# What is the definition of of 'compound', 'mixture', 'element' and 'molecule'?

I am looking for the precise definitions, as I am very confused as to what they are exactly, because although I mostly understand what they mean, I have encountered some conflicting definitions that confused me.

As it stands, this is what I understand them to mean:

• Compound: two or more different atoms bonded together.
• Mixture: two or more different atoms together but not joined.
• Molecule: two particles (same or different) bonded together.
• Element: only 1 type of atom; this definition is applied to things both bonded and not to itself.

I don't know whether these terms apply microscopically, macroscopically, or both. For example: I think 1 $H_2 O$ molecule is a compound, but is a bathtub of them called a mixture (as it contains more than 1 atom type), a compound, or both? That is, to be a compound, do all the atoms in the compound have to be bonded physically together? Generally, to what extent can these four terms overlap?

I would very much like a systematic way of thinking about these terms.

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If one atom, then not molecule and not mixture and not compound. If two atoms and same element, then molecule and not compound and not mixture. If two atoms and not same element, then compound and molecule and not mixture. If two different molecules and evenly mixed, then homogeneous mixture. If two different substances and differentiable on the molecular level, then heterogeneous mixture. The air is a homogeneous mixture. Your oatmeal is a heterogeneous mixture. –  Leonardo Jan 10 '13 at 6:17

I can't really provide a systematic approach, but I can attempt to clarify (as a student myself).

• Elements are classes of atoms. Atoms of the same element are similar (if not identical) in their physical and chemical properties (but be aware of Isotopes which are physical variations among atoms of the same element).
A definite (I suppose, systematic) way to distinguish elements is that their corresponding atoms have different numbers of electrons orbiting the nucleus.
The term is also used to describe a collective of the same atom (element). A bar of gold (Gold being an 'element'; a chemical class) is said to be an element itself.

• Molecules are a group of atoms covalently bonded to each other (which can be considered a 'direct connection', if you will). The molecule can consist of atoms of the same element, or atoms of different elements.
If you joined to molecules via covalent bonding, you'd have created a new molecule (it's nothing special)

• Compounds are (in the chemical sense of the term) made of two or more elements. This means, a lot of molecules (like H2O) are considered compounds (though H2 is not). 'Compound' is often used interchangeably with the word substance, which describes a collective of molecules (water is a substance. It is a large quantity of neighbouring H2O molecules).
A substance (and a compound, when compound is used to describe a quantity of molecules) only consists of the same molecule. In water, all the molecules are H2O. If there were also some H2 molecules among them, it would no longer be considered a compound/substance (but instead; a mixture).
These molecules are not directly bonded to each other (or they'd be one big molecule), they are just in close* proximity to one another and are attracted to one another, keeping them in proximity. Even when the substance is a solid, like ice, they are not directly connected, the molecules are just slower (less energetic) and closer together (and in some special cases, like ice, stronger, such as Hydrogen Bonding. Don't be confused, though it is referred to as 'bonding', there is no direct covalent bond, just an attraction).

• Mixtures are collectives of different molecules or atoms. The molecules and atoms are like that in a substance; they're not bonded to each other, but are attracted, and in a mixture specifically, the molecules/atoms are not all the same. Juice, for instance, is a mixture. It contains some Vitamin C molecules, some water molecules, some sugar molecules, et c.

Here is a horribly drawn diagram to help clarify the distinctions.

Reflecting on this, here is what in your understanding needs specific attention.

Compound: two or more different atoms bonded together A molecule or a group of identical molecules.
Mixture: two or more different atoms or molecules together but not joined (they're not covalently bonded).
Molecule: two or more particles atoms (same or different) bonded together.

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In a sample of water, you'll find the compound $H_2O$. In a sample of hydrogen gas, you'll find the element hydrogen, though it'll take the form of the molecule $H_2$. In a mixture, you may find different molecules, compounds, and elements.