Atoms, Isotopes, Ions & Molecules

The building blocks of everything — from water to DNA to the lactic acid in your yogurt vat

BIO 111 — Chemistry of Life
Every biological process — enzyme function, DNA replication, muscle contraction, fermentation — is chemistry. And chemistry is atoms doing things to each other. Start here and everything in biology makes more sense.
An atom is the smallest unit of an element that retains the chemical properties of that element. It has a nucleus (protons + neutrons) surrounded by electrons in shells. The number of protons = atomic number = the element's identity.

Click an element above to explore its atomic structure

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Protons, neutrons, electrons — what each does
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Protons (+): In the nucleus. Positive charge. The number of protons = atomic number = defines which element it is. Change the proton count and you have a different element entirely — that's nuclear chemistry.

Neutrons (neutral): In the nucleus. No charge. Add mass but not charge. Same element can have different neutron counts — those are isotopes. Neutrons contribute to nuclear stability.

Electrons (−): In shells around the nucleus. Negative charge. In a neutral atom, electrons = protons. Electrons determine chemical behavior — bonding, reactivity, ionic charge. The outermost shell (valence shell) is everything in chemistry.
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Electron shells and the octet rule
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Electrons fill shells from innermost outward. Shell 1 holds up to 2 electrons. Shell 2 holds up to 8. Shell 3 holds up to 18 (but first fills to 8 before the next shell starts). The octet rule: atoms are most stable with 8 electrons in their outermost (valence) shell — a full outer shell. Atoms bond with other atoms specifically to achieve this stable configuration. Carbon has 4 valence electrons (needs 4 more) — which is exactly why it forms 4 bonds and is the backbone of all organic molecules.
💡 Hydrogen is the exception — its first shell holds only 2, so hydrogen is "happy" with 2 electrons (the duet rule). That's why H₂O has two hydrogens — oxygen needs 2 more electrons, each hydrogen provides 1.
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The 4 elements of life — CHON + more
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96% of the human body (and most living things) is made of just 4 elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N) — CHON. Add Phosphorus (P) and Sulfur (S) and you have the backbone of almost every biological molecule. Carbon is the master builder — 4 valence electrons means 4 bonds means infinite structural variety. Hydrogen provides the lightweight filler. Oxygen drives most of the energy-releasing reactions. Nitrogen is essential for proteins and DNA. The rest of the periodic table fills supporting roles — calcium for bones, iron for hemoglobin, sodium and potassium for nerve signals, magnesium for chlorophyll.
Isotopes are atoms of the same element with different numbers of neutrons. Same atomic number (same element), different mass number. Most elements have multiple naturally occurring isotopes.

