Here's why:

* Solid State: Atoms in solids are tightly packed and have a fixed arrangement. This strong attraction between atoms gives solids their rigidity and definite shape.

* Liquid State: Liquids have weaker attractions between atoms than solids, allowing them to flow and take the shape of their container.

* Gaseous State: Gases have very weak attractions between atoms, so they move freely and expand to fill their container.

Examples:

* Solids: Most metals (iron, copper, gold, etc.), nonmetals like carbon (diamond, graphite), and many others.

* Liquids: Mercury, bromine.

* Gases: Oxygen, nitrogen, hydrogen, helium, neon, argon, krypton, and xenon.

Exceptions:

* Noble Gases: The noble gases (He, Ne, Ar, Kr, Xe, Rn) are all gases at room temperature. This is because they have a very stable electron configuration and don't readily form bonds with other atoms.

* Other exceptions: There are a few other elements like phosphorus and sulfur that exist in different allotropes (different forms of the same element) with different properties. For example, white phosphorus is a solid, while red phosphorus is a powder.

Note: "Room temperature" is typically considered to be around 20-25 degrees Celsius (68-77 degrees Fahrenheit).