Atoms: the building blocks
Everything made of ordinary matter, from a glass of water to a steel beam, is built from atoms. An atom has a tiny, dense nucleus containing protons and neutrons, surrounded by a cloud of electrons. The number of protons decides which element the atom is: every atom with one proton is hydrogen, every atom with six is carbon, and every atom with eight is oxygen. The deeper story of subatomic particles belongs to physics, but chemistry mostly cares about one thing: the electrons, especially the outermost ones.
Elements are organized in the periodic table, which groups them by repeating patterns in how their electrons are arranged. Elements in the same column tend to behave in similar ways, which is why sodium and potassium are both soft, reactive metals, and why helium and neon are both unreactive gases.
Why atoms join together
Most atoms are more stable when their outer electrons are arranged in certain ways, often described as a “full” outer shell. Atoms can get there by sharing, giving or taking electrons. The result is a chemical bond. Three types are worth knowing:
- Covalent bonds form when atoms share electrons. Water, carbon dioxide and most of the molecules in living things are held together this way.
- Ionic bonds form when one atom hands electrons to another, leaving oppositely charged ions that attract each other. Table salt, sodium chloride, is the classic example.
- Metallic bonds hold metals together, with electrons moving fairly freely among a lattice of atoms. This freedom is a big part of why metals conduct electricity and can be bent without shattering.
Molecules and compounds
A molecule is a group of atoms held together by covalent bonds. Two hydrogen atoms and one oxygen atom make a water molecule, H2O. The formula is a recipe: the small number after each symbol tells you how many atoms of that element are present. A compound is any substance made of more than one element in a fixed ratio. Water is a compound; so is salt, although salt forms a repeating crystal of ions rather than separate molecules.
Shape matters as much as ingredients. Water molecules are bent rather than straight, and the oxygen end carries a slight negative charge while the hydrogen ends are slightly positive. That small imbalance explains a surprising amount: why water dissolves many substances, why it forms droplets, and why it behaves so differently from similar-sized molecules.
What a chemical reaction is
In a chemical reaction, bonds between atoms break and new bonds form, producing different substances. The atoms themselves are the same before and after; they are only rearranged. This is why a balanced equation has the same number of each kind of atom on both sides. When methane burns, for example:
CH4 + 2 O2 → CO2 + 2 H2O
Count the atoms: one carbon, four hydrogens and four oxygens on the left; one carbon, four hydrogens and four oxygens on the right (two in the carbon dioxide and two in the two water molecules). The mass is conserved, which is a chemical expression of the broader idea of energy conservation and its companion rule, conservation of mass.
Energy in reactions
Breaking bonds takes energy and forming bonds releases it. If the new bonds release more energy than was needed to break the old ones, the reaction gives off energy, usually as heat, and is called exothermic. Burning fuel and the reaction inside a hand warmer are examples. If the reaction absorbs more than it releases, it is endothermic, like the dissolving of some salts that makes a cold pack feel cold.
Many reactions that are favorable overall still need a push to get started, called activation energy. A match supplies it for a fire. A catalyst lowers that barrier without being used up, which is how enzymes in your digestive system break food down at body temperature when the same reactions would otherwise be far too slow.
Speed and signs of a reaction
Reactions go faster with higher temperature, higher concentration of reactants, more surface area (powdered sugar dissolves faster than a sugar cube) and the presence of a catalyst. Common clues that a reaction has taken place include a color change, gas bubbling out, a solid forming in a liquid, or noticeable heat or cooling. None of these is proof on its own, since dissolving and boiling can look similar, but together they are a useful guide.
Chemistry in the world around you
Once you start looking, reactions are everywhere. Plants use light to turn carbon dioxide and water into sugar through plant photosynthesis, and living things later release that stored energy by reacting the sugar with oxygen. A battery works because a reaction moves electrons through an outside circuit, which is the heart of battery storage. Reactions even shape which materials we can reuse, as the recycling process often depends on separating one substance from another.
Other articles in this cluster follow individual threads: how acids and bases behave, why iron rusts, how soap lifts grease, how glass and plastic are made, and what is happening when food browns or bread rises. All of them rest on the same few ideas covered here: atoms are rearranged, bonds are the currency, and energy changes decide what tends to happen.
A note on reading formulas
A chemical formula can look intimidating, but it is just bookkeeping. Subscripts show how many atoms are in a molecule, and coefficients in front of a formula show how many molecules take part. The arrow means “produces.” Written that way, almost any reaction becomes a simple statement of which atoms moved where. Learning to count atoms on each side is the single most useful skill for getting comfortable with the subject.