Select an isotope above to compare nuclear structures

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Radioactive isotopes — unstable nuclei
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Some isotopes are stable (like C-12 and C-13). Others are unstable — too many or too few neutrons relative to protons makes the nucleus unstable. These are radioactive isotopes that spontaneously emit particles or energy (radiation) as they decay toward a more stable configuration. Carbon-14 is radioactive — it decays with a half-life of about 5,730 years. Half-life is the time for half of a sample to decay. After one half-life, 50% remains. After two, 25%. After three, 12.5%. This is exponential decay — the same log mathematics as microbial death curves, just on a geological timescale.
💡 Radiocarbon dating uses C-14 half-life. By measuring how much C-14 remains in an organic sample vs how much it started with, you can calculate how long ago the organism died. The math is: t = (t₁/₂ / ln2) × ln(N₀/N) — a logarithm.
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Radioactive isotopes in medicine and biology
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Radioactive isotopes have enormous practical uses. PET scans use fluorine-18 to detect cancer — cancer cells consume glucose rapidly, so radioactive glucose accumulates in tumors and lights up on the scan. Iodine-131 is used to treat thyroid cancer — the thyroid naturally concentrates iodine, so radioactive iodine kills thyroid cells selectively. Carbon-14 is used in metabolic studies — researchers feed organisms labeled molecules and track where the carbon goes. Phosphorus-32 labels DNA and RNA to track replication. The same nuclear instability that makes these atoms dangerous in large doses makes them useful as tracers in tiny amounts.
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Atomic mass — why it's not a whole number
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The atomic mass on the periodic table (Carbon = 12.011, not 12) is the weighted average of all naturally occurring isotopes. Carbon-12 makes up about 98.9% of natural carbon, Carbon-13 makes up about 1.1%, and Carbon-14 is a tiny trace amount. The average: (12 × 0.989) + (13 × 0.011) ≈ 12.011. This is why atomic masses are decimals — they're population averages, not the mass of any single atom. A single carbon atom is always exactly 12, 13, or 14 amu — never 12.011.
Ions are atoms (or groups of atoms) that have gained or lost electrons, giving them a net electrical charge. Cations are positive (lost electrons). Anions are negative (gained electrons).
Sodium loses an electron → Na⁺ Na 11p, 11e⁻ valence e⁻ −e⁻ Na⁺ 11p, 10e⁻ net charge: +1 smaller (lost outer shell) Chlorine gains an electron → Cl⁻ Cl 17p, 17e⁻ needs 1 e⁻ +e⁻ Cl⁻ 17p, 18e⁻ net charge: −1 larger (gained extra shell) Na⁺ + Cl⁻ → NaCl (table salt) opposite charges attract → ionic bond
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Why ions matter biologically
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Ions are essential for virtually every biological process. Na⁺ and K⁺ gradients across cell membranes generate nerve impulses and muscle contractions — including your heartbeat. Ca²⁺ triggers muscle contraction, blood clotting, and cell signaling. Mg²⁺ is required by over 300 enzymes and is the central atom in chlorophyll. Fe²⁺/Fe³⁺ in hemoglobin carries oxygen. H⁺ concentration IS pH — and pH regulates enzyme function, fermentation, protein folding, and almost every biochemical reaction.
💡 When S. thermophilus produces lactic acid during fermentation, it's releasing H⁺ ions into the milk. The accumulation of those H⁺ ions is what drops the pH. The pH drop is literally an ion concentration change.
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Ionic bonds — opposites attract
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When a cation and anion form, the opposite charges attract each other — this electrostatic attraction is an ionic bond. NaCl is the classic example: Na⁺ and Cl⁻ attract and form a crystal lattice. In water, ionic compounds dissociate — the polar water molecules pull the ions apart. This is why NaCl dissolves in water, and why dissolved ions (electrolytes) conduct electricity. The salt you add to your yogurt production for flavor or texture is providing Na⁺ and Cl⁻ ions to the product matrix.
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Common biological ions to know
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Na⁺ — nerve impulses, osmotic balance
K⁺ — nerve and muscle function, inside cells
Ca²⁺ — bone, muscle contraction, cell signaling
Mg²⁺ — enzyme cofactor, chlorophyll center
Fe²⁺/³⁺ — hemoglobin oxygen transport
H⁺ — pH, proton gradients, ATP synthesis
Cl⁻ — osmotic balance, nerve function
PO₄³⁻ — DNA backbone, ATP, bone
Molecules form when atoms share or transfer electrons to achieve stable outer shells. The type of bond depends on how electrons are shared — equally, unequally, or transferred completely.
Three types of chemical bonds Ionic bond Na⁺ lost 1e⁻ Cl⁻ gained 1e⁻ ↔ electrostatic attraction electron transferred dissolves in water Nonpolar covalent H H 2e⁻ H₂ electrons shared equally same electronegativity no partial charges Polar covalent (water) O δ⁻ δ⁻ H δ⁺ H δ⁺ electrons pulled toward O partial + and − charges this makes water POLAR Hydrogen bonds — the weak force that holds life together Hydrogen bonds form between the δ⁺ hydrogen of one polar molecule and the δ⁻ oxygen (or N or F) of another. Each H-bond is weak (~1/20 the strength of a covalent bond). But millions of them together = strong. Water's H-bonds explain: surface tension, high specific heat, cohesion, adhesion, ice floating. DNA's two strands are held together by H-bonds between base pairs — strong enough to store info, weak enough to unzip. Protein secondary structure (α-helices, β-sheets) is held together by H-bonds between backbone atoms.
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Why water is the molecule of life
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Water's polarity makes it the universal solvent — ionic and polar molecules dissolve in it, while nonpolar molecules (fats, oils) don't. This is why your cell membrane works: phospholipid heads are polar (face water), tails are nonpolar (avoid water). Everything else in biology happens in water or at a water interface. The polarity of water also means it has unusually high specific heat — it resists temperature change — which stabilizes biological environments. And ice is less dense than liquid water (H-bonds in ice form a more spread-out lattice), which is why ice floats and aquatic ecosystems survive winters.
💡 Your yogurt vat is mostly water. The casein micelles, the whey proteins, the lactic acid — all of it is dissolved in or interacting with water molecules. The pH drop during fermentation is H⁺ ions accumulating in that water. Water isn't the backdrop — it's the medium everything plays out in.
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Bond strength hierarchy
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Not all bonds are equal. From strongest to weakest:
Covalent bonds: ~350 kJ/mol. Atoms sharing electrons. Breaking these requires serious energy — like denaturation or combustion. DNA backbone is covalent.
Ionic bonds: ~250 kJ/mol in crystal, much weaker in water. Dissociate easily in aqueous solution — that's why salts dissolve.
Hydrogen bonds: ~20 kJ/mol each. Weak individually — but DNA holds together, proteins fold, water has special properties because of the sheer number of them. Disrupted by heat — which is partly why pasteurization works on protein structure.
Van der Waals: <5 kJ/mol. Temporary induced dipoles between all molecules. Weak but ubiquitous — important for molecular recognition in enzymes and receptors.

