Question

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

Short answer

The Lewis symbols for the given atoms and ions are: a) Ca: \( \ \bullet \mathrm{Ca} \bullet \) b) P: \[\begin{array}{c} \\ \bullet & \phantom{-} & \bullet\\ \mathrm{P} & \bullet & \bullet\\ & \bullet & \\ \end{array}\] c) Mg\(^{2+}\): Mg d) S\(^{2-}\): \[\begin{array}{c} \bullet & \bullet & \bullet\\ \mathrm{S} & \bullet & \bullet\\ \bullet & \bullet & \\ \end{array}\]

Step-by-step solution

Determine the number of valence electrons for each atom or ion

First, we need to check the periodic table and find the group number of each element, which corresponds to the number of valence electrons for each atom. For ions, we will need to adjust the number of valence electrons based on the charge. a) Ca: Calcium is in group 2, so it has 2 valence electrons. b) P: Phosphorus is in group 15, so it has 5 valence electrons. c) Mg2+: Magnesium is in group 2, but it has lost 2 electrons to form a 2+ ion, so it has 0 valence electrons. d) S2-: Sulfur is in group 16, but it has gained 2 electrons to form a 2- ion, so it has 8 valence electrons.

Draw the Lewis symbol for each atom or ion

Now, we can draw the Lewis symbols for each atom or ion by placing the elemental symbol in the middle and surrounding it with the appropriate number of dots to represent the valence electrons. a) Ca: • Ca• b) P: • • P •• • c) Mg2+: Since Mg2+ doesn't have any valence electrons, the Lewis symbol is simply Mg without any dots: Mg d) S2-: ••• S •••• •• So the Lewis symbols for the given atoms and ions are: a) Ca: Ca•• b) P: P with 5 dots c) Mg2+: Mg d) S2-: S with 8 dots

Key concepts

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom that determine its chemical properties and bonding behavior. These are the electrons that are involved when atoms form chemical bonds with other atoms to create compounds. In simple terms, they can be thought of as the 'social butterflies' of the atom, seeking interactions with others to create a more stable arrangement.

Every element's valence electrons can be quickly identified by referring to its group on the periodic table. Elements in the same group often share similar properties because they have the same number of valence electrons. For instance, calcium (Ca), which is in group 2, has 2 valence electrons. When visualizing this with a Lewis symbol, dots are placed around the element's symbol to represent each valence electron.

However, when elements gain or lose electrons to become ions, their number of valence electrons changes. For example, magnesium (Mg) loses two electrons to form the Mg²⁺ ion, resulting in no valence electrons, which is reflected in its Lewis symbol having no dots. On the other hand, sulfur (S) gains two electrons to form the S^2- ion, which gives it a total of 8 valence electrons, depicted with eight dots around the S symbol in its Lewis structure. Understanding valence electrons is crucial as it's the first step in predicting the chemical behavior and bonding possibilities of an element or ion.

Periodic Table

The periodic table is a fundamental tool in chemistry, organizing all known elements into a tabular arrangement based on their atomic number, electron configurations, and recurring chemical properties. The table is structured in rows called periods and columns known as groups. Elements in the same group usually share similar characteristics due to having the same number of valence electrons, which influences how they interact with other atoms.

To determine an element's valence electrons using the periodic table, simply look at the group number for elements in Groups 1 to 18. For example, phosphorus (P) is found in Group 15 and typically has 5 valence electrons. These electrons are the basis for its Lewis symbol, which features five dots encompassing the element's symbol.

Bridging the Groups and Electron Configuration

An intriguing aspect of the periodic table is how it showcases the arrangement of electrons in an element's orbitals. Starting from hydrogen at the top left, with a single electron, each subsequent element adds one more electron, following a specific path across different orbitals as guided by the rules of quantum mechanics. This predictable addition allows for chemical behavior to be anticipated and provides us with the means to infer bonding patterns. Gaining expertise in using the periodic table is crucial for students, as it not only helps in identifying valence electrons but also in understanding the periodic trends such as electronegativity, atomic radius, and ionization energy.

Chemical Ions

Chemical ions are atoms or molecules that have gained or lost one or more electrons, and this imbalance between protons and electrons results in a net electric charge. There are two types of ions: cations, which are positively charged ions, and anions, which are negatively charged ions. When forming ions, atoms either achieve a noble gas configuration by losing or gaining electrons, or achieve a pseudo-noble gas configuration, as observed in transition metals.

Looking at specific examples:

• Magnesium tends to lose two electrons and becomes a cation with a 2+ charge, represented as Mg²⁺.
• Sulfur tends to gain two electrons to fill its valence shell and becomes an anion with a 2- charge, shown as S^2-.

These transformations have immense implications for the Lewis symbols we draw for each ion. An ion's Lewis symbol will reflect any gain or loss of electrons. For magnesium's cation, Mg²⁺, the Lewis symbol is simply 'Mg' with no dots since it has no valence electrons after losing them. For the sulfur anion, S^2-, the Lewis symbol consists of the 'S' surrounded by eight dots - two on each side, indicating the complete octet reflecting its gain of two electrons.

Understanding chemical ions is vital because they are often involved in forming ionic compounds, crucial to many chemical reactions, especially in electrolytes and cellular processes. Their behavior in electric fields and how they interact through ionic bonds are foundational concepts for students learning chemistry.