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First, I would like to quote sentences from a book introducing elements and atoms:

An element is a fundamental (pure) form of matter that cannot be broken down to a simpler form.

Elements are made up of particles called atoms. An atom is the smallest unit of any element [...]

The first quote states that element cannot be broken down to a simpler form, but the second quote says elements are formed by atoms, which means element can be split into a simpler form (atoms)?

The difference between atom and element confuses me.

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The wording in your quote is a bit unclear, I agree. A very nice definition is given by the Jefferson Lab page What is the simplest way of explaining what atoms, elements, compounds and mixtures are?

Atoms

are the smallest bits of ordinary matter

Whereas an element is

a substance that is made entirely from one type of atom

Atoms themselves can be 'broken down' into smaller sub-atomic particles (protons, neutrons and electrons) - the amounts of these (particularly the number of protons) define each element.

For example, in their un-ionised form, the element carbon is made entirely from carbon atoms, which have 6 protons and 6 electrons each, and cadmium atoms, which have 48 protons and 48 electrons, make up the element cadmium.

The amount of neutrons can vary with atoms of a particular element, forming isotopes, which (from the link):

For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7 and 8 respectively.

Different arrangements of atoms of single elements can form allotropes, such as for carbon, with diamond and graphite (among others - shown below). They both are composed of atoms of the element carbon, but in different configurations.

Below are diagrams of the allotropes of carbon featured in the article Work Function Engineering of Graphene (Garg et al. 2014)

enter image description here

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    $\begingroup$ Probably worth clarifying that atoms can be broken down into smaller particles. $\endgroup$ – bon Sep 5 '15 at 10:10
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    $\begingroup$ Since "an element is a substance that is made entirely from one type of atom" and since isotopes involve different atoms, would a block of lead containing a natural abundance of isotopes be considered a mixture of elements? $\endgroup$ – ron Sep 5 '15 at 15:50
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    $\begingroup$ @ron It is a mixture, but it is not considered a mixture of different elements, since all of the isotopes are isotopes of lead. (The category boundaries are not perfectly crisp. Deuterium, for instance, is different enough from ¹H that in some contexts it makes sense to treat it as a separate element.) $\endgroup$ – zwol Sep 5 '15 at 16:54
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    $\begingroup$ @IncnisMrsi I don't know what you mean by "not in the sense of elemental substance, but elements in general", but water definitely does contain both hydrogen and oxygen, which are elements. There are things in nature that are not composed of elements; the most familiar example is light. But you have to go pretty far afield to find something material that is not composed of elements, such as a neutron star. $\endgroup$ – zwol Sep 7 '15 at 21:27
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    $\begingroup$ @ron that would be a mixture of isotopes (using the broad definitions from the Jefferson Labs link at the least) - that could make for an interesting question in itself. $\endgroup$ – user15489 Sep 7 '15 at 23:09
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Given that you seem to be a beginner in Chemistry, I thought you might benefit from a description from somebody who only studied up to high school chemistry.

We'll use water as an example because you've probably heard it referred to as H2O.


A single "piece" of water, H2O is a molecule.

It contains 2 elements: Hydrogen, and Oxygen.

But it contains 3 atoms, 2 Hydrogen, and 1 Oxygen.


Atoms contain even smaller pieces called subatomic particles. There are three kinds: Protons, Neutrons, and Electrons. the number of protons defines what the element is.

1 proton - Hydrogen
2 protons - Helium
12 protons - Carbon, etc.

The total number of protons + neutrons is the atom's atomic weight

Elements can contain different numbers of neutrons. These are called isotopes.

Hydrogen-1 has 1 proton
Hydrogen-2 has 1 proton 1 neutron
Hydrogen-3 has 1 proton 2 neutrons

These different forms of hydrogen are isotopes of one another.


enter image description here Obviously it gets more complicated that this, but for you at the moment, it doesn't need to :)

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The word element has two slightly different meanings in chemistry:

  • an element as an abstraction, an “atomic species” (ex.: $\ce{{}_8O}$ oxygen),   and
  • an elemental substance (ex.: $\ce{O2}$ dioxygen gas), that represents an element in practice.

