Difficulty: Introductory. No prerequisites beyond basic algebra and a periodic table.
Big picture: This material is the foundation for everything else in general chemistry. You need to measure things, report those measurements properly, and understand what atoms are made of before you can talk about how they bond or react. If you cannot identify significant figures or write an electron configuration, later topics (stoichiometry, bonding, molecular geometry) will not make sense. This is where the course begins and where exam marks are easiest to collect if you put the time in early.
General chemistry starts with how we measure and report physical quantities (metric prefixes, significant figures, accuracy vs precision) and what matter is made of at the atomic level (protons, neutrons, electrons, isotopes, electron configurations). Master the sig fig rules and the difference between valence and core electrons, and you have the scaffolding for every topic that follows.
Accuracy
How close a measurement is to the true (accepted) value. In simple terms, accuracy is about hitting the bullseye.
Precision
How close repeated measurements are to each other, regardless of whether they are near the true value. Think of it as: a tight cluster of darts, even if they are all off-centre.
Density
Mass per unit volume (mass / volume). A physical property used to identify substances or convert between mass and volume. In simple terms, it tells you how much stuff is packed into a given space.
Significant figures (sig figs)
The digits in a measurement that carry meaning about its precision. Rules: all non-zero digits count, sandwiched zeros count, trailing zeros after a decimal count, leading zeros do not. Think of it as: sig figs tell you how confident you are in your measurement.
Isotopes
Atoms of the same element (same number of protons) that differ in their number of neutrons, giving them different mass numbers. In simple terms, isotopes are siblings: same element, different weight.
Atomic number (Z)
The number of protons in the nucleus of an atom. It defines which element the atom is.
Mass number (A)
The total number of protons plus neutrons in the nucleus.
Valence electrons
The electrons in the outermost energy level of an atom. These are the electrons involved in bonding and chemical reactions. Think of it as: valence electrons are the sociable ones that interact with other atoms.
Core electrons
All the electrons that are not valence electrons. They sit in filled inner shells and do not participate in bonding.
Electron configuration
The arrangement of electrons in an atom's orbitals, written using subshell notation (e.g. 1s² 2s² 2p⁶). In simple terms, it is the address system for where each electron lives.
Prefix | Symbol | Value | Power of 10 |
|---|---|---|---|
Mega | M | 1,000,000 | 10⁶ |
Kilo | k | 1,000 | 10³ |
Deci | d | 0.1 | 10⁻¹ |
Centi | c | 0.01 | 10⁻² |
Milli | m | 0.001 | 10⁻³ |
Micro | μ | 0.000001 | 10⁻⁶ |
Nano | n | 0.000000001 | 10⁻⁹ |
Pico | p | 0.000000000001 | 10⁻¹² |
All non-zero digits are significant.
Sandwiched zeros (zeros between non-zero digits) are significant.
Trailing zeros to the right of the decimal point on numbers greater than one are significant.
Leading zeros (between the decimal point and the first non-zero digit) are not significant.
Zeros clarified in scientific notation are significant.
Rounding in calculations:
Addition and subtraction: the result must have the same number of decimal places as the measurement with the fewest decimal places.
Multiplication and division: the result must have the same number of significant figures as the measurement with the fewest sig figs.
Accuracy is closeness to the true value. Precision is the reproducibility of repeated measurements. You can be precise without being accurate (consistently wrong), and you can get one accurate result without being precise.
Range calculation: subtract the lowest value from the highest. To express as a percentage, divide the range by each extreme value separately and multiply by 100.
Particle | Charge | Location | Relative mass |
|---|---|---|---|
Proton | +1 | Nucleus | ~1 amu |
Neutron | 0 | Nucleus | ~1 amu |
Electron | -1 | Around nucleus | ~0 (negligible) |
Isotopes share the same atomic number (Z, number of protons) but differ in neutron count (N), which changes the mass number (A = Z + N).
