4pt}\ddot{\imath} \Longrightarrow \ddot{\imath}\)

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom that participate in chemical bonding. These electrons reside in the outermost energy level or shell and determine the reactivity and the types of bonds an atom can form. The periodic table is structured in such a way that elements with the same number of valence electrons are placed in the same group (vertical columns). For example, all elements in Group 1 have one valence electron, while those in Group 2 have two, and this pattern continues across the table.

For atoms like calcium (Ca) and phosphorus (P), their group number directly indicates their number of valence electrons. However, for ions such as Mg²⁺ and S^2-, you must consider the charge; magnesium has lost two electrons, leaving it with none, while sulfide has gained two, giving it a total of eight valence electrons. The concept of valence electrons is central to drawing Lewis symbols, which are simple representations of an atom's valence electrons.

Periodic Table

The periodic table is an organized chart of elements that displays them in increasing atomic number and groups them by similar chemical properties. One of the critical insights provided by the periodic table is the periodicity of valence electrons; this pattern helps determine reactivity and bonding behavior.

The table is divided into rows (periods) and columns (groups). Elements within the same group commonly have the same number of valence electrons, making them behave similarly in chemical reactions. By referencing the periodic table, students can quickly identify the number of valence electrons in an element, which is essential when predicting how elements will interact and bond with each other.

Electron Configuration

Electron configuration refers to the arrangement of electrons in an atom's orbitals. It follows a set of principles and notations, one of which details how electrons fill the orbitals from lower to higher energy levels. For example, calcium has the electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², indicating that its two valence electrons are located in the 4s orbital.

Electron configurations are important in determining the chemical behavior of an atom. The valence shell configuration, in particular, determines how an atom will interact with others. For ions, the electron configuration changes when gaining or losing electrons. By knowing an element's electron configuration, one can deduce its Lewis symbol, representing the valence electrons graphically.

Chemical Bonding

Chemical bonding is the force that holds atoms together in molecules and compounds. At its most basic level, bonding involves the interaction of atoms' valence electrons. There are several types of bonds, including ionic, covalent, and metallic bonds. Ionic bonds form between atoms that transfer electrons, like in the formation of Mg²⁺ from magnesium losing two electrons, leading to an ionic bond with elements that can accept those electrons, such as sulfide (S^2-).

Covalent bonds happen when atoms share valence electrons, as seen in the diatomic molecules of oxygen (O₂) or nitrogen (N₂). The Lewis symbols are often used to predict bonding patterns, as the dots represent potential sites for bonding. For example, a Lewis symbol with one dot suggests that an atom can form one bond, while two paired dots (a lone pair) may indicate that the atom is less likely to form a bond at that site.