Unit 2: Atoms, Molecules, and Ions
How the atomic model was pieced together from clues nobody could see directly, and the rules for naming what atoms build once they combine.
11 topics · 41 questions
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2.1 Dalton's Atomic Theory and Its Limits
- How it worksHow does the law of definite proportions force every sample of a compound to have the same fixed recipe?
- How it worksWalk me through how the law of multiple proportions revealed that atoms combine in whole-number ratios.
- Common mistakeWhy isn't Dalton's claim that atoms are indivisible still considered true today?
Your turn1 of 2
Two separate samples of pure water, one from a lake and one made in a lab, are both broken down into hydrogen and oxygen. Why does the law of definite proportions guarantee both samples give the same mass ratio of hydrogen to oxygen?2.2 Discovering the Electron: Cathode Rays and the Charge-to-Mass Ratio
- How it worksHow did J.J. Thomson bend cathode rays with electric and magnetic fields to measure their charge-to-mass ratio?
- How it worksWalk me through Millikan's oil drop experiment pinning down the actual charge of a single electron.
- Common mistakeWhy isn't the plum pudding model consistent with an atom that has a concentrated positive core?
- Going furtherWhat changes about the atomic model once the electron's charge and mass are both known separately?
Your turn1 of 2
Thomson deflected cathode rays with electric and magnetic fields and could only measure one combined quantity: the charge-to-mass ratio of the particles. Why couldn't that experiment alone reveal the electron's charge and mass as separate numbers?2.2 Rutherford's Gold Foil Experiment and the Nuclear Atom
- How it worksHow does almost every alpha particle passing straight through the foil point to atoms being mostly empty space?
- How it worksWalk me through how the rare, large-angle deflections locate a small, dense, positive nucleus.
- Common mistakeWhy isn't the plum pudding model able to explain even one alpha particle bouncing straight back?
- Going furtherWhat happens to the deflection pattern if the gold foil is swapped for a much lighter element?
Your turn1 of 2
In Rutherford's gold foil experiment, the vast majority of alpha particles passed straight through the foil as if nothing were there. What does that observation tell you about the atom's structure?2.3 Atomic Structure: Protons, Neutrons, and Isotopes
- How it worksHow do the numbers of protons, neutrons, and electrons combine to fix an atom's identity, mass, and charge?
- Worked exampleFind the number of protons, neutrons, and electrons in a neutral atom of chlorine-37.
- Common mistakeWhy isn't the mass number the same thing as an element's atomic number?
- Going furtherWhat happens to an atom's identity, as an element, when the number of neutrons in it changes?
Your turn1 of 2
An atom of an element with atomic number 20 has a mass number of 49. How many neutrons does it contain?2.3 Atomic Symbolism and Average Atomic Mass from Isotope Abundances
- How it worksHow does a weighted average pull the reported atomic mass toward the more abundant isotope?
- Worked exampleFind the average atomic mass of boron given 19.9 percent boron-10 at 10.01 amu and 80.1 percent boron-11 at 11.01 amu.
- Common mistakeWhy isn't the average atomic mass just the plain midpoint between an element's two isotope masses?
Your turn1 of 2
An element has two isotopes: one with a mass of 45 amu at 85 percent natural abundance, and the other with a mass of 184 amu making up the rest of the abundance. Find the element's average atomic mass.2.3 Forming Ions: Losing and Gaining Electrons
- How it worksHow does removing or adding electrons change an atom's charge without touching its identity as an element?
- Worked exampleFind the charge on an ion formed from a magnesium atom that has lost two electrons.
- Common mistakeWhy isn't losing an electron the same thing as losing a proton?
- Going furtherWhat happens to an atom's radius when it loses enough electrons to become a cation?
Your turn1 of 2
A neutral atom loses 2 electrons entirely, forming a cation. What charge, in elementary charge units, does the resulting ion carry?2.5 Organizing Elements: The Modern Periodic Table
- How it worksHow does arranging elements by increasing atomic number, not mass, fix the few places Mendeleev got out of order?
- How it worksWalk me through why elements in the same column of the periodic table react in such similar ways.
- Common mistakeWhy isn't the periodic table just a list sorted by atomic mass?
- Going furtherWhat happens to an element's classification as metal, nonmetal, or metalloid as you scan left to right across a row?
Your turn1 of 2
The modern periodic table is arranged by increasing atomic number. Why isn't it simply a list sorted by atomic mass instead?2.4 Chemical Formulas: Molecular, Empirical, and Structural
- How it worksHow does a structural formula capture information a molecular formula like C2H6O leaves out entirely?
- Worked exampleFind the empirical formula that corresponds to the molecular formula C6H12O6.
- Common mistakeWhy isn't a molecular formula enough to tell ethanol and dimethyl ether apart?
Your turn1 of 2
Glucose (C6H12O6) and formaldehyde (CH2O) are very different substances with very different properties, yet they share the exact same empirical formula. How is that possible?2.6 Molecular and Ionic Compounds
- How it worksHow do you predict whether two elements will form an ionic compound or a molecular one just from their positions on the periodic table?
- Worked exampleWalk me through writing the formula for the ionic compound formed from aluminum and oxygen.
- Common mistakeWhy isn't there such a thing as a single molecule of sodium chloride?
- Going furtherWhat changes about a compound's formula when a polyatomic ion needs more than one copy to balance charge?
Your turn1 of 2
How can you predict, just from their positions on the periodic table, whether two elements will form an ionic compound or a molecular one?2.7 Naming Ionic Compounds, Including Transition Metals
- How it worksHow does a Roman numeral in a compound's name pin down the charge on a transition metal that can form more than one ion?
- Worked exampleWalk me through naming the compound formed from copper and the sulfate ion.
- Worked exampleFind the correct name for the compound with formula Fe2O3.
- Common mistakeWhy isn't a Roman numeral needed when naming a compound made with sodium or calcium?
Your turn1 of 2
A transition metal M forms the ionic compound with formula MCl3 (chloride ions each carry a charge of -1). What Roman-numeral charge belongs on the metal cation in its name?2.7 Naming Molecular Compounds and Acids
- How it worksHow do Greek numerical prefixes remove all the ambiguity that ionic naming handles with charges instead?
- How it worksWalk me through naming the acid formed when HCl dissolves in water versus the acid formed from HClO3.
- Worked exampleFind the name of the molecular compound with the formula P2O5.
- Common mistakeWhy isn't mono- ever used as a prefix on the first element's name in a molecular compound?
Your turn1 of 2
N2O4 is named dinitrogen tetroxide, while NO2 (a different compound made of the very same two elements) is named nitrogen dioxide. Why do prefixes need to specify BOTH elements' subscripts here, unlike in ionic naming?