Polyatomic Ions and the Octet Rule

Polyatomic Ions

  • Polyatomic ions are ions formed from two or more atoms.

    • Example:

    • Ammonium ion: NH_4^+

    • Carbonate ion: CO_3^{2-}

  • Charge of a polyatomic ion is due to an imbalance between the total number of protons and electrons in the atoms that constitute it.

List of Common Ions

Cations

  • H_3O^+ - Hydronium ion

  • NH_4^+ - Ammonium ion

  • ## Anions

    • OH^- - Hydroxide ion

    • HSO_4^- - Hydrogen sulfate (bisulfate) ion

    • CO_3^{2-} - Carbonate ion

    • PO_4^{3-} - Phosphate ion

    • HCO_3^- - Hydrogen carbonate (bicarbonate) ion

    • HPO_4^{2-} - Hydrogen phosphate ion

    • NO_2^- - Nitrite ion

    • H2PO4^- - Dihydrogen phosphate ion

    • NO_3^- - Nitrate ion

    • Cr2O7^{2-} - Dichromate ion

    • SO_3^{2-} - Sulfite ion

    • CH3CO2^- - Acetate ion

    • SO_4^{2-} - Sulfate ion

    • CN^- - Cyanide ion

The Octet Rule

  • The Octet Rule is a fundamental concept in chemistry that states that atoms tend to gain, lose, or share valence electrons to achieve a stable configuration of eight valence electrons.

  • This stability is analogous to the electron configuration of noble gases, which are inherently stable due to their complete valence shells.

Understanding Electron Arrangement

  • Knowing the arrangement of electrons around an atom's nucleus is crucial for determining the type of ion that is formed.

  • Electron dot structures are used to represent the valence electrons of an atom.

Group 8A Elements (Noble Gases)

  • Group 8A consists of inert (noble) gases.

    • Characteristics:

    • Inert: These elements are resistant to changes such as losing or gaining electrons.

    • The resistance to change is primarily due to their stable arrangement of valence electrons.

Electron Configuration and Octet Rule

  • Atoms will gain, lose, or share valence electrons to achieve 8 valence electrons, thereby attaining a stable electron configuration like that of noble gases.

Electron Dot Structures and Valence Electrons

  • Understanding electron configurations is essential to predict the behavior of nonmetal and metal atoms regarding ion formation.

  • For example:

    • Nonmetals:

    • Gain electrons to achieve a stable octet.

    • Example: Oxygen (O) with 6 valence electrons can gain 2 electrons to form the oxide ion O^{2-}.

    • Metals:

    • Lose valence electrons to reach stability.

    • Example: Magnesium (Mg) with 2 valence electrons can lose 2 electrons, forming the magnesium ion Mg^{2+}.

Representative Elements and Their Electrons

  • The number of valence electrons varies among groups in the periodic table:

    • Representative Elements:

    • Group 1A (Alkali Metals) - 1 valence electron

    • Group 2A (Alkaline Earth Metals) - 2 valence electrons

    • Group 3A - 3 valence electrons

    • … up to Noble Gases (Group 8A) with 8 valence electrons.

Predicting Ions from Elements

  • Example: What ion would you predict Barium (Ba) to form?

    • Options:

    • a. Ba^{2-}

    • b. Ba^+

    • c. Ba^{2+}

    • d. Cannot form an ion

Ionic Compounds

  • Pure substances, including elements and compounds, can consist of either one kind of atom or multiple elements chemically combined.

  • Compounds:

    • Defined as matter made of two or more chemically combined elements.

    • Each compound maintains consistent proportions of the constituent elements.

    • Example: Water (H_2O) consists of 2 hydrogen atoms and 1 oxygen atom.

Chemical Changes and Properties

  • When atoms in an element or compound undergo a chemical change, they can form new compounds.

    • Example: Iron can rust in the presence of oxygen in the air.

  • Chemical properties refer to the traits that describe how an element or compound can change or react chemically.

Nature of Ionic and Covalent Compounds

  • Depending on the nature of the elements involved in the reaction, either ionic or covalent compounds will form.

  • Mixtures are simply a combination of two or more pure substances without a chemical change taking place.

Note: All content has been adapted from © 2014 John Wiley & Sons, Inc. All rights reserved.