Problem 2 - General Chemistry: Principles and Modern Applications

Question

Write Lewis symbols for the following ions. (a) \(\mathrm{H}^{-}\) (b) \(\operatorname{Sn}^{2+} ;\) (c) \(\mathrm{K}^{+} ;\) (d) \(\mathrm{Br}^{-} ;\) (e) \(\mathrm{Se}^{2-} ;\) (f) \(\mathrm{Sc}^{3+}\).

Step-by-Step Solution

Answer

The Lewis symbols for \( \mathrm{H}^{-},\operatorname{Sn}^{2+},\mathrm{K}^{+}, \mathrm{Br}^{-},\mathrm{Se}^{2-}, \mathrm{Sc}^{3+} \) are \( \stackrel{..}{\mathrm{H}}^{-},\stackrel{: :}{\mathrm{Sn}}^{2+}, \mathrm{K}^{+}, \stackrel{.. : .. : .. :}{\mathrm{Br}}^{-},\stackrel{.. : .. : .. :}{\mathrm{Se}}^{2-}, \mathrm{Sc}^{3+} \) respectively.

Step 1: Draw the Lewis symbol for \( \mathrm{H}^{-} \)

The hydrogen atom \( \mathrm{H} \) has one valence electron. Since the ion \( \mathrm{H}^{-} \) is negatively charged, it means it has gained an electron. Therefore, \( \mathrm{H}^{-} \) has two electrons. The Lewis symbol is : \( \stackrel{..}{\mathrm{H}}^{-} \)

Step 2: Draw the Lewis symbol for \( \operatorname{Sn}^{2+} \)

The tin atom \( \operatorname{Sn} \) has four valence electrons. Since the ion \( \operatorname{Sn}^{2+} \) is positively charged, it means it has lost two electrons. Therefore, \( \operatorname{Sn}^{2+} \) has two electrons. The Lewis symbol is \( \stackrel{: :}{\mathrm{Sn}}^{2+} \)

Step 3: Draw the Lewis symbol for \( \mathrm{K}^{+} \)

The potassium atom \( \mathrm{K} \) has one valence electron. Since the ion \( \mathrm{K}^{+} \) is positively charged, it means it has lost an electron. Therefore, \( \mathrm{K}^{+} \) has no electrons. The Lewis symbol is \( \mathrm{K}^{+} \)

Step 4: Draw the Lewis symbol for \( \mathrm{Br}^{-} \)

The bromine atom \( \mathrm{Br} \) has seven valence electrons. Since the ion \( \mathrm{Br}^{-} \) is negatively charged, it means it has gained an electron. Therefore, \( \mathrm{Br}^{-} \) has eight electrons. The Lewis symbol is \( \stackrel{.. : .. : .. :}{\mathrm{Br}}^{-} \)

Step 5: Draw the Lewis symbol for \( \mathrm{Se}^{2-} \)

The selenium atom \( \mathrm{Se} \) has six valence electrons. Since the ion \( \mathrm{Se}^{2-} \) is negatively charged, it means it has gained two electrons. Therefore, \( \mathrm{Se}^{2-} \) has eight electrons. The Lewis symbol is \( \stackrel{.. : .. : .. :}{\mathrm{Se}}^{2-} \)

Step 6: Draw the Lewis symbol for \( \mathrm{Sc}^{3+} \)

The scandium atom \( \mathrm{Sc} \) has three valence electrons. Since the ion \( \mathrm{Sc}^{3+} \) is positively charged, it means it has lost three electrons. Therefore, \( \mathrm{Sc}^{3+} \) has no electrons. The Lewis symbol is \( \mathrm{Sc}^{3+} \)

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and they play a critical role in chemical bonding. These electrons are able to be gained, lost, or shared during chemical reactions.
You may wonder why these electrons are so special. Think of them as the handshake atoms use to connect with one another. They determine how atoms like to combine with others to make molecules and compounds.
In the periodic table, you can find the number of valence electrons based on an atom's group number. For example:

• Hydrogen (H) has 1 valence electron, as it's in the first group.
• Potassium (K) also has 1, sharing the same group with hydrogen.
• Selenium (Se) sits in group 16, holding 6 valence electrons.

When drawing Lewis symbols, these valence electrons are depicted as dots around the element's symbol. It provides a simple visual representation of the electron configuration and helps explain how atoms bond.

Ionic Charge

Ionic charge arises when atoms gain or lose electrons to become more stable.
Atoms usually aim to have a full set of valence electrons, like the noble gases. When they achieve this, they become charged ions. The charge is determined by the difference between protons (positive) and electrons (negative).
If an atom:

• Gains electrons, it becomes negatively charged (anion).
  For example, a bromine atom gains an electron, resulting in a charge of \(-1\) as \( ext{Br}^-\).
• Loses electrons, it becomes positively charged (cation).
  Potassium loses its single valence electron and results in a charge of \(+1\), becoming \( ext{K}^+\).

Predicting ionic charge helps in drawing accurate Lewis symbols and understanding the formation of ionic compounds.

Electron Gain and Loss

The exchange of electrons is at the heart of forming ions.
For an atom to achieve a more electrically stable state, it undergoes electron gain or loss:

• Electron Gain: Atoms attract extra electrons; this increases the number of electrons to a complete outer shell, making them anion. Selenium, for instance, picks up two electrons to fill up its shell, transitioning to \( ext{Se}^{2-}\).
• Electron Loss: Atoms shed their valence electrons, aiming for a full inner shell and becoming cations.
  For example, scandium loses three valence electrons, resulting in \( ext{Sc}^{3+}\).

This process of gain and loss not only stabilizes the atom but also dictates the kinds of chemical bonds it will form, such as ionic bonds between metals and non-metals.