The Chemistry You Need

Atoms, Elements, and Bonds

Everything is made of atoms. An atom has a nucleus containing protons (positive charge) and neutrons (no charge), with electrons (negative charge) orbiting around it.

The number of protons determines what element the atom is. One proton = hydrogen. Six protons = carbon. Eight protons = oxygen. Seventy-nine protons = gold. There are 118 known elements; about 25 are essential for life.

Atoms can bond to each other to form molecules. There are three main types of bonds you need to know:

Covalent bonds — Two atoms share a pair of electrons. These bonds are strong. Water (H₂O) is held together by covalent bonds between hydrogen and oxygen. So is your DNA. So is every protein.

Ionic bonds — One atom donates an electron to another. The atom that lost an electron becomes positively charged; the one that gained it becomes negatively charged. They stick together because opposites attract. Table salt (NaCl) is ionic — sodium gives up an electron to chlorine.

Hydrogen bonds — Much weaker than the other two. Form when a hydrogen atom already covalently bonded to one atom is also weakly attracted to another nearby atom. Individually, hydrogen bonds are flimsy. Collectively, they hold biology together — they're what makes water weird, what holds DNA's two strands in their double helix, and what gives proteins their precise three-dimensional shapes.

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Why Water Is Weird

A water molecule has an oxygen atom covalently bonded to two hydrogen atoms. Oxygen pulls electrons more strongly than hydrogen does, so the oxygen end of the molecule carries a slight negative charge and the hydrogen ends carry a slight positive charge.

This makes water polar — it has a positive side and a negative side. Polar molecules form hydrogen bonds with each other.

Every water molecule is hydrogen-bonded to several of its neighbors. Those bonds are constantly breaking and reforming, but at any given moment, water is a tightly networked liquid. This explains everything:

• Cohesion — water sticks to itself, creating surface tension
• Adhesion — water sticks to other polar surfaces, which is how it climbs up plant stems
• High heat capacity — water absorbs a lot of energy before it heats up, which stabilizes Earth's climate and your body temperature
• Universal solvent — water dissolves anything else that's polar or charged, which is why blood, cytoplasm, and oceans are water-based
• Ice floats — when water freezes, the hydrogen bonds lock into a crystal that takes up more space than liquid water, making ice less dense

Without these properties, life would not exist. The chemistry of water is not a side note. It's the foundation.

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The Four Categories of Biological Molecules

Almost every molecule in your body falls into one of four categories. You'll meet all of these again in every track.

Carbohydrates — Sugars and starches. Built from carbon, hydrogen, and oxygen. Used for energy storage and structural support. Examples: glucose (a single sugar), starch (a chain of glucose), cellulose (the structural material in plants).

Lipids — Fats and oils. Mostly carbon and hydrogen. Used for long-term energy storage, building cell membranes, and signaling. The reason lipids form membranes is because they're hydrophobic — they avoid water — which makes them spontaneously cluster into sheets.

Proteins — Built from amino acids. There are 20 amino acids in biology, and proteins can be hundreds or thousands of amino acids long. The order of amino acids determines how the protein folds, and the fold determines what the protein does. Proteins do most of the work in cells.

Nucleic acids — DNA and RNA. Built from nucleotides. Store and transmit genetic information. Covered in F1.

These four — carbs, lipids, proteins, nucleic acids — make up nearly everything in living organisms. If you can name those four and remember roughly what each does, you've cleared the chemistry bar for biology.

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pH: Acids, Bases, and Why It Matters

In any sample of water, a tiny fraction of the molecules spontaneously split into a hydrogen ion (H⁺) and a hydroxide ion (OH⁻). The balance between these two ions determines how acidic or basic a solution is.

Acids are substances that release H⁺ ions into water. They make a solution more acidic. Examples: stomach acid, vinegar, lemon juice.

Bases are substances that release OH⁻ ions or absorb H⁺ ions. They make a solution more basic (or alkaline). Examples: bleach, baking soda, soap.

pH is a scale from 0 to 14 measuring this balance:

• 0–6 = acidic (lower = more acidic)
• 7 = neutral (pure water)
• 8–14 = basic (higher = more basic)

The scale is logarithmic — each whole number represents a 10x change. pH 3 is 10x more acidic than pH 4, and 100x more acidic than pH 5.

Why this matters for biology: most proteins only work properly within a narrow pH range. Your blood pH is tightly regulated at 7.35–7.45. Drop it to 7.0 or push it to 7.8 and you'll be dead within minutes — enzymes stop functioning, oxygen transport breaks down, the entire system collapses. pH isn't an abstract chemistry concept. It's a life-or-death parameter.

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Wait, Actually...

Carbon is the chemical reason life exists.

Of the 118 known elements, carbon is the only one that can form four stable covalent bonds in nearly any combination — including bonds with itself. This means carbon can build chains, rings, branches, and three-dimensional structures of arbitrary complexity. A carbon atom can be part of a glucose molecule, a steroid hormone, a strand of DNA, or a diamond.

Silicon, directly below carbon on the periodic table, has the same four-bond ability. Science fiction has speculated for decades about silicon-based life. The problem: silicon bonds are less stable, silicon-oxygen compounds are solid rocks instead of breathable gases, and silicon can't form the same diversity of complex molecules. As far as we know, every living thing in the universe is carbon-based — and the universal periodic table makes a strong case that it has to be.

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