Textbooks on chemistry, possibly, downplay the distinction since they are not textbooks on philosophy.

Different elements are distinguished by their atomic number (ex.: 8 for oxygen). Atomic number is a characteristic of an atom, whether is it free or bound, neutral or ionized. Atomic number is the number or protons in the nucleus (that is chemically immutable) and, the same, electric charge of the nucleus (in appropriate unit). It is an invariant preserved in all chemical processes. Elements are a classification of atoms. For each natural Z, all atoms with atomic number Z form a chemical element: A generalized atomic species (although different atoms of the same element are not always identical). Any atom represents (pertains to) some element, and to only one element.

An elemental substance consists of atoms of the same element, and doesn't contain atoms of other elements. It may be formed of free atoms (as for noble gases), molecules (as for $\ce{O2}$), or form a crystal. When a chemist tries to isolate an element, i. e. to gather its atoms in one place, an elemental substance results. Hence chemists use one word “element” with two slightly different denotations.

Again, the difference between abstract elements (atomic species) and elemental substances. When we ask: “of which elements does water consist?”, we assume species (hydrogen and oxygen, of whose atoms does the water consist), and don’t assume the water contains $\ce{H2}$ or $\ce{O2}$ (although it might, a slight quantity).

And how are atoms different from all this? An atom is a physical body. We can count them as things: There is one atom, two atoms, three atoms, etc.. How many atoms are in a water $\ce{H2O}$ molecule? There are three: Two atoms of hydrogen and one atom of oxygen. How many elements are in water (molecule)? Two elements: Hydrogen and oxygen.

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An elements is a substance which is made out of atoms that all have the same proton number. So an element can consist of multiple atoms (like oxygen which goes around in pairs). An atom consists of a nucleus and some electrons.

Source: IGCSE Chemistry

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  • $\begingroup$ Does water (H₂O) contain elements? Your source, apparently, defines an elemental substance, not an element in general. $\endgroup$ – Incnis Mrsi Sep 5 '15 at 21:02
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    $\begingroup$ The elements that make up water are hydrogen and oxygen. Hydrogen and oxygen are elements on the periodic table. The oxygen element goes around in pairs in air but can be broken to form other molecules and compounds. $\endgroup$ – Andrew Wang Sep 5 '15 at 21:13
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Here is the IUPAC definition of an element:

https://doi.org/10.1351/goldbook.C01022

  1. A species of atoms; all atoms with the same number of protons in the atomic nucleus.

  2. A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. Sometimes this concept is called the elementary substance as distinct from the chemical element as defined under 1, but mostly the term chemical element is used for both concepts.

Now that we have had evidence of atoms as nucleus plus electrons for over 100 years, there is no more need for the classic definition ("smallest particle, can't separate") that is now known not to be true if you allow for nuclear reactions.

Atom is a term at the particular scale, it refers to one particle. Element traditionally is a term at the macroscopic scale, it refers to the nature of a substance (either element or compound).

However, it is also used to refer to an atom-type, such as in the statement that "acetic acid contains oxygen", meaning that it contains oxygen atoms. Of course, acetic acid is not an element, and it is not a mixture of elements. In order to make it from the elements, you need a chemical reaction, so then the elements are no longer elements (but the oxygen is still an oxygen).

The confusion comes from the first part of the definition: An element is a species of atoms. We call those atoms that have 8 protons in their nucleus "oxygen atoms". It would be nice to have separate words for the particular and the macroscopic scale, but we have not.

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An atom is a thing; an element is a type of thing.

