Lesson 1 of 6 · Count what is actually present
Elements and molecules
9 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish atoms, ions, isotopes, molecules, and mixtures, then keep particle counts separate from mass.
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An element is an identity, not a location
The same can occur in a star, a rock, an atmosphere, or your body. Its identity follows the number of protons in an atomic nucleus. Carbon has six; oxygen has eight. An has a different neutron count while retaining that elemental identity. An ion has gained or lost electrons. These changes answer different questions: neither an extra neutron nor a missing electron automatically turns carbon into oxygen.
Ordinary chemical reactions rearrange electrons and connections between ; they do not change nuclei into different elements. That distinction makes an atom inventory useful. If a sealed chemical experiment seems to lose oxygen atoms, first check what was counted: perhaps gas escaped, a was omitted, or the measurement missed oxygen in a solid. Producing another element would require a nuclear process, outside this chemical model.
A formula tells you which particles to count
A is a discrete bonded group of atoms. Oxygen gas contains O₂ molecules, so a molecule need not contain different elements. Water, H₂O, is both molecular and a : it contains hydrogen and oxygen. A mixture contains multiple substances without requiring them to become one new chemical species. Not every compound consists of separate molecules; an ionic salt can form an extended lattice of charged constituents.
A belongs to the chemical formula. The 2 in CO₂ means two oxygen atoms per carbon dioxide molecule. A counts whole units: 3 CO₂ represents three such molecules, containing three carbon and six oxygen atoms. Changing that coefficient changes the amount. Changing CO₂ into CO changes the substance. This is why balancing an equation cannot be done by casually editing the small numbers inside each formula.
Check 1 of 3
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Work two different inventories
Consider an invented of 3 CO₂ molecules and 4 H₂O molecules, with no reaction specified. There are seven molecules, but 21 atoms: three carbon, eight hydrogen, and ten oxygen. The carbon dioxide molecule fraction is 3/7. Asking what fraction of the atoms are carbon instead gives 3/21. Both calculations describe the same collection; they divide different counted quantities by different totals.
Using deliberately rounded atomic masses H = 1, C = 12, and O = 16 , one CO₂ has mass 44 and one H₂O has mass 18. The collection therefore has mass 3 × 44 + 4 × 18 = 204 units. Carbon dioxide contributes 132/204, about 64.7% of the mass, although it contributes only 3/7 of the molecules. A report must identify which fraction it means.
Check 2 of 3
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Work through a decision
From an inventory to a planetary claim
Imagine a sample report listing carbon, hydrogen, and oxygen. That list does not uniquely identify its molecules. The atoms might occupy water, carbon dioxide, larger organic compounds, several , or some combination. NASA’s Bennu analyses required measurements of particular compounds rather than treating an elemental list as a complete recipe. Even an accurately counted inventory leaves questions about bonding and physical arrangement unanswered.
When comparing two claims, underline the noun after each number: atoms, molecules, , mass, or concentration. Then ask whether the instrument distinguishes the proposed species and whether the sample represents the place being discussed. The same atom accounting that prevents a classroom counting error also prevents a planetary headline from turning the detection of an element into the detection of a particular gas—or of life.
Count molecules, atoms, and mass separately
Original instructional diagram using an invented mixture. Particle number and mass use different weighting; the labeled boxes are not molecular shape models.
Open full-size diagram in a new tab: Count molecules, atoms, and mass separatelyCheck 3 of 3
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Put it together
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