Short, approachable lessons for curious learners. Read at your pace, then test your understanding with interactive activities.
Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons described by quantum mechanics. The atomic number is the number of protons.
Elements are arranged in order of increasing atomic number. Elements in the same column often share some chemical behavior because of similar valence-electron arrangements.
Atoms can be joined by bonds. Covalent bonds involve shared electron density; ionic bonding involves electrostatic attraction among ions. Bonding is more nuanced than simplistic transfer/share pictures.
pH is a measure related to hydrogen-ion activity. The scale is logarithmic; a pH change of one unit corresponds to about tenfold change in hydrogen-ion activity.
Mass measures the amount of matter, volume measures the space an object occupies, and density is mass divided by volume. At a given temperature, density is an intensive property.
Scientific models simplify complex systems. Evidence, measurement quality, and risk assessment help us decide what a model can—and cannot—tell us.