An atom is a collection of protons, neutrons and electrons. A single, isolated atom in its neutral state has some number of protons, the same number of electrons and some number neutrons (about as many as protons for the lighter elements, up to about 50% more, for heavier elements). The number of neutrons or protons in an atom only changes as a result of radioactive processes or very high-energy interactions such as you get in particle accelerators. And I mean really high energy: even if you think about blowing up sticks of dynamite, that isn't nearly enough energy to start messing with the protons and neutrons. Chemistry happens when atoms come together and share electrons or give electrons to each other. Chemical reactions happen all the time and many of them don't need a whole lot of energy: moving electrons from atom to atom is often very easy.

So, the chemistry of an atom depends on the number of electrons and the number of electrons in an isolated atom depends directly on the number of protons. Electrons are so easy to add and remove from atoms (just rub a balloon against your hair: the static electricity is because you transferred electrons between your hair and the balloon) so we classify atoms according to the number of protons in them. Neutrons aren't so relevant: I'll put a note about them at the end.

So, the element of an atom is determined by the number of protons. All atoms of hydrogen have one proton, and all atoms with one proton are hydrogen. Two protons is helium, three is lithium, seventeen is chlorine, 79 is gold, etc. A pure sample of an element contains only atoms of that type: for example, a pure sample of iron contains only atoms with 26 protons. Water, on the other hand, is not an element: a molecule of water consists of two hydrogen atoms (one proton each) sharing electrons with an oxygen atom (eight protons).

Now, what does it mean to claim that an element "cannot be broken down into a simpler form," and why aren't atoms "a simpler form"? Well, they're not a simpler form because an iron atom is iron: it's the same form, not simpler. Think of it this way. If I give you a lump of pure iron, all you can do is break it into smaller lumps of iron, or make it into a more complex substance, for example by letting it rust – rust is formed of iron and oxygen. The smallest possible lump of iron you could make is a single iron atom, but that's still just an unbelievably tiny lump of iron. If you wanted to break down a lump of iron beyond single iron atoms, you'd need to use a nuclear reactor or a particle accelerator or something like that and then, finally, you'd be able to get something that wasn't iron, because you'd have changed the number of protons in the atoms.

Let's contrast that with water. If I give you a bucket of pure water then, just like the lump of iron, you can divide it into smaller and smaller samples, ultimately ending up with a single water molecule. But you can do something else: if you pass electricity through the water, it splits into pure hydrogen and pure oxygen. These are "simpler" substances because each consists of atoms of just one element, whereas water has atoms of two elements.


What about neutrons? Well, in terms of chemistry, they don't do a whole lot and atoms that have the same number of protons but different numbers of neutrons are much more alike (they have essentially the same chemistry, for example) than atoms that have the same number of neutrons but different numbers of protons. It makes much more sense to classify by the number of protons, since that determines the number of electrons and that determines the chemistry.

Suppose you tried instead to classify atoms according to the number of neutrons. Well, most argon atoms (18 protons) have 22 neutrons, but some chlorine atoms (17 protons) and a decent fraction of potassium atoms (19 protons) also have 22 neutrons. As you probably know, argon, chlorine and potassium are absolutely nothing like each other. On the other hand, potassium atoms with 22 neutrons behave almost identically to the most common kind of potassium atoms, which have 21 neutrons.

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    $\begingroup$ Did not downvote, but I think this answer is inaccurate, in the sense that it ignores molecular elements/allotropes, which can be broken down, changing the substance but not the element. $\endgroup$ – March Ho Sep 5 '15 at 13:47
  • $\begingroup$ @MarchHo That's a good point, though I'd describe it as incomplete, rather than inaccurate. $\endgroup$ – David Richerby Sep 5 '15 at 14:05
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    $\begingroup$ I didn't downvote either, but in your first paragraph, "approximately the same number of neutrons" is flat-out wrong for the heavier elements. I'd say "some number of protons, some (usually a little larger) number of neutrons, and the same number of electrons as protons". $\endgroup$ – zwol Sep 5 '15 at 16:57
  • $\begingroup$ Is element “a form of matter”? If it is, then which namely, and is it a substance of something else? You made a lengthy lecture on neutrons and radioactivity, but didn’t address these questions. $\endgroup$ – Incnis Mrsi Sep 5 '15 at 20:55

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