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Ions, Ionic and Covalent Compounds: GOB Chemistry Study Notes

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Ions

Elements in Nature

Elements are the fundamental building blocks of matter, and only a few are found in their pure form in nature. Most elements exist as compounds due to their chemical reactivity.

  • Noble gases are rarely found in compounds due to their stable electron configurations.

  • Many elements combine to form compounds, which are more stable than the pure elements.

  • Chemical changes occur when atoms rearrange their electrons to form new substances.

  • Subatomic particles (protons, neutrons, electrons) contribute to the properties of elements.

Stable Electron Arrangements/Configurations

Atoms seek stability by achieving a full valence shell, often resembling the electron configuration of noble gases.

  • Noble gases have stable electron arrangements (8 valence electrons, except for Helium with 2).

  • Atoms of different elements will lose, gain, or share electrons to achieve stability.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom and determine chemical reactivity.

  • Elements in the same group have the same number of valence electrons.

  • Examples:

    • Alkali metals: 1 valence electron

    • Alkaline earth metals: 2 valence electrons

    • Halogens: 7 valence electrons

    • Noble gases: 8 valence electrons (2 for Helium)

Octet Rule

The octet rule states that atoms tend to lose, gain, or share electrons to achieve a stable set of eight valence electrons.

  • Atoms with a full valence shell are generally more stable.

  • Exceptions include hydrogen and helium, which are stable with two electrons.

Loss and Gain of Electrons

Atoms can lose or gain electrons to form ions, which are charged particles.

  • Loss of electrons: Forms positive ions called cations (e.g., Na+, Mg2+).

  • Gain of electrons: Forms negative ions called anions (e.g., Cl-).

Ion Formation & Electron Arrangement

Ion formation involves the transfer of electrons to achieve a stable electron configuration.

Element

Symbol

Protons

Electrons

Ion Formed

Sodium

Na

11

10

Na+

Magnesium

Mg

12

10

Mg2+

Chlorine

Cl

17

18

Cl-

Ionic Charges from Group Numbers

The charge of ions formed by representative elements can be predicted from their group number.

Group

Common Ion Charge

1A

+1

2A

+2

3A

+3

5A

-3

6A

-2

7A

-1

Ions with Variable Charge

Some metals can form more than one type of cation with different charges, known as variable charge ions.

  • Roman numerals are used to indicate the charge (e.g., Nickel(II) ion: Ni2+).

  • Transition metals commonly exhibit variable charges.

Polyatomic Ions

Polyatomic ions are groups of atoms covalently bonded together that carry an overall charge.

  • Examples include NO3- (nitrate), SO42- (sulfate).

Compounds

Types of Bonds

Atoms combine through chemical bonds, which can be classified as ionic or covalent.

  • Ionic bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions.

  • Covalent bonds: Formed by the sharing of electrons between atoms.

Ionic Compounds

Ionic compounds consist of positive and negative ions held together by electrostatic forces.

  • Names are based on the ions present.

  • Formulas are written to balance the total positive and negative charges.

Ionic Compound Naming

Ionic compound names indicate the ions present but not the number of each ion.

  • Example: Calcium phosphate

Ionic Compound Formula

Formulas for ionic compounds are written to ensure the total charge is zero.

  • Example:

Practice: Writing Formulas

Compound Name

Formula

Iron(III) chloride

FeCl3

Calcium nitride

Ca3N2

Zinc sulfate

ZnSO4

Chromium(VI) phosphide

CrP2

Cesium telluride

Cs2Te

Cobalt(III) nitrite

Co(NO2)3

Covalent Bond-Sharing Electrons

Covalent bonds involve the sharing of electrons between nonmetal atoms to achieve stability.

  • Molecules such as H2, N2, O2, F2, Cl2, Br2, I2 are examples of diatomic molecules.

  • Each covalent bond represents a shared pair of electrons.

Polarity of Covalent Bonds

The polarity of a covalent bond depends on the difference in electronegativity between the bonded atoms.

  • Nonpolar covalent bond: Electrons are shared equally.

  • Polar covalent bond: Electrons are shared unequally, creating partial charges.

  • Examples: HCl is polar, Cl2 is nonpolar.

Covalent Compound Names and Formulas

Covalent compounds are named using prefixes to indicate the number of atoms of each element.

Prefix

Number

Mono-

1

Di-

2

Tri-

3

Tetra-

4

Penta-

5

Hexa-

6

Hepta-

7

Octa-

8

Nona-

9

Deca-

10

  • Example: is sulfur trioxide

  • Example: is dinitrogen tetroxide

Polarity of Molecules

Molecules can be polar or nonpolar depending on their shape and the distribution of charge.

  • Molecules with one polar bond are polar.

  • Molecules with symmetrical charge distribution are nonpolar.

  • Example: H2O is polar, CO2 is nonpolar.

Summary of Naming Compounds

Naming compounds requires understanding the type of bonding and the rules for naming ionic and covalent compounds.

  • Ionic compounds: Name the cation first, then the anion.

  • Covalent compounds: Use prefixes to indicate the number of atoms.

  • Polyatomic ions: Use the name of the ion as is.

Chapter Summary

Upon completion of this material, students should be able to:

  • Understand the formation and properties of ions and ionic compounds.

  • Explain electron arrangements and the octet rule.

  • Predict names and formulas for ionic and covalent compounds.

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