Key vocabulary

Atom
Smallest unit of an element. Nucleus (protons + neutrons) + electron shells. Atomic number = proton count = element identity.
Atomic number
Number of protons. Defines which element it is. Carbon = 6, Nitrogen = 7, Oxygen = 8. Cannot change without becoming a different element.
Mass number
Protons + neutrons. Changes between isotopes of the same element. Carbon-12 vs Carbon-14 — same protons, different neutrons.
Isotope
Atoms of the same element with different neutron counts. Same chemical behavior, different mass. Some isotopes are radioactive.
Ion
Atom with net electrical charge from gaining (anion −) or losing (cation +) electrons. Na⁺, Cl⁻, Ca²⁺, H⁺.
Electronegativity
Tendency of an atom to attract electrons toward itself. Oxygen is highly electronegative — pulls electrons away from H in water, creating polarity.
Valence electrons
Electrons in the outermost shell. Determine bonding behavior. Carbon has 4, making it ideal for building complex organic molecules.
Hydrogen bond
Weak attraction between δ⁺ hydrogen and δ⁻ atom (O, N, F). Individually weak but collectively crucial for DNA structure, protein folding, and water's properties.

Exam reminders

Atomic number = protons. Mass number = protons + neutrons. Electrons = protons (in a neutral atom).
Isotopes differ in neutron count only. Same element, same chemistry, different mass.
Cation = lost electrons = positive charge. Anion = gained electrons = negative charge.
Covalent bonds share electrons. Ionic bonds transfer electrons. H-bonds are attractions between already-formed polar molecules.
Water is polar because oxygen pulls electrons — creating δ⁻ on O, δ⁺ on H. This drives hydrogen bonding and makes water the universal solvent.
Carbon's 4 valence electrons let it form 4 bonds — the basis of all organic chemistry and life's molecular diversity.

The CHON elements

C — Carbon (6p) — 4 valence e⁻, forms backbone of all organic molecules
H — Hydrogen (1p) — 1 valence e⁻, fills bonds, makes water polar
O — Oxygen (8p) — 6 valence e⁻, highly electronegative, drives polarity
N — Nitrogen (7p) — 5 valence e⁻, essential for proteins and DNA