Atoms, Elements, and Chemical Compounds: Foundations of General Chemistry

Atoms and Experimental Evidence

What are atoms and what experimental evidence exists for them?

An atom is the smallest identifiable unit of an element, retaining the chemical properties of that element. The existence and properties of atoms are supported by several key experimental laws:

• Law of Conservation of Mass: The total mass remains constant during a chemical reaction. $\text{Total mass before reaction} = \text{Total mass after reaction}$

• Law of Definite Proportions: Elements combine in fixed, unique mass ratios to form compounds.

These laws provide strong evidence for the existence of atoms and their role in chemical reactions.

History and Models of the Atom

How has the model of the atom changed over time?

• Indivisible Atoms: Early models considered atoms as solid, indivisible spheres unique to each element.

• "Plum Pudding" Model: Proposed after the discovery of the electron (1897), this model depicted electrons embedded in a positively charged substance, like raisins in pudding.

• Nuclear Model: The Gold Foil Experiment (1911) revealed a small, dense, positively charged nucleus surrounded by electrons, with most of the atom being empty space.

Each model was developed in response to new experimental evidence, refining our understanding of atomic structure.

Subatomic Particles

What are the properties of protons, neutrons, and electrons?

• Proton: Located in the nucleus, mass ≈ 1.00727 amu, charge +1, symbol p+

• Neutron: Located in the nucleus, mass ≈ 1.00866 amu, charge 0, symbol n0

• Electron: Located in the atomic volume (outside nucleus), mass ≈ 0.000549 amu, charge -1, symbol e-

Elementary charge: $e = 1.602 \times 10^{-19} \ \text{C}$

Atomic Number, Mass Number, and Isotopes

What are the atomic and mass number?

• Atomic Number (Z): Number of protons in the nucleus.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

What is an isotope?

• Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).

How do I write a nuclide symbol?

Nuclide symbols are written as:

$^{A}_{Z}\text{X}$

• X = element symbol

• A = mass number

• Z = atomic number

Atomic Mass and Its Calculation

What is the atomic mass and how is it calculated?

Atomic mass is the weighted average mass of all naturally occurring isotopes of an element, based on their relative abundances.

Formula:

$\text{Atomic Mass} = \sum \left( \frac{\%\ \text{Abundance}}{100\%} \times \text{Mass of Isotope} \right)$

Example: For neon isotopes:

Isotope	Mass (amu)	% Abundance
Ne-20	19.992435	90.48%
Ne-21	20.993843	0.27%
Ne-22	21.991382	9.25%

Calculation:

$\text{Atomic Mass} = (0.9048 \times 19.992435) + (0.0027 \times 20.993843) + (0.0925 \times 21.991382) = 20.18$ amu

The Periodic Table and Classification of Elements

How are elements in the Periodic Table arranged?

• Elements are arranged by increasing atomic number (Z).

• Rows are called periods; columns are called groups or families.

• Elements in the same group have similar chemical properties.

What are metals, metalloids, and nonmetals?

Metals	Metalloids	Nonmetals
Lustrous, easy to shape Mostly solids Good conductors of heat and electricity	Properties intermediate between metals and nonmetals Exist as solids Can sometimes conduct heat and electricity	Brittle and dull as solids Exist as solids, liquids, or gases Poor conductors of heat and electricity

What are some key groups of elements in the Periodic Table?

• Alkali Metals (Group 1)

• Alkaline Earth Metals (Group 2)

• Transition Metals (Groups 3-12)

• Halogens (Group 17)

• Noble Gases (Group 18)

• Lanthanides and Actinides (f-block)

Ions and Ionic Compounds

What are ions?

• Ions: Atoms or groups of atoms with an unequal number of protons and electrons.

• Cations: Positively charged ions (fewer electrons than protons).

• Anions: Negatively charged ions (more electrons than protons).

What are some common monatomic ions?

• Monatomic ions: Single atoms that carry a charge (e.g., Na+, Cl-).

What are some common polyatomic ions?

• Polyatomic ions: Groups of atoms covalently bonded that carry a net charge (e.g., SO42-, NH4+).

+1 Charge	-1 Charge	-2 Charge	-3 Charge
Ammonium (NH4+)	Hydroxide (OH-) Nitrate (NO3-) Acetate (C2H3O2-) Chlorate (ClO3-) Perchlorate (ClO4-) Cyanide (CN-) Bicarbonate (HCO3-)	Carbonate (CO32-) Sulfate (SO42-) Sulfite (SO32-) Oxalate (C2O42-) Thiosulfate (S2O32-)	Phosphate (PO43-)

Chemical Formulas and Compounds

What is the basic structure of a chemical formula?

• A chemical formula represents a compound by listing all constituent elements and their relative proportions using subscripts.

• Elements are listed in order of decreasing metallic character (metals first).

• Example: CO2 (2 oxygen atoms per carbon atom)

What is the difference between ionic and molecular compounds?

• Ionic Compounds: Composed of cations and anions held together by electrostatic forces; smallest unit is the formula unit (e.g., NaCl).

• Molecular Compounds: Composed of groups of atoms held together by covalent bonds; smallest unit is the molecule (e.g., H2O).

Naming and Writing Formulas for Compounds

How do I write chemical formulas for ionic compounds?

• The formula unit is electrically neutral, with cations and anions in the simplest whole-number ratio.

• Superscripts (charges) are used to determine subscripts (number of ions needed for neutrality).

• Example: Mg2+ and Cl- combine to form MgCl2.

How do we transition between the name and chemical formula of an ionic compound?

• Name the cation first, then the anion.

• For transition metals, indicate the charge with Roman numerals.

• Polyatomic ions retain their names in compounds.

How do we transition between the name and chemical formula of a binary molecular compound?

• Use Greek prefixes to indicate the number of each atom (mono-, di-, tri-, etc.).

• The first element is named first; the second element ends in "-ide." Prefix "mono-" is omitted for the first element.

• Example: CO = carbon monoxide, CO2 = carbon dioxide

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5
hexa-	6
hepta-	7
octa-	8
nona-	9
deca-	10

Acids: Structure and Nomenclature

What is an acid?

• Acids are molecular compounds that produce H+ ions when dissolved in water.

How do we transition between the name and chemical formula of an acid?

• Binary acids: Contain hydrogen and one other nonmetal. Named as "hydro-" + base name + "-ic acid" (e.g., HCl = hydrochloric acid).

• Oxyacids: Contain hydrogen, oxygen, and another element (usually a polyatomic ion). Naming depends on the polyatomic ion:

  • If the ion ends in "-ate": acid name ends in "-ic acid" (e.g., H2SO4 = sulfuric acid).

  • If the ion ends in "-ite": acid name ends in "-ous acid" (e.g., H2SO3 = sulfurous acid).

Example: HNO3 (nitrate ion) → nitric acid; HNO2 (nitrite ion) → nitrous acid.

Summary Table: Properties of Subatomic Particles

	Proton	Neutron	Electron
Location	nucleus	nucleus	atomic volume
Mass (amu)	1.00727	1.00866	0.000549
Charge (xe)	+1	0	-1
Symbol	p+	n0	e-

Additional info: The notes provide a comprehensive overview of foundational atomic theory, periodic trends, and chemical nomenclature, suitable for introductory General Chemistry courses.