A starting reference for every new chemistry student
Fundamentals of Chemistry
Chemistry is the study of matter and the changes it undergoes. This page walks through the core ideas, in plain language, in the order most courses build on them — from atoms to reactions to the lab bench.
Chapter 1 of 11
What Is Chemistry?
Chemistry is the science of — anything that has mass and takes up space — and of the changes matter goes through. It asks two questions about the world: what is this made of, and what happens when it changes?
Everything around you is built from a small set of basic materials called , combined in different ways. Chemistry gives you the tools to describe those materials precisely — through properties like color, hardness, and — and to predict how they will behave, whether that's rust forming on iron, bread rising, or medicine working in the body.
In simple terms
If biology studies living things and physics studies energy and motion, chemistry sits in the middle: it studies the substances everything else is made of, and the reactions that turn one substance into another.
Like other sciences, chemistry works by observing, forming a testable idea (a ), running controlled experiments, and adjusting the idea based on results. Every chapter that follows builds on this same habit of careful observation and evidence.
One distinction is worth making from the very start: a alters how a substance looks or is arranged — ice melting, paper being torn — without changing what it's made of. A turns a substance into something genuinely new, with different properties — wood burning to ash, iron rusting. Most of this reference is about that second kind of change.
Key terms in this chapter
Chapter 2 of 11
States of Matter
Matter usually exists as a , , or . The difference between them is how tightly the particles are packed and how freely they move.
- Solid — particles are packed closely in a fixed pattern; they vibrate but don't move around. A solid keeps its own shape and volume.
- Liquid — particles are close together but can slide past each other. A liquid keeps its volume but takes the shape of its container.
- Gas — particles are far apart and move freely in every direction. A gas takes both the shape and the volume of its container.
A fourth state, , exists at very high energy — it's what makes up stars and lightning — but the first three are what most early chemistry focuses on. A substance can also skip the liquid stage entirely: solid carbon dioxide ("dry ice") turns straight into gas through , and a gas can turn straight back into a solid through , such as frost forming on a cold window.
Heating pushes matter from solid to liquid to gas; cooling reverses each step.
Try it: heat a substance up
Solid — cold, and holding a fixed shape. Drag the slider to add heat.
Key terms in this chapter
Chapter 3 of 11
Atoms & Their Parts
Every element is made of — particles far too small to see, even with most microscopes. Each atom has a tiny, dense center called the , surrounded by a much larger, mostly empty cloud where electrons move.
- Proton — positively charged, sits in the nucleus.
- Neutron — no charge, also sits in the nucleus.
- Electron — negatively charged, moves around the nucleus in shells.
The number of protons is an atom's — it's what makes one element different from another. Atoms of the same element can still have different numbers of neutrons; these variants are called .
Protons and neutrons form the nucleus; electrons occupy shells around it.
Electron shells fill in order, from the inside out, before the next one starts — the first shell fills at 2 electrons, then the second and third each fill at 8 (the third shell can hold more once heavier elements start filling it, but for the first 20 elements it stops at 8 before the next shell begins). The electrons in the outermost shell, called , are the ones involved in bonding, which is why they matter so much in the next few chapters.
Try it: build an atom
Hydrogen (H)
Mass number 1 · charge 0 · electron shells: 1
Change the counts to build different elements and ions — protons decide the element, electrons decide the charge.
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Chapter 4 of 11
The Periodic Table
The arranges every known element by atomic number so that elements with similar behavior line up in the same column. Rows are called ; columns are called .
Elements fall into broad families: metals (shiny, conduct electricity, tend to lose electrons) fill most of the table; nonmetals (poor conductors, tend to gain electrons) sit toward the upper right; metalloids (properties in between) form a diagonal staircase between them. All 118 known elements are shown below — tap any of them for its name and category.
Tap an element to see its name and category.
The two detached rows are the lanthanides and actinides — they belong between groups 3 and 4 of periods 6 and 7, but are moved below so the table stays a manageable width.
Key terms in this chapter
Chapter 5 of 11
Elements, Compounds & Mixtures
A pure , like oxygen or iron, contains only one type of atom. When atoms of different elements join chemically in fixed proportions, they form a — water (H2O) and carbon dioxide (CO2) are compounds, and they behave completely differently from the elements that make them.
A is different again: two or more substances that are simply combined, not chemically joined, so each keeps its own properties and can usually be separated again by physical means (filtering, evaporating, or distilling).
In simple terms
Element = one ingredient. Compound = ingredients chemically locked together into something new. Mixture = ingredients stirred together but still themselves underneath.
More examples: common compounds include table salt (NaCl), ammonia (NH3), and carbon dioxide (CO2); common mixtures include air (mostly nitrogen and oxygen), salad, granite, and salt water.
