BIOB 160 Principles of Living Systems · Unit 1: The chemistry of life · on Exam 1
1.2 The chemical context of life
Study notes for The chemical context of life in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- About 96% of living matter is C, H, O and N. An atom has protons and neutrons in the nucleus and electrons in shells; the atomic number is the number of protons and the mass number is protons + neutrons.
- Isotopes have the same number of protons but different neutrons. Radioactive isotopes decay at a fixed half-life (carbon-14 about 5,730 years) and are used as tracers and for dating.
- Chemical behavior depends on valence electrons in the outer shell. Atoms form bonds to fill it: H makes 1 bond, O 2, N 3, C 4.
- Covalent bonds share electrons: nonpolar when shared equally, polar when one atom is more electronegative (O and N pull hard). Ionic bonds form between ions with opposite charges after an electron is transferred (Na⁺Cl⁻).
- Weak bonds shape biology: hydrogen bonds (a partially positive H attracted to an electronegative O or N), van der Waals interactions and ionic attractions in water. Molecular shape determines function, as with enzymes, receptors and drugs.
Key formulas
Mass number
$$A = \text{protons} + \text{neutrons}$$
Worked example
Oxygen has atomic number 8. How many protons, neutrons and electrons does a neutral atom of oxygen-18 have, and how many covalent bonds does oxygen usually make?
Show the solution
8 protons (atomic number), 10 neutrons (18 − 8) and 8 electrons (neutral). With 6 valence electrons it needs 2 more, so it usually forms 2 covalent bonds.
Common mistakes
- Mixing up atomic number (protons) and mass number (protons + neutrons).
- Calling a hydrogen bond a covalent bond. It is a weak attraction between molecules or parts of a molecule.
- Forgetting that a polar covalent bond is unequal sharing, not a transfer of electrons.
Exam tipReading: Campbell ch. 2 (pp. 28–43). Rank bond strength: covalent > ionic (in water, ionic bonds are weakened) > hydrogen > van der Waals.
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