Chap 2Atoms and Molecules: Structure, Properties, and Calculations

Atoms and Molecules

Course Objectives

This section introduces the foundational concepts of atomic structure and chemical formulas, essential for understanding GOB Chemistry. Students should be able to:

• Use symbols for chemical elements to write formulas for chemical compounds.

• Describe the characteristics of protons, neutrons, and electrons.

• Apply the concepts of atomic number and mass number to determine the number of subatomic particles, identify isotopes, and write correct symbols for isotopes.

• Use atomic weights of the elements to calculate molecular weights of compounds.

• Utilize the mole concept to relate number of moles, grams, and number of atoms for elements and compounds, and perform factor-unit calculations.

Skills for Elements and Molecules

• For a given element, be able to:

  1. Give the mass number rounded to the nearest whole number.

  2. Give the atomic number.

  3. Tell how many protons, neutrons, and electrons are present.

  4. Identify if the atom is an isotope or ion.

  5. Give its appropriate classification (e.g., metal, nonmetal, metalloid).

• For a given molecule, be able to:

  1. Identify the number of each atom present.

  2. Calculate the molecular weight.

Atomic Structure

Atoms and Elements

Atoms are the basic building blocks of matter. Elements consist of only one type of atom and are organized in the periodic table.

• Atoms are composed of subatomic particles:

  • Protons (p+): Positively charged, found in the nucleus, mass = 1 amu.

  • Neutrons (n): No charge (neutral), found in the nucleus, mass = 1 amu.

  • Electrons (e-): Negatively charged, found outside the nucleus in shells, mass ≈ 0 amu.

Example: Carbon Atom

• 6 protons, 6 neutrons, 6 electrons.

• Electrons are arranged in shells around the nucleus.

Periodic Table of the Elements

The periodic table organizes all known elements by their chemical properties (groups/columns) and energy levels (periods/rows).

• Groups (columns): Elements with similar chemical properties.

• Periods (rows): Elements with the same number of electron shells.

• Classification of groups:

  • Alkali Metals

  • Alkaline Earth Metals

  • Transition Metals

  • Metalloids (Semimetals)

  • Nonmetals

  • Noble Gases

  • Rare Earth Metals

Atomic Number, Mass Number, and Isotopes

Atomic Number (Z)

The atomic number is the number of protons in the nucleus of an atom and determines the element's identity.

• If the number of protons changes, the element changes.

• Atomic number is always a whole number.

• In a neutral atom, number of protons = number of electrons.

Example

• Li: 3 protons, 3 electrons (neutral atom)

Mass Number (A)

The mass number is the sum of the number of protons and neutrons in the nucleus.

• Mass number = number of protons + number of neutrons

• Used to distinguish between isotopes of the same element.

Ions

An ion is an atom that has lost or gained electrons, resulting in a net positive or negative charge.

• Charge is written as a superscript.

• Example:

  • Li+: 3 protons, 2 electrons

  • S2-: 16 protons, 18 electrons

Isotopes

Isotopes are atoms of the same element (same number of protons) but different numbers of neutrons.

• Isotope notation: mass numberSymbol (e.g., 13C)

• Atomic mass is the weighted average of the mass numbers of naturally occurring isotopes.

Atomic and Molecular Mass Calculations

Atomic Mass and Formula Weight

The atomic mass (in amu) is used to calculate the formula weight of compounds.

• Formula Weight = Sum of (number of each atom × atomic weight)

Examples

• H2O: 2 × 1.0 amu (H) + 16.0 amu (O) = 18.0 amu

• NaCl: 23.0 amu (Na) + 35.5 amu (Cl) = 58.5 amu

• NiCl2·6H2O: 58.7 amu (Ni) + 2 × 35.5 amu (Cl) + 12 × 1.0 amu (H) + 6 × 16.0 amu (O) = 237.7 amu

• C9H8O4: 9 × 12.0 amu (C) + 8 × 1.0 amu (H) + 4 × 16.0 amu (O) = 180.0 amu

The Mole Concept

The mole is a counting unit used in chemistry to relate mass, number of particles, and volume.

• 1 mole = $6.022 \times 10^{23}$ particles (Avogadro's Number)

• Molar mass (g/mol) is numerically equal to atomic/molecular mass (amu).

• Example:

  • 12.011 g C = 1 mol C = $6.022 \times 10^{23}$ atoms of C

Conversions Using the Mole

Conversion factors allow calculation between mass, moles, and number of particles.

• Mass (g) → Moles → Number of atoms/molecules

• Example: How many atoms of C are in 4.0 g of C?

  • Step 1: Calculate moles: $\text{moles C} = \frac{4.0\ g}{12.011\ g/mol}$

  • Step 2: Calculate atoms: $\text{atoms C} = \text{moles C} \times 6.022 \times 10^{23}$

Molecular Formulas and Molar Mass

Molecular Formulas

Molecules contain more than one atom. The number of atoms of each element is indicated by subscripts in the formula.

• Examples:

  • O2: 2 atoms of oxygen

  • H2O: 2 atoms of hydrogen, 1 atom of oxygen

  • C2H4O2 (Acetic Acid): 2 atoms of carbon, 4 atoms of hydrogen, 2 atoms of oxygen

Molar Mass of Compounds

The molar mass of a compound is the sum of the molar masses of its constituent atoms.

• Calculate by multiplying the number of each atom by its molar mass and summing.

• Example:

  • H2O: $2 \times 1.0\ g + 1 \times 16.0\ g = 18.0\ g/mol$

  • C2H4O2: $2 \times 12.0\ g + 4 \times 1.0\ g + 2 \times 16.0\ g = 60.0\ g/mol$

Sample Calculation

• How many moles of H are in 10.0 g of C2H4O2?

  1. Calculate moles of C2H4O2: $\text{moles} = \frac{10.0\ g}{60.0\ g/mol}$

  2. Multiply by number of H atoms per molecule: $\text{moles H} = \text{moles C}_2\text{H}_4\text{O}_2 \times 4$

Summary Table: Subatomic Particles

The following table summarizes the properties of subatomic particles:

Particle	Symbol	Charge	Location	Approximate Mass (amu)
Proton	p+	+1	Nucleus	1
Neutron	n	0	Nucleus	1
Electron	e-	-1	Electron shells	0

Summary Table: Periodic Table Classifications

Group	Examples	Properties
Alkali Metals	Li, Na, K	Highly reactive, 1 valence electron
Alkaline Earth Metals	Mg, Ca	Reactive, 2 valence electrons
Transition Metals	Fe, Cu, Ni	Variable oxidation states, colored compounds
Metalloids	B, Si	Semi-conductive, intermediate properties
Nonmetals	C, O, N	Poor conductors, various states
Noble Gases	He, Ne, Ar	Inert, full valence shell

Additional info: Some content was inferred and expanded for clarity and completeness, including sample calculations and classification tables.