1) Answer is: molecule.
A chemical compound is a substance composed of many identical molecules.
Element bonded to an identical chemical element is not a compound.
For example, emerald (beryl) is chemical compound (beryllium aluminium cyclosilicate) with the chemical formula Be₃Al₂(SiO₃)₆. Pure beryl is colorless.
2) Answer is: atoms.
For example, polar molecule water (H₂O) is made of two hydrogen atoms and one oxygen atom connected with covalent bonds.
Other example, molecule carbon(IV) oxide (CO₂) is made of one carbon atom and two oxygen atoms bonded with covalent bonds.
Atom is the smallest constituent unit of matter that has the properties of a chemical element.
3) Answer is: element.
Pure substance is made of only one type of atom (element) or only one type of molecule, it has definite and constant composition with distinct chemical properties.
Compounds can be decomposed into other substances by chemical means, elements cannot.
There are 118 elements discovered so far.
4) Answer is: pure substance.
Pure substances can be separated chemically, not physically, that is difference between pure substances and mixtures.
For example, electrolysis of water (pure substance) is the chemical separation, because from one molecule (water) two molecules (hydrogen and oxygen) are produced. Water is separeted into two molecules:
Reaction of reduction at cathode: 2H⁺(aq) + 2e⁻ → H₂(g).
Reaction of oxidation at anode: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻.
5) Answer is: molecule.
A compound is a pure substance because its molecule cannot be broken down into simpler particles by physical means.
Molecule can be broken by chemical means (chemical reaction).
For example, balanced chemical reaction od decomposition of calcium carbonate:
CaCO₃(s) → CaO(s) + CO₂(g).
Calcium carbonate is made of calcium, carbon and oxygen atoms.
6) Answer is: atoms.
In water, atoms of hydrogen (H) and oxygen (O) are connected by polar covalent bonds.
Covalent bond is bond between nonmetals. Hydrogen and oxygen are nonmetals.
Oxygen is an element (nonmetal) with atomic number 8 (8 protons and 8 electrons). Oxygen can form anion (O²⁻) when it gain two electrons to have stable electron configuration like closest noble gas neon (Ne) with atomic number 10.
Electron configuration of oxygen atom: ₈O 1s² 2s² 2p⁴.
