An element is a pure substance made up of only one type of atom. Compounds are made up of two or more different types of elements chemically bonded together.
Atoms are arranged in the periodic table according to their atomic number, which is the number of protons in the nucleus. Elements with similar properties are placed in the same group, and elements in the same period have the same number of electron shells.
A physical change is a change in the physical state or appearance of a substance, such as melting or boiling. A chemical change involves the rearrangement of atoms to form new substances, such as burning or rusting.
The Law of Conservation of Mass states that matter cannot be created or destroyed, only transformed. In chemical reactions, the total mass of the reactants is equal to the total mass of the products, as atoms are rearranged but not created or destroyed.
An exothermic reaction releases energy in the form of heat, while an endothermic reaction absorbs energy from the surroundings. Exothermic reactions often feel warm or produce light, while endothermic reactions may feel cold or require heat to occur.