Formation of the Ionic Bond: Electrostatic Attraction Between Opposites
Why Do Atoms Form Ions?
Definition
Octet rule
Atoms are driven by their quest for stability, often achieved by attaining a noble gas electron configuration, commonly referred to as the octet rule.
- Metals, with only a few electrons in their outermost shells, tend to lose electrons, forming positively charged ions called cations.
- In contrast, non-metals gain electrons to fill their outer shells, forming negatively charged ions, or anions.
How Ionic Bonds Form
- When a metal and a non-metal interact, the metal atom transfers one or more electrons to the non-metal atom.
- This transfer creates oppositely charged ions, which are held together by electrostatic forces, which is the attraction between positive and negative charges.
- This force is the essence of an ionic bond.
Definition
Ionic bond
An ionic bond is the electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion.
Example
Formation of Sodium Chloride ($ \text{NaCl} $)
- A sodium atom ($ \text{Na} $) loses one electron, becoming a sodium ion ($ \text{Na}^+ $).
- A chlorine atom ($ \text{Cl} $) gains that electron, becoming a chloride ion ($ \text{Cl}^- $).
- The $ \text{Na}^+ $ and $ \text{Cl}^- $ ions are then attracted to each other, forming the ionic compound sodium chloride.
Tip
- The strength of an ionic bond depends on the charges of the ions and the distance between them.
- Higher charges and smaller ionic radii result in stronger bonds.
Naming Binary Ionic Compounds: Cation First, Anion Second
- When naming ionic compounds, the cation (positive ion) is always named first, followed by the anion (negative ion).
- The name of the cation is the same as the name of the metal, while the name of the anion is derived from the non-metal with the suffix -ide.
Example
- $ \text{NaCl} $: Sodium chloride
- $ \text{MgO} $: Magnesium oxide
- $ \text{AlN} $: Aluminum nitride
Common Mistake
- Do not reverse the naming order.
- Always name the cation first, even though it is written to the left of the anion in the chemical formula.
Self review
What is the name of the compound $ \text{CaS} $?
Polyatomic Ions: Groups of Atoms Acting as a Single Ion
- Not all ions are single atoms.
- Some ions consist of multiple atoms bonded together, carrying an overall charge.
- These are called polyatomic ions, and they behave as a single unit in chemical reactions.
Common Polyatomic Ions You Should Know:
- Sulfate: $SO_4^{2-}$ (found in gypsum and detergents)
- Nitrate: $NO_3^-$ (common in fertilizers and explosives)
- Carbonate: $CO_3^{2-}$ (present in limestone and baking soda)
- Hydroxide: $OH^-$ (important in bases like sodium hydroxide)
- Ammonium: $NH_4^+$ (found in ammonium salts and fertilizers)
- Phosphate: $PO_4^{3-}$ (essential in biological molecules like DNA and ATP)
- Chlorate: $ClO_3^-$ (used in disinfectants and bleaching agents)
- Acetate: $CH_3COO^-$ (present in vinegar and food preservatives)
- Permanganate: $MnO_4^-$ (used as a disinfectant and oxidizing agent)
- Bicarbonate: $HCO_3^-$ (found in baking soda and blood buffer systems)
Naming Ionic Compounds with Polyatomic Ions
- The rules for naming ionic compounds remain the same: the cation is named first, followed by the anion.
- If the compound contains a polyatomic ion, use the name of the ion as it appears in the table above.
Example
- $ \text{NaNO}_3 $: Sodium nitrate
- $ \text{CaCO}_3 $: Calcium carbonate
- $ \text{NH}_4\text{Cl} $: Ammonium chloride
Tip
- When writing formulas with polyatomic ions, enclose the ion in parentheses if more than one is needed.
- For example, $ \text{Mg(NO}_3\text{)}_2 $ indicates two nitrate ions for every magnesium ion.
Self review
What is the formula for potassium sulfate?
Writing Formulas for Ionic Compounds: Balancing Charges
- The formula of an ionic compound reflects the ratio of cations to anions necessary to achieve electrical neutrality.
- The total positive charge must equal the total negative charge.
Steps to Determine the Formula:
- Identify the charges of the cation and anion.
- Balance the charges by determining the smallest whole-number ratio of ions.
- Write the formula, placing the cation first and the anion second.
Example question
Deduce the formula for magnesium chloride.
Solution
- Magnesium ($ \text{Mg} $) forms $ \text{Mg}^{2+} $, and chlorine ($ \text{Cl} $) forms $ \text{Cl}^- $.
- To balance the charges, two $ \text{Cl}^- $ ions are needed for every $ \text{Mg}^{2+} $ ion.
- The formula is $ \text{MgCl}_2 $.
Example question
Deduce the formula for aluminum sulfate.
Solution
- Aluminum ($ \text{Al} $) forms $ \text{Al}^{3+} $, and sulfate ($ \text{SO}_4^{2-} $) forms $ \text{SO}_4^{2-} $.
- To balance the charges, two $ \text{Al}^{3+} $ ions (total charge $ +6 $) combine with three $ \text{SO}_4^{2-} $ ions (total charge $ -6 $).
- The formula is $ \text{Al}_2(\text{SO}_4)_3 $.
Example question
Deduce the formula for calcium nitrate.
Solution
- Calcium forms $ \text{Ca}^{2+} $, and nitrate forms $ \text{NO}_3^- $.
- Two nitrate ions are needed to balance the charge of one calcium ion.
- The formula is $ \text{Ca(NO}_3\text{)}_2 $.
Common Mistake
- Do not forget to balance the charges.
- Incorrect balancing leads to incorrect formulas.
Self review
What is the formula for barium phosphate?