Mixtures can be (uniform throughout, like salt water) or (you can see the different parts, like sand and gravel). In a homogeneous mixture like salt water, the substance being dissolved (salt) is the and the substance doing the dissolving (water) is the ; how much solute is packed into the solvent is the .
Key terms in this chapter
Chapter 6 of 11
Chemical Bonding
Atoms bond because a full outer shell of electrons is more stable than a partly-filled one. There are three main ways atoms achieve this together:
- — one atom gives up an electron and another takes it, creating charged particles called that attract each other. Typical of a metal bonding with a nonmetal, like sodium chloride (table salt).
- — atoms share electrons instead of giving them away. Typical of two nonmetals bonding, like the oxygen atoms in O2 or the atoms in water.
- Metallic bonds — in a block of metal, electrons move freely between many atoms at once, like a shared "sea" of electrons, which is why metals conduct electricity.
More examples of each: ionic compounds include magnesium oxide (MgO), calcium chloride (CaCl2), and potassium bromide (KBr); covalent compounds include methane (CH4), ammonia (NH3), and carbon dioxide (CO2).
Left: an electron transfers, forming two oppositely-charged ions. Right: two pairs of electrons are shared between the atoms — a double bond, as in real O2.
Key terms in this chapter
Chapter 7 of 11
Reactions & Equations
A turns starting substances, or , into new substances, or . We write this as a :
Example
2H2 + O2 → 2H2O — two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.
Matter is never created or destroyed in a reaction, so a must show the same number of each type of atom on both sides. That's why the numbers in front of each formula (like the "2" in 2H2O) matter so much.
Common signs that a reaction has happened include a color change, bubbles of gas, a solid forming out of a solution (a ), or a change in temperature. Reactions that release heat are ; reactions that absorb heat are .
Most reactions you'll meet early on fall into a few recognizable patterns:
- — two or more substances combine into one: A + B → AB. Example: 2Mg + O2 → 2MgO.
- — one substance breaks down into two or more: AB → A + B. Example: 2H2O2 → 2H2O + O2.
- — one element takes the place of another in a compound. Example: Zn + CuSO4 → ZnSO4 + Cu.
- — a substance reacts quickly with oxygen, releasing heat and usually light. Example: CH4 + 2O2 → CO2 + 2H2O.
Some reactions barely happen on their own until a is added — a substance that speeds the reaction up without being used up itself, such as the enzymes that drive reactions in your body.
Key terms in this chapter
Chapter 8 of 11
Acids, Bases & pH
An is a substance that releases hydrogen ions when dissolved in water; acids taste sour and react with many metals (lemon juice, vinegar). A is a substance that releases hydroxide ions in water, or otherwise accepts hydrogen ions; bases feel slippery and taste bitter (soap, baking soda).
The runs from 0 to 14 and measures how acidic or basic a solution is: below 7 is acidic, 7 is neutral, above 7 is basic.
Mixing an acid and a base in the right amounts causes , producing a salt and water. Chemists check pH with an , a dye that changes color depending on acidity, such as litmus paper.
Key terms in this chapter
Chapter 9 of 11
The Mole
Atoms are too small and too numerous to count one by one, so chemists count them in bundles called , the same way a baker counts eggs in dozens. One mole of anything contains particles (to four significant figures) — a number chosen so it connects neatly to the masses on the periodic table.
The of a substance, in grams per mole, is numerically the same as the atomic or formula mass shown on the periodic table. That link lets chemists move between "how many grams" and "how many particles" — the starting point for weighing out exact amounts in a lab, a skill called .
In simple terms
A mole is just a very large counting unit. Once you know how many moles you have, the balanced equation from Chapter 7 tells you how much of everything else is involved.
Try it: grams → moles
That's 1.00 mole(s).
moles = mass (g) ÷ molar mass (g/mol)
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Chapter 10 of 11
Lab Safety & Equipment
A chemistry lab has its own toolkit. A beaker and flask hold and mix liquids; a test tube is used for small-scale reactions; a graduated cylinder and pipette measure exact volumes; a Bunsen burner provides controlled heat.
Chemicals and equipment can be hazardous, so labs use standard warning symbols. Learning to recognize them is one of the first safety habits any student builds:
These are simplified illustrations of the idea — always follow the official GHS hazard pictograms printed on real chemical labels and safety data sheets.
Basic safety rules
Wear eye protection, never taste or smell a chemical directly, keep the work area ventilated, tie back long hair, and always know where the emergency shower and eyewash station are before you start.
Key terms in this chapter
Chapter 11 of 11
Glossary
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