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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 $\ce{H2O}$ 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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  • $\begingroup$ 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. $\endgroup$
    – Leonardo
    Jan 10, 2013 at 6:17
  • $\begingroup$ Have a look at the IUPAC Gold book. It defines what Chemists consider these things to be. goldbook.iupac.org $\endgroup$
    – porphyrin
    Jul 1, 2016 at 7:30

6 Answers 6

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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 made of two or more elements bound together. This means a lot of phases made of molecules (like H2O) are considered compounds (though H2 is not). While in mixtures proportions of constituents are highly variable, in compounds they are not. If a compound is molecular, all it's molecules are virtually identical (this doesn't account for different isotopes, or molecular conformations).

  • Mixtures are broad group created when you mix different substances, as long as they don't react to make a single pure compound in the process. For example juice is a mixture. It contains some Vitamin C, some water, some sugar, etc.

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

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

Compound: A molecule or a group of identical molecules.
Mixture: two or more different atoms or molecules together but not as a single compound Molecule: two or more atoms (same or different) bonded together.

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    $\begingroup$ What about ionic substances? Is NaCl a compound? I always thought it was but if we take your definition there is no such thing as an ionic compound. $\endgroup$
    – lagrange103
    Oct 14, 2014 at 13:53
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    $\begingroup$ @AntiEarth we can still have monoatomic molecules, e.g. neon atoms. I think the definition of a molecule is that its the smallest possible particle capable of independent existence or something. $\endgroup$
    – Sherlock Holmes
    Oct 15, 2014 at 1:10
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    $\begingroup$ @SherlockHolmes It is my understanding that single atoms are not molecules; the wikipedia definition reads "A molecule /ˈmɒlɪkjuːl/ is an electrically neutral group of two or more atoms held together by chemical bonds" $\endgroup$
    – Anti Earth
    Oct 15, 2014 at 2:12
  • $\begingroup$ I think there is an error in your definition of atoms: "...way to distinguish elements is that their corresponding atoms have different numbers of electrons orbiting the nucleus" (emphasis mine). By your definition, $\ce{O^{-2}}$, $\ce{F-}$ and $\ce{Ne^0}$ (all isoelectronic) would be equal atoms, which is not true. Atoms are defined by their atomic number (number of protons), not the number of electrons. $\endgroup$
    – schneiderfelipe
    Apr 18, 2020 at 13:31
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    $\begingroup$ @FelipeS.S.Schneider yea I recoil at most of what my highschool self has written here $\endgroup$
    – Anti Earth
    Apr 19, 2020 at 19:49
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In a sample of water, you'll find the compound $\ce{H2O}$. In a sample of hydrogen gas, you'll find the element hydrogen, though it'll take the form of the molecule $\ce{H2}$. In a mixture, you may find different molecules, compounds, and elements.

At this point, you may find it helpful to look at the following classification of matter. Matter tree
The left side of the tree denotes chemical bonds/changes. The right is physical. (i.e., you can get a heterogeneous mixture to convert to a homogeneous mixture by changing physical things about the mixture like adding in another element. You can't do the same to elements and compounds.)

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I was also confused when I learned this all stuff first time. When you get any element, molecule, compound or mixture don't think about complex stuff but think on the basic level.

I have given the most basic definition (properties) of an element, molecule, compound or mixture.

Element:

  • A substance which cannot be broken down chemically is called elements.

  • All substances listed in Periodic Table are elements.

Molecule:

  • Two or more atoms or elements combined together are called a molecule.

Compound:

  • Two or more elements combine together to form a compound.

  • The smallest unit of a compound is a molecule.

  • Compounds possess different characteristics compared to constituent elements.

  • Example: $\ce{H2O}$.

    • $\ce{H}$ is highly combustible

    • $\ce{O}$ helps in combustion

    • $\ce{H2O}$ is used in fire extinguishers (different characteristics than $\ce{H}$ and $\ce{O}$ )

Mixture:

  • When two or more matter mix without changing their original property this is called a mixture.

  • Mixtures can be separated by a simple process, like filtration.

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Compound: a pure substance formed from the bonding of two or more different elements together. It can be decomposed into its constituent elements by chemical means.

Element: a pure substance consisting of one type of atom only.

Mixture: two or more different substances are mixed but they are not chemically bonded together and may be separated by a physical process such as filtration, distillation and so on.

Molecule: I'm actually not sure about this - I always read that it's a group of atoms (same or different) bonded together and is the smallest particle of an element/compound capable of independent existence; however, we treat monoatomic gases as a molecule, e.g. Neon.

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I'm only a student but I just did some research on it (for myself) but I thought I could maybe help you so this is what I understand.

Compound: two or more different atoms chemically bonded together.

Molecule: two or more different or same atoms chemically bonded together.

Element: one single atom.

A mixture contains two or more substances (elements, lattices, molecules, compounds), that are not chemically bonded together.

So basically a mixture is like you said a bath tub with elements, compounds, lattices, and/or molecules in it but they are not bonded together chemically. Otherwise, it would be a molecule. I just think of a mixture as a box with things chucked in it like toys can be molecules and books can be elements, you can separate them easily. This is what I understood so it might not be 100 % right.

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    $\begingroup$ I disagree with your definition of element — I think you mean only atoms of a single type. I would call a single atom an atom. $\endgroup$
    – Jan
    Oct 27, 2015 at 12:00
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Everything in the universe is made up of something, and when we try to refine or filter and figure out what those are at the very basic level we come to a certain number of things.

These certain number of things when combined in different ways and quantities make up anything and everything matter. Humans, plants, air, water everything. These things are well classified, studied and placed in the modern periodic table and have a well-defined number of them and are called the elements' of the modern periodic table. And the tiniest possible particle of an element is called an 'atom' of that element. why tinniest?.

Here is a better explanation for all your queries. Hope it helps.

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