Matter and Pure Substances: Elements, Compounds, and Mixtures

Matter in general

• The first lecture focuses on matter in general and what counts as matter in this context.
• The talk centers on pure substances first: elements, compounds, and mixtures.

Pure Substances

• Elements

  • Elements are found in the periodic table of elements.
  • In a chemistry lecture hall you typically have access to a periodic table; a personal copy is provided for exams.
  • An element in macroscopic form (a lump) is still an element in its pure form when isolated.
  • Elements are described using their symbols from the periodic table; e.g., something that contains only carbon can be described as C (carbon).
  • Periodic table symbols may be one or two letters (and sometimes three in old tables or placeholders). For example, element 112 was once shown as Uub in some representations before an official name was agreed.
  • The periodic table is an ongoing project; scientists can produce new artificial elements under extreme conditions, but these elements are often fleeting in time (fractions of a nanosecond).
  • The symbols for elements are internationally agreed, and names are permanent once agreed.

• Molecules, molecular substances, and compounds

  • Compounds contain at least two different elements that are chemically bonded together.
  • Chemical formulas express the ratio of the elements in a compound in the bulk, and this ratio is fixed for a given compound.
  • Molecular compounds: the chemical formula typically describes the makeup of one molecule. Example: water, H_2O, consists of two hydrogens and one oxygen per molecule.
  • Network (covalent network) compounds: the formula does not describe a single molecule; it describes the makeup of a repeating network unit. Example: silicon dioxide, SiO_2.
  • Ionic compounds: formulas describe the makeup of one charge-neutral unit composed of positive and negative ions; e.g., sodium chloride, NaCl.
  • It is useful to know whether a compound is molecular or ionic, because the formula’s meaning changes depending on that classification.
  • Ethanol as an example of a molecular compound: C2H6O, which contains two carbons per molecule along with hydrogens and oxygens.

• Distinctions and guidance

  • When you see a formula like the above, the likelihood is that it’s a molecular compound (unless noted otherwise).
  • In pure elements, you can have either discrete molecules or single atoms that are chemically bound in the bulk; e.g., nitrogen gas is diatomic, N_2.
  • Elements such as nitrogen in air exist as diatomic molecules (N≡N), not as single isolated atoms.
  • Sulfur can exist as discrete molecules that form rings (e.g., S8) and can be envisioned as a ring or soccer-ball-like arrangement.
  • The index in a formula defines: (i) the number of atoms in a molecule for molecular compounds, (ii) the number of each type of atom in a charge-neutral unit for ionic compounds, and (iii) the number of repeating units in a network formula like SiO2.
  • In everyday language, elements and compounds are distinguished, but the formulas reveal the composition and bonding characteristics.

• Examples and common formulas

  • Water: H_2O (two hydrogens, one oxygen per molecule)
  • Silicon dioxide: SiO_2 (network compound; unit-based description, not a single molecule)
  • Sodium chloride: NaCl (ionic compound; unit consists of one Na+ and one Cl– in the neutral unit)
  • Ethanol: C2H6O (molecular compound with a fixed molecular composition)
  • Nitrogen gas: N_2 (two nitrogen atoms per molecule)
  • Oxygen gas: O_2 (two oxygen atoms per molecule)
  • Carbon dioxide: CO_2 (two oxygens per carbon; typical molecular compound)
  • Calcium hydroxide: Ca(OH)_2 (a solid ionic compound; the formula describes the neutral unit with Ca2+ and two OH− groups)

• Pure elements and discrete molecular forms

  • Some elements occur as discrete molecules rather than isolated atoms; e.g., nitrogen forms N_2 in the air.
  • Some elements can form molecular clusters (e.g., sulfur forming S8 rings).
  • In the case of metals like gold (Au), atoms are present but are typically bound in the bulk metallic structure rather than as isolated atoms.

• Periodic table symbols and naming nuances

  • Element symbols can be one or two letters; newer elements have two-letter symbols, sometimes with controversy over naming.
  • In older representations, three-letter symbols or placeholders (e.g., Uub) have appeared; once an element’s name is agreed upon, a standard symbol is assigned.

Mixtures

• Definition

  • A mixture contains more than one component (i.e., two or more substances) that are not chemically bonded into one new substance.
  • A common everyday example: tap water, which is not pure H2O; it contains dissolved minerals and ions.

