Compound

Recap.. Compound are molecules made up of two or more different type of atom chemically joined together.
Compound have different properties as of their element root. What is element root?
CO2 - carbon dioxide Carbon and oxygen is the root of the compound. It is not tested in syllabus but is good to know.
For example Oxygen is combustable and Carbon is used to strengthen the molecule to make the substance harder. However the compund is not combustable and it's a stable molecule by it's own unlike carbon and oxygen.
Maybe you're wandering why elements chemically combine? Quite troublesome huh?
Because they want to be more stable. Alone by itself an atom is not chemically stable because their valence shell or you can call it outermost shell is not fully filled with electron. For instance, Carbon's electronic configuration is 2.4 because it has 6 electron. First shell can only take up to two electrons. The Second and Third can be filled up to 8. because the outermost shell is still lack of 4 electrons it will want to chemically combine with another atom to form a stable particle. You maybe wandering STILL why Carbon combine with two oxygen instead of one is because oxygen's outermost shell has 6 electrons(2.6) and it needs 2 more electrons to form a stable particle. The Carbon would then give up 2 of it electrons to one oxygen atom and other 2 to another oxygen atom thus fully filling their outermost shell to form a stable particle known as Carbon dioxide(CO2).
The chemical reaction is called covalent bonding. I'm still not really good at it but I can tell you briefly. Covalent bonding is the sharing of electrons between atoms to form a stable particle. There is also double bond and triple bond. Double bond shares 2 pairs of electron(4electrons) and triple bond shares 3 pairs of electrons(6electrons) << probably at sec 4 pure science would learn combine science don't have to I think.
Examples of compound
NaCl(sodium chloride salt)
H2SO4(sulfate)
HCl(hydrochloride)

this are the more common ones.