All substances consist of tiny, invisible particles. The smallest units are atoms, and particles formed when atoms join together are molecules.
Basic Knowledge
When a substance is divided into smaller and smaller parts, we eventually reach the smallest particles that cannot be divided any further. These are called atoms. Atoms are extremely small (about 10-10 m in diameter), and there are as many types as there are elements. About 118 types are currently known and are represented by element symbols (H, C, O, Na, Fe, and so on).
A particle made when two or more atoms join together is called a molecule. For example, a hydrogen molecule is H2, formed from two hydrogen atoms, and water is H2O, formed from two hydrogen atoms and one oxygen atom. Substances made of molecules are called molecular substances; substances that do not form molecules, such as iron and sodium chloride, consist of atoms or ions directly bonded together.
Main element symbols: H(hydrogen), He(helium), C(carbon), N(nitrogen), O(oxygen), Na(sodium), Mg(magnesium), Al(aluminum), S(sulfur), Cl(chlorine), K(potassium), Ca(calcium), Fe(iron), Cu(copper), Ag(silver), Zn(zinc).
Atom（the smallest particle making up a substance. Its mass and size are determined for each type）
Element（the concept representing a type of atom. Represented by an element symbol）
Molecule（a particle made of two or more bonded atoms. The smallest unit showing the properties of a substance）
Element symbol（one or two alphabetic characters representing a type of atom）
Chemical formula（a formula representing a substance with element symbols and numbers. Examples include
H2O and CO2）
Further Study
Atomic theory was proposed by the ancient Greek Democritus, but modern atomic theory was established by John Dalton (1803). Dalton claimed that “atoms of the same element have the same mass” and “atoms are neither newly created nor destroyed in a chemical change.”
An atom has a nucleus (protons + neutrons) at its center, with electrons moving around it. In chemical changes, electrons are transferred or shared, while the nucleus does not change. Therefore, the number of atoms remains unchanged before and after a reaction, and the law of conservation of mass holds.
Examples of molecules: hydrogen H2, oxygen O2, nitrogen N2, carbon dioxide CO2, water H2O, ammonia NH3, and chlorine Cl2.
- Atom = the smallest particle that cannot be divided further
- The first letter of an element symbol is uppercase, and the second letter is lowercase
- Molecule = a particle made of two or more bonded atoms
- Iron, copper, and sodium chloride do not form molecules
- The small number at the lower right of a chemical formula = the number of atoms
H2O: 2 H atoms and 1 O atom- Atoms are neither newly created nor destroyed in chemical changes
Points to Note
① Do not confuse an element (the concept of a type) with an atom (an actual particle). “Oxygen atom” refers to an atom, while “oxygen element” refers to an element. ② In middle school, molecular formula and chemical formula are treated as the same, but iron Fe is not a molecule and is treated as one atom. ③ A hydrogen molecule H2 is a combination of two hydrogen atoms and is different from a hydrogen atom H. ④ Uppercase and lowercase letters in element symbols must be distinguished exactly (Co is cobalt, while CO is carbon monoxide).
Practice
- Write the chemical formula for water and state the types and numbers of atoms it contains.
- Write the element symbol for iron. Does iron form molecules?
- Name the substances represented by the following chemical formulas: CO2, NH3, O2.