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Chemistry Notes: Isotopes, Ionic notation, and Binary Molecular Nomenclature

Percent Abundance and System of Equations

  • Let x represent the percent abundance of the first isotope and y represent the percent abundance of the second isotope. Since these are the only two isotopes considered, their total must add up to 100%: x + y = 100\%.
  • It’s acceptable to express the abundances either as fractions or percentages, as long as they sum to 100%.
  • Maximum combination of abundances: since there are only two contributors, the total cannot exceed 100% (i.e., the sum is bounded by 100%).
  • When solving a system with a second equation, you can rearrange that second equation to solve for one variable (either x or y). It does not matter which variable you solve for first. For example, if the second equation is of the form A - x = B, you can solve for x directly; similarly you could solve for y if it’s written as a function of y. The transcript notes: “I can rearrange that second equation to solve for one of the variables. It doesn’t matter which.”
  • The process will involve a bit of algebra and some number crunching, but is manageable.
  • When performing these calculations, it’s common to specify the desired precision in significant figures (sig figs). The speaker asks, “Do we agree? Is that the numbers you got?”
  • In practice, there’s often a tolerance or acceptable range. If you’re close to the answer but off by a decimal place, that’s usually considered close enough for many problems.
  • The speaker mentions gamma/o five and looks ahead to lattice-like structures that will be seen in future material. In this context, lattice-like structures often refer to extended ionic or covalent networks rather than discrete molecules.
  • A repeating pattern is described as positively charged, then the charge symbol (positive and negative charges). The number precedes the charge symbol (e.g., 2+, 3−). The speaker notes that there can be two ways to represent charge notation and that some instructors (e.g., Alex) may require a specific form.
  • Example given in passing: there is a line stating “36 electrons and I’m told the charge of one minus,” illustrating how electron count and charge information feed into determining molecular formulas or ions.
  • The idea of deriving empirical formulas from a larger molecular formula is illustrated: a common multiple of six can be reduced to CH(2)O, i.e., the empirical formula CH(2)O. The speaker then mentions hexamine with the formula C(6)H({12})N(_4):
    • Empirical reduction example: ext{From a compound with a multiple of six, the empirical formula can be CH}_{2} ext{O}.
    • Hexamine has the molecular formula ext{C}6 ext{H}{12} ext{N}_4.
    • These examples illustrate how empirical formulas relate to molecular formulas and how reductions can reveal simplest whole-number ratios.

Charge Notation and Ionic Species

  • The transcript describes a repeating pattern of charge notation: first the numerical value, then the symbol for the charge (e.g., 2+, 3−). There is a preference for the form where the charge is written after the numeral.
  • The speaker notes that there are two common notations for writing charge and that he will usually accept either. Some instructors may be more specific about which form is expected for input.
  • The example discussed includes a case with 36 electrons and a −1 charge, illustrating how electron count and charge sign are used to determine the species.
  • The line: “I know the difference between those. Alex may be a little bit more specific on what it's looking for in terms of when you enter in charge” highlights that input conventions can vary by instructor.
  • Practical takeaway: when writing or interpreting ionic charges, be aware of both representations and follow the preferred convention for your course or assignment.

Nomenclature of Binary Molecular Compounds

  • Focus: binary molecular compounds are composed of two nonmetal elements (nonmetal–nonmetal) and have no overall ionic charge.
  • Naming rules for binary molecular (nonmetal–nonmetal) compounds:
    • The first element listed is named first and kept with its usual name.
    • The second element’s name is changed to end with the suffix -ide.
    • Prefixes are used to indicate the number of atoms of each element (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-). When the prefix ends in o or a and the second element begins with a vowel, the final vowel of the prefix is dropped to avoid awkward pronunciation (e.g., mono- with a vowel-starting second element loses the final o/a).
    • The example provided: for P(4)O({10}), the name is tetraphosphorus decaoxide:
    • First element: phosphorus (P), with the prefix tetra- indicating four atoms: tetr(a) or tetra-.
    • Second element: oxygen, with the suffix -ide and the prefix deca- indicating ten atoms: decaoxide.
    • Result: tetraphosphorus decaoxide.
  • The transcript also emphasizes that for the binary molecular naming rules, the first element’s name is left as-is and the second element’s name is modified to end with -ide; prefixes are used for both elements.

Special Considerations: Transition Metals and Oxidation States

  • A key complication in naming and formula assignment arises with transition metals because many have multiple common oxidation states (charges).
    • Example: chromium can be +3 or +6 (Cr3+ vs Cr6+ in various compounds).
    • Manganese may show +2, +4, +5, +7 among others (e.g., Mn2+, Mn4+, Mn5+, MnO7+ in some contexts).
  • This variability makes it harder to determine a unique ionic formula without additional information (stoichiometry, charges balancing, or oxidation-state indicators).
  • Practical consequence: in systematic naming for transition metal compounds, numerals in parentheses are used to denote the oxidation state (e.g., chromium(III) oxide, Cr(2)O(3)), to avoid ambiguity.
  • The transcript ends with an acknowledgment of this issue: “Oh, you’re right. I didn’t [address that]” indicating a correction or clarification was expected about how to handle transition metals in nomenclature.

Practical Tips: Sig Figs, Rounding, and Acceptable Notations

  • When performing abundance calculations or any quantitative chemical problem, it’s important to agree on significant figures (sig figs) in advance.
  • The instructor suggests that there’s often a range of acceptable values, and being off by a decimal place may still be close enough in many contexts.
  • Be mindful of notation conventions for charges and how instructors expect inputs to be formatted (e.g., 2+ vs +2, or whether to use subscripts or parentheses for oxidation states).
  • If you encounter ambiguous or variable conventions, follow the course-specific guidelines or ask for clarification from the instructor.

Connections, Examples, and Real-World Relevance

  • The concept of percent abundance and isotope balancing connects to mass spectrometry, isotopic labeling, and calculating average atomic masses used in periodic tables and molecular mass determinations.
  • Understanding lattice-like structures and ionic charge patterns underpins solid-state chemistry, crystal lattices, and ionic compound formation.
  • Binary molecular nomenclature is foundational for communicating about covalent compounds (e.g., CO as carbon monoxide, CO(_2) as carbon dioxide) and is widely used in chemistry, pharmacology, environmental science, and materials science.
  • Recognizing the limitations of naming when transition metals are involved highlights the need for standardized nomenclature (e.g., Stock system with Roman numerals, classical names, or newer IUPAC conventions) to ensure precise understanding across disciplines.

Key Formulas and Notation (LaTeX)

  • Isotope abundance relation: x + y = 100\%
  • Empirical formula reduction example: \mathrm{CH_2O}
  • Hexamine formula: \mathrm{C6H{12}N_4}
  • Binary molecular compound example: \mathrm{P4O{10}} \rightarrow \text{tetraphosphorus decaoxide}
  • Binary molecular compound general naming rules summary:
    • First element name unchanged
    • Second element name ends with -ide
    • Use prefixes for the number of atoms (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-)
    • Drop final vowel of prefix if second element starts with a vowel (to avoid awkward pronunciation)

Quick Reference Examples

  • P4O10: tetraphosphorus decaoxide
  • CH_2O: empirical formula for some carbohydrate-like empirical units
  • C6H12N4: hexamine (molecule with six carbons, twelve hydrogens, and four nitrogens)
  • MgO: magnesium oxide (note: ionic compound; naming conventions differ from binary molecular rules)
  • For transition metal oxides, use oxidation-state indicators when needed (e.g., chromium(III) oxide, Cr2O3) to avoid ambiguity.