• Homogeneous vs. heterogeneous mixtures

  • Homogeneous mixture: uniform composition down to the atomic level; no observable regions with different compositions.
  • A homogeneous mixture does not spontaneously separate into its components over time (e.g., water kept in a sealed bottle for a long time).
  • Heterogeneous mixture: non-uniform distribution with distinct regions of differing composition that can be identified microscopically or macroscopically.
  • Example of a heterogeneous mixture: a slab of granite showing different colored grains; sampling different regions yields different compositions.

• Examples and exploration

  • Tap water: not pure water; contains calcium, magnesium, chloride, sulfate, and other dissolved species; the presence of these ions is often valued (e.g., minerals in drinking water).
  • Air as a mixture: not a fixed ratio of its components; composition can vary slightly from sample to sample and source to source.
  • Standard composition of air in this context (as a representative example): roughly 78\% nitrogen, 21\% oxygen, and about 1\% other gases (including CO2 and argon).
  • Oxygen content can vary due to processes like oxygen generation or purification of air; the ratio need not be fixed for every sample of air.
  • The concept of a fixed or variable ratio is a hallmark of mixtures, not compounds.

• Gas and liquid mixtures

  • Gas mixtures (like air) are typically homogeneous.
  • Liquid mixtures (e.g., dissolved substances in water) are also often homogeneous at the molecular level, but can be heterogeneous if phase separation occurs.
  • Dry ice, which is solid CO2, sublimates directly to a gas; when placed in water it forms a fog due to rapid condensation of water vapor and sublimation effects.
  • Safety note: do not swallow dry ice or inhale CO2 gas in enclosed spaces; CO2 can displace oxygen and cause asphyxiation.

• Alloys and homogeneous mixtures in everyday materials

  • White gold is not pure gold; it is an alloy, which is a type of homogeneous mixture at the atomic level.
  • In white gold, other metallic elements (e.g., palladium, nickel) are alloyed with gold to achieve a silvery appearance.
  • Alloys can produce different colors and properties (white gold, gray gold, rose gold) depending on the alloy composition.
  • The alloy is distributed homogeneously at the atomic level rather than existing as separate chunks of different metals.

• Practical and real-world relevance

  • The discussion connects to everyday substances (tap water, air, gold jewelry) to illustrate how matter can be classified as pure substances or mixtures.
  • The examples show how chemical formulas reflect composition and bonding, and how this differs between molecular, ionic, and network compounds.
  • The content emphasizes critical thinking about what constitutes a single substance versus a mixture in real-world contexts.

The Periodic Table and Element Symbols (Additional context from the lecture)

• The periodic table lists fundamental materials that constitute all matter.
• Some elements exist in forms that are not single atoms (e.g., diatomic or polyatomic molecules) in nature.
• The periodic table arrangement reflects underlying periodicity and bonding trends, which helps explain properties and behaviors of elements and compounds.
• The periodic table is an evolving catalog as new artificial elements are produced under extreme conditions, often with extremely short lifetimes.
• The symbols for elements are internationally standardized; naming often reflects historical discovery and nomenclature conventions.
• The speaker notes that the periodic table is an ongoing project and hints at the practical importance of having a current reference during exams.

Summary of Key Formulas and Numbers (recap)

• Molecular formulas and examples:
  • Water: H_2O
  • Ethanol: C2H6O
  • Nitrogen gas: N_2
  • Oxygen gas: O_2
  • Silicon dioxide: SiO_2 (network compound)
  • Sodium chloride: NaCl (ionic compound)
  • Calcium hydroxide: Ca(OH)_2
  • Carbon dioxide: CO_2
• Common air composition (representative): 78\%\,N2, 21\%\,O2, 1\%\,others (including CO2, Ar)
• Percentages printed with LaTeX-friendly notation: 78\%, 21\%, 1\%

Connections to broader themes

• Distinguishing element, compound, and mixture helps in predicting properties, reactivity, and separation techniques.
• Understanding when a formula refers to a molecule versus a unit in a crystal lattice or ionic network is crucial for interpreting chemical behavior.
• Real-world relevance includes safety (dry ice), material design (white gold alloys), and environmental considerations (air composition and purification).