Chemistry is the science of what things are made of and how they interact. Every material you encounter — water, steel, plastic, medicine, food — is governed by chemical principles. This guide covers the foundational concepts that everything else builds on: atomic structure, bonding, chemical reactions, the mole concept, and solutions. Whether you are in a chemistry course or just want to understand the science behind everyday materials, these fundamentals are the starting point.
An atom is the smallest unit of an element that retains that element's chemical properties. Atoms consist of a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in orbitals. The atomic number (number of protons) determines the element — hydrogen has 1 proton, carbon has 6, iron has 26, gold has 79.
The periodic table organizes all 118 known elements by atomic number. Elements in the same column (group) share similar chemical properties because they have the same number of valence (outer-shell) electrons. Group 1 elements (lithium, sodium, potassium) are all highly reactive metals. Group 18 elements (helium, neon, argon) are all inert noble gases. Use the Interactive Periodic Table to explore element properties.
| Subatomic Particle | Charge | Mass (amu) | Location |
|---|---|---|---|
| Proton | +1 | ~1.007 | Nucleus |
| Neutron | 0 | ~1.009 | Nucleus |
| Electron | -1 | ~0.00055 | Orbitals (electron cloud) |
The electron is approximately 1,836 times lighter than a proton. Virtually all of an atom's mass is in the nucleus, while the electron cloud determines the atom's volume and chemical behavior.
Atoms bond to achieve a stable electron configuration (usually a full outer shell of 8 electrons, known as the octet rule). There are three primary types of chemical bonds:
Ionic bonds form when one atom transfers electrons to another. Sodium (Na) has 1 valence electron; chlorine (Cl) needs 1 electron. Sodium donates its electron to chlorine, creating Na⁺ and Cl⁻ ions that attract each other electrostatically. Ionic compounds (like table salt, NaCl) tend to form crystal lattices, have high melting points, and conduct electricity when dissolved in water.
Covalent bonds form when atoms share electrons. Two hydrogen atoms each contribute one electron to form H₂, sharing the pair equally. Water (H₂O) forms when oxygen shares electrons with two hydrogen atoms. Covalent compounds can be gases (CO₂), liquids (H₂O), or solids (sugar), generally have lower melting points than ionic compounds, and usually do not conduct electricity.
Metallic bonds form in metals, where valence electrons are delocalized into a “sea of electrons” shared among all the metal atoms. This explains metals' properties: electrical conductivity (electrons flow freely), thermal conductivity, malleability (atoms can slide past each other without breaking bonds), and luster (free electrons reflect light).
Bond polarity and water: In a water molecule, oxygen is more electronegative than hydrogen, pulling the shared electrons closer to itself. This creates a polar molecule with a slightly negative oxygen end and slightly positive hydrogen end. This polarity is why water is such an excellent solvent (dissolves ionic compounds and other polar molecules), has a high boiling point for its molecular weight, and exhibits surface tension. Water's polarity is arguably the most important chemical property for life on Earth.
Atoms are far too small to count individually. A single drop of water contains approximately 1.67 × 10²¹ molecules. The mole bridges the atomic world and the laboratory world.
One mole = 6.022 × 10²³ items (Avogadro's number). The mole is defined so that one mole of any element has a mass in grams equal to its atomic mass from the periodic table. One mole of carbon-12 weighs exactly 12 grams. One mole of water (H₂O, molecular mass 18.015) weighs 18.015 grams — roughly one tablespoon.
Molar mass is the mass of one mole of a substance. To find the molar mass of a compound, add up the atomic masses of all atoms in its formula. For glucose (C₆H₁₂O₆): (6 × 12.011) + (12 × 1.008) + (6 × 15.999) = 180.156 g/mol. Use the Molar Mass Calculator for quick calculations.
A chemical reaction rearranges atoms from reactant molecules into product molecules. The law of conservation of mass requires that the same number of each type of atom appears on both sides of the equation.
Balancing example: The combustion of methane (natural gas): CH₄ + O₂ → CO₂ + H₂O. Unbalanced, this has 4 hydrogen on the left and 2 on the right. Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O (1 carbon, 4 hydrogen, and 4 oxygen on each side). Use the Chemical Equation Balancer to check your work.
Stoichiometry uses the mole ratios from balanced equations to calculate quantities. From the methane combustion equation above: 1 mole of CH₄ reacts with 2 moles of O₂. If you burn 3 moles of CH₄, you need 6 moles of O₂ and produce 3 moles of CO₂ and 6 moles of H₂O. Converting to grams: 3 moles of CH₄ (48 grams) requires 192 grams of O₂ and produces 132 grams of CO₂ and 108 grams of H₂O. Total mass in (240 g) equals total mass out (240 g) — conservation of mass confirmed.
A solution is a homogeneous mixture of a solute (the substance dissolved) in a solvent (the dissolving medium, usually water). Concentration describes how much solute is present. Molarity (M) is the most common unit: moles of solute per liter of solution. A 1 M NaCl solution contains 1 mole (58.44 grams) of NaCl per liter. Use the Concentration Calculator and Dilution Calculator to work with solutions.
pH measures the hydrogen ion concentration on a logarithmic scale: pH = -log[H⁺]. The scale runs from 0 (most acidic) to 14 (most basic), with 7 being neutral. Each pH unit represents a 10-fold change in H⁺ concentration. Use the pH Calculator to explore acid-base chemistry.
| Substance | Approximate pH | Classification |
|---|---|---|
| Battery acid | ~0.8 | Strong acid |
| Stomach acid | 1.5–3.5 | Strong acid |
| Lemon juice | ~2.0 | Acid |
| Coffee | ~5.0 | Weak acid |
| Pure water | 7.0 | Neutral |
| Human blood | 7.35–7.45 | Slightly basic |
| Baking soda solution | ~8.3 | Weak base |
| Household bleach | ~12.5 | Strong base |
pH values are approximate and vary with concentration and temperature. Human blood pH outside the 7.35–7.45 range is a medical emergency.
Calculate molar masses, balance equations, and work with solutions. Use the free Molar Mass Calculator to solve chemistry problems — no signup required.
Related tools: Chemical Equation Balancer · Periodic Table · Concentration Calculator · Dilution Calculator · pH Calculator · Scientific Notation Calculator · Significant Figures Calculator