Hydrogen isotopes as an example:
Protium: 1 proton, 0 neutrons (the common form)
Deuterium (heavy hydrogen): 1 proton, 1 neutron
Tritium (radioactive): 1 proton, 2 neutrons
Electron configurations describe which orbitals electrons occupy. Fill orbitals in order of increasing energy (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on).
Example: Br = [Ar] 4s² 3d¹⁰ 4p⁵. The valence electrons are those in the highest principal energy level: 4s and 4p (7 valence electrons). The 3d electrons are not valence electrons for main-group classification purposes.
Main-group elements: the number of valence electrons equals the number of electrons in the highest energy s and p sublevels. This is why the group number on the periodic table tells you the valence electron count for main-group elements.
Significant figures matter in any lab or engineering context where measurement uncertainty propagates through calculations. Reporting too many digits implies false precision. Isotopes are the basis of radiocarbon dating, medical imaging (PET scans use radioactive isotopes), and nuclear energy.
Density: d = m / V (mass divided by volume)
Mass number: A = Z + N (protons + neutrons)
Percentage range: ((highest - lowest) / value) × 100
Students often think leading zeros are significant. They are not. The number 0.0042 has only two significant figures (the 4 and the 2).
Precision and accuracy are frequently confused. A set of measurements can be highly precise (tightly clustered) yet completely inaccurate (far from the true value). They are independent qualities.
Many students assume that all electrons in a configuration are valence electrons. Only those in the highest principal energy level (the outermost s and p subshells for main-group elements) count as valence.
Isotopes are sometimes mistakenly thought to be different elements. They are the same element with different neutron counts.
⚠️ Sig fig rules come up on nearly every calculation problem. If you round incorrectly or report the wrong number of sig figs, you lose marks even when the maths is right.
⚠️ Expect a question asking you to distinguish accuracy from precision, often using a dart-board or target analogy.
⚠️ Electron configuration and identifying valence electrons is a recurring exam topic. Know the shorthand noble-gas notation and be able to count valence electrons for any main-group element.
⚠️ Isotope notation (mass number, atomic number) frequently appears in multiple-choice sections.
True or False: The number 0.00320 has four significant figures.
Fill in the blank: The SI prefix for 10⁻³ is ______.
True or False: Precision means the same thing as accuracy.
Fill in the blank: The atomic number (Z) tells you the number of ______ in an atom.
True or False: Deuterium and protium are different elements.
Answers: 1. False (3 sig figs). 2. Milli. 3. False. 4. Protons. 5. False (same element, different isotopes).
Q: How many significant figures are in the number 4,500?
A: Two (the 4 and the 5). The trailing zeros are ambiguous without a decimal point. Writing it as 4.500 × 10³ would clarify four sig figs.
Q: What is the electron configuration of bromine (Br, Z = 35)?
A: [Ar] 4s² 3d¹⁰ 4p⁵. The valence electrons are the seven in the 4s and 4p subshells.
Q: A student measures a density of 2.70 g/mL, 2.69 g/mL, and 2.71 g/mL. The accepted value is 2.50 g/mL. Are these measurements precise, accurate, or both?
A: Precise but not accurate. The measurements are tightly clustered (good precision) but consistently far from the true value of 2.50 g/mL (poor accuracy).
Q: Tritium has a mass number of 3. How many neutrons does it contain?
A: Two. Mass number (3) minus atomic number (1 for hydrogen) equals 2 neutrons.
Q: For a main-group element, how do you determine the number of valence electrons from the periodic table?
A: The group number gives you the valence electron count. For example, oxygen is in group 16 (or group 6A), so it has 6 valence electrons.
Valence electrons connect directly to chemical bonding (Document 2): the number and arrangement of valence electrons determine whether atoms form ionic or covalent bonds. Electron configuration also feeds into VSEPR theory for predicting molecular shapes. Significant figures carry through to every stoichiometry calculation in this course, so getting comfortable with them now saves headaches later.
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