Chapter 3 Study Guide: Compounds, Electron Arrangements, and the Mole

Compounds and Electron Arrangements

Electron Arrangements and Energy Levels

Atoms are composed of electrons, protons, and neutrons. Electrons occupy specific energy levels around the nucleus, and their arrangement determines the chemical properties of the atom.

• Electrons are negatively charged particles in constant motion, possessing energy.

• Lower energy levels are more stable than higher energy levels.

• Electrons can only occupy certain energy levels (quantized), not energies in between.

• The maximum number of electrons in a given energy level is calculated by the formula $2n^2$, where n is the energy level.

• Atoms rearrange their electrons to increase stability.

Energy Level Diagram

• Each step represents a principal energy level (n=1, n=2, etc.).

• Electrons fill lower energy levels first.

Valence Shells and Valence Electrons

The highest energy level containing electrons is called the valence shell. Electrons in this shell are known as valence electrons and are responsible for chemical bonding.

• Valence electrons are farthest from the nucleus.

• They participate in the formation of compounds.

Valence Electrons and the Periodic Table

The periodic table helps determine the number of valence electrons for main-group elements (Groups 1A–8A).

• The group (column) number indicates the number of valence electrons.

• The period (row) indicates the outermost energy level containing electrons.

The Octet Rule

Atoms are most stable when they have eight valence electrons, known as the octet rule. Noble gases naturally have a full valence shell and are chemically inert.

• Atoms achieve an octet by:

  • Losing electrons to form cations (positive ions)

  • Gaining electrons to form anions (negative ions)

  • Sharing electrons to form covalent bonds

Periodic Trends

Atomic Radius

The atomic radius is the distance from the nucleus to the outermost electron shell. It varies across the periodic table due to changes in energy levels and nuclear charge.

• Down a group: atoms gain energy levels, increasing atomic radius.

• Across a period (left to right): atoms have more protons, pulling electrons closer and decreasing atomic radius.

• Larger atoms lose electrons more easily.

Electronegativity

Electronegativity is the ability of an atom to attract shared electrons in a chemical bond.

• Across a period (right): increases due to more protons.

• Up a group: increases as valence electrons are closer to the nucleus.

Ionization Energy

Ionization energy is the energy required to remove an electron from an atom.

• Across a period (right): increases due to stronger nuclear attraction.

• Up a group: increases as electrons are closer to the nucleus.

• Low ionization energy means easier formation of positive ions.

Chemical Bonds and Compounds

Types of Chemical Bonds

Chemical bonds are attractions between charged particles (electrons and protons) in atoms. They are classified as:

• Ionic bonds: Transfer of electrons between metals and nonmetals.

• Covalent bonds: Sharing of electrons between nonmetals.

• Metallic bonds: Delocalized electrons among metal atoms.

Ionic Bonds

Ionic bonds occur when a metal atom transfers electrons to a nonmetal atom, forming oppositely charged ions that attract each other.

• Metal becomes a cation (positive ion).

• Nonmetal becomes an anion (negative ion).

• Example: Sodium (Na) transfers an electron to chlorine (Cl), forming Na+ and Cl-.

Common Ionic Charges by Group

Group	Valence Electrons	Most Likely Charge	Example
1A	1	+1	Na
2A	2	+2	Mg
3A	3	+3	Al
5A	5	-3	N
6A	6	-2	O
7A	7	-1	Cl
8A	8	0	-

Covalent Bonds

Covalent bonds form between nonmetal atoms by sharing electrons, allowing each atom to achieve a full valence shell (octet).

• Atoms share electrons to achieve stability.

• Example: Water (H2O) forms by sharing electrons between hydrogen and oxygen.

Molecular Shapes

Three-Dimensional Arrangement of Atoms

The shape of a molecule is determined by the arrangement of bonds around the central atom. Atoms arrange themselves so that bonds are as far apart as possible.

• Linear: Atoms arranged in a straight line (e.g., CO2).

• Bent: Atoms form an angle (e.g., H2O).

• Tetrahedral: Four bonds arranged symmetrically (e.g., CH4).

Molecular Weight and the Mole

Molecular Weight

The molecular weight of a compound is the sum of the atomic masses of its constituent atoms.

• Formula: $\text{Molecular Weight (g/mol)} = \sum (\text{Atomic Mass} \times \text{Number of Atoms})$

• Example: CO2 has 1 carbon (12.01 g/mol) and 2 oxygen (16.00 g/mol each), so its molecular weight is 44.01 g/mol.

The Mole and Avogadro's Number

A mole is a counting unit in chemistry, representing 6.022 × 1023 particles (Avogadro's number).

• 1 mole of any substance contains 6.022 × 1023 representative particles (atoms, molecules, or formula units).

• Used to relate mass, number of particles, and volume in chemical calculations.

Mole Conversions

Converting between moles and number of particles uses Avogadro's number as a conversion factor.

• 1 mol atoms = 6.022 × 1023 atoms

• Example: To find the number of atoms in 2.45 mol of copper, multiply 2.45 by 6.022 × 1023.

Representative Particles Table

Substance	Representative Particle
He	atoms
Cu	atoms
Fe	atoms
O2	molecules
H2O	molecules
CO2	molecules
NaCl	formula units

Calculating Moles and Mass

To calculate the mass of a substance from moles, use the molecular weight as a conversion factor.

• Formula: $\text{Mass (g)} = \text{Moles} \times \text{Molecular Weight (g/mol)}$

• Example: 1 mol of glucose (C6H12O6) has a molecular weight of 180.16 g/mol.

• To find the number of moles in 100 g of glucose: $\text{Moles} = \frac{100\ \text{g}}{180.16\ \text{g/mol}}$

Summary Table: Key Concepts

Concept	Definition	Example
Valence Electrons	Electrons in the outermost shell	Group 1A has 1 valence electron
Octet Rule	Atoms are stable with 8 valence electrons	Noble gases
Ionic Bond	Transfer of electrons between metal and nonmetal	NaCl
Covalent Bond	Sharing of electrons between nonmetals	H2O
Mole	6.022 × 1023 particles	1 mol of O2 = 6.022 × 1023 molecules

Additional info: Academic context was added to clarify periodic trends, molecular shapes, and mole calculations for completeness.