Ionic vs Covalent Bonding and Lewis Structures

Ionic vs Covalent Bonding and Nomenclature

• Valence electrons are in the outermost electron shell of an atom.
• These electrons participate in bonds and reactions with other atoms/molecules, being furthest from nucleus and least tightly held.

Lewis Structures

• Valence electrons of main-group elements are represented as dots around the element symbol in Lewis structures.
• Lewis structures depict main group elements and interactions with other atoms/molecules.
• To draw a Lewis dot symbol:
• Determine the number of valence electrons.
• Write the atomic symbol.
• Draw dots around the 4 sides of the symbol, with 1 dot representing 1 electron.
• Don't pair electrons until there are 5 or more valence electrons (except Helium).

Lewis Theory

• A chemical bond involves sharing/transferring electrons between atoms/molecules.
• Transferred electrons cause atoms to become ions, which are unbalanced with protons in the nucleus and electrons in outer shells, resulting in a charge (+ or -).
• Metals lose electrons to form cations.
• Nonmetals gain electrons to form anions.
• Cations and anions stick together, forming ionic bonds because opposite charges attract.
• Atoms will transfer electrons until each atom in the compound has 8 valence electrons (octet rule).
• Covalent bonds form when nonmetal atoms share electrons, resulting in covalent bonds.
• Atoms share electrons until each atom in the compound has 8 valence electrons in the outermost shell (octet rule).

Rules for Covalent Bonding Using Lewis Theory

• Count the total valence electrons of all atoms in the compound.
• Determine the central atom.
• Bond all other atoms to the central atom; 1 bond = 2 electrons.
• Fill in the valence electrons of terminal atoms.
• Any leftover electrons become lone pairs on the central atom.
• With a central atom lacking a full octet, create double/triple bonds using terminal O, N, or S atoms.
• Calculate the formal charge of each atom in the molecule.

Example: Water (H2O)

• Hydrogen has 1 valence electron, and oxygen has 6.
• Its skeleton structure is H-O-H
• Place remaining electrons: H : O : H (with two lone pairs on Oxygen).

Polar Covalent Bonds

• Form when atoms combine with differing electronegativity.
• Electronegativity is an atom's ability to attract shared electrons to itself.
• Elements vary in electronegativity, a characteristic varying in periods and organizing the periodic table.

Electronegativity Differences (ΔEN) and Bond Types

• If ΔEN is less than 0.5, the bond is pure (nonpolar) covalent, like N2.
• If ΔEN is between 0.5 and 2.0, the bond is polar covalent, like HF.
• If ΔEN is greater than 2.0, the bond is ionic, like NaCl.
• With ΔEN less than 0.5, electrons are equally shared, creating a pure or nonpolar covalent bond.
• With atoms of different electronegativity where ΔEN is 0.5-2.0, electrons aren't equally shared.
• If ΔEN is greater than 2.0, one atom steals electrons, which occurs between a metal and a nonmetal.

Intermolecular Forces and States of Matter

• Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another.
• They range from 5 kJ to 20 kJ per mole.
• These forces are weaker than ionic/covalent bonds and have an effect only when molecules are close.

Mole Concept Calculations

• The mole is used to work easily with subatomic particles.
• One mole represents 6.022x10^23 items (atoms, molecules, or formula units).
• Defining the mole in this way allows conversion from grams to moles or moles to particles, even though the particles are tiny.
• 1 mole of an element = 6.022x10^23 atoms = Molar / average atomic / mass in grams (found on the Periodic Table).

Balancing Equations

• A chemical reaction is a process where reactants convert into products.
• Substances are either chemical elements or compounds.
• A chemical reaction rearranges the constituent atoms of reactants to create different products.
• Evidence of a chemical reaction include:
• Unexpected color change
• Formation of a solid or gas
• Change of temperature
• Emission of light.
• A chemical change might not always be visible.
• Observation of a change doesn't prove a chemical change occurred: chemical analysis must provide conclusive evidence.
• Balancing chemical equations involves several steps

Mass and Moles Stoichiometry

• A balanced equation provides information like a recipe.
• It indicates reactant amounts and expected product yields.
• Stoichiometry helps quantify relationships, determining the amounts of products/reactants in a reaction.
• Describing these relationships is reaction stoichiometry.
• Mole-to-mole factors enable conversion between compound moles in a reaction.
• We can measure grams in the lab, but not moles.
• Determine mass of each substance in a chemical reaction using molar mass and mole-to-mole factors.
• The ratios in a balanced equation are used for determining quantities by weight, which is composition stoichiometry.