Question

Write a Lewis symbol for each atom or ion. $$ \text { a. }\mathrm{s}^{2-} \quad \text { b. } \mathrm{Mg} \quad \text { c. } \mathrm{Mg}^{2+} \quad \text { d. } \mathrm{P} $$

Short answer

a. :S:^{2-}:: b. :Mg: c. Mg^2+ (no dots) d. :P:..

Step-by-step solution

Understanding Lewis Symbols

Lewis symbols (also known as Lewis dot diagrams) represent the valence electrons of an atom or ion. Valence electrons are shown as dots around the elemental symbol.

Determining Valence Electrons for Sulfur Ion (S^2-)

Sulfur (S) has 6 valence electrons as a neutral atom. The sulfur ion S^2- has gained 2 additional electrons, so it has a total of 8 valence electrons. Place 8 dots around the symbol for S, pairing the electrons as needed.

Determining Valence Electrons for Magnesium (Mg)

Magnesium (Mg) has 2 valence electrons. Place 2 dots around the symbol for Mg, without pairing them.

Determining Valence Electrons for Magnesium Ion (Mg^2+)

The magnesium ion Mg^2+ has lost 2 valence electrons compared to a neutral magnesium atom. Therefore, there are no valence electrons to represent; the Lewis symbol will just be the Mg symbol with no dots.

Determining Valence Electrons for Phosphorus (P)

Phosphorus (P) has 5 valence electrons. Place 5 dots around the symbol for P, with one pair and three unpaired electrons.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial to understanding chemical reactivity and bonding. They determine how an atom will interact with other atoms to form chemical compounds. For instance, in a sulfur ion (S^2-), the sulfur atom typically has six valence electrons in its neutral state, but when it gains two extra electrons, becoming an ion, it has eight valence electrons. On the other hand, magnesium (Mg) has two valence electrons which it can lose to form the magnesium ion (Mg²⁺), resulting in zero valence electrons for the ion. Understanding the count and arrangement of these valence electrons is the first step in predicting the chemical properties of elements and their ions.

Valence electrons can participate in the formation of chemical bonds, either by sharing electrons with other atoms in covalent bonds, or by transferring electrons to or from other atoms in ionic bonds. Some atoms, like noble gases, have complete valence electron shells and are generally non-reactive.

Electron Dot Diagrams

Electron dot diagrams, also known as Lewis dot structures, are a visual representation of the valence electrons as dots around the symbol of an element. These diagrams are indispensable tools for depicting the valence shell electron configuration of an atom or ion. For example, with sulfur in the S^2- ion, eight dots would surround the 'S' to represent the extra electrons it gained. For the magnesium ion (Mg²⁺), no dots are depicted since magnesium loses its valence electrons upon ionization.

Each dot represents one valence electron, and these diagrams can show unpaired electrons as well as paired electrons (lone pairs). The aim is to either fill the valence shell, achieving a noble gas configuration, or to have the valence shell either emptied (in the case of cations) or filled (in the case of anions). Understanding how to draw these can aid in visualizing how atoms bond and form compounds. When electrons are shared between atoms in covalent bonds, the dots are drawn between the elements to signify the shared pair.

Chemical Ions

Chemical ions are charged atoms or molecules formed when an atom gains or loses valence electrons. Cations are positively charged ions formed by the loss of valence electrons, while anions are negatively charged ions formed by the gain of valence electrons. As shown in the example with magnesium, when Mg loses its two valence electrons, it becomes a cation (Mg²⁺), resulting in a positive charge due to the deficit of electrons. Conversely, sulfur becomes the anion S^2- when it gains two electrons, thus carrying a negative charge.

The formation of ions is an essential process in chemistry because ions are highly reactive and participate in a wide range of chemical reactions. The charge of an ion is a critical factor in determining its reactivity, how it will interact with other ions and molecules, and how it will behave in different environments, such as aqueous solutions or crystal lattices. Understanding the formation and properties of ions provides insight into the fundamentals of ionic bonding and electrochemistry, as well as the behavior of salts, acids, and bases in various reactions.