As Se Br Kr Rb Sr Y Zr etc. Xe Cs Ba La etc. Ra Ac etc. A basic Periodic Table. With the exception of elements that exist as monatomic molecules, e.g., krypton K, all elements and compounds contain atoms that are bonded together.
Bonds hold atoms together.
Gases , e.g., methane, nitrogen and carbon dioxide, are made up of small covalent molecules which behave independently from one another. Liquids , e.g., water, alcohol and petrol, are usually smallish covalent molecules that are held together by weak intermolecular forces , e.g., van der Waals forces or dipole-dipole attractions or hydrogen bonding . Solid covalent compounds arise when the intermolecular forces are strong enough to prevent individual movement of molecules. If covalent bonding results in very long chains of atoms then these chain molecules are known as polymers . This simple system of classification allows to distinguish most materials by consideration of a few basic physico-chemical properties that are related to the underlying atomic arrangement.