Basic concepts: elements.

I figured it was time I weighed in on a basic concept from chemistry, so let’s talk about what defines an element.

As far as chemists are concerned, the world is made up of atoms and various assemblies and modifications thereof. Those atoms and modifications of atoms are, in turn, made up of protons, neutrons, and electrons. Protons have a +1 charge and a mass of 1.0073 amu [1]. Neutrons have zero charge and a mass of 1.0087 amu. And electrons have a -1 charge and a mass of 5.49 x 10-4 amu. Various combinations of these three will give you atoms, radicals, and ions. Protons and neutrons hang out together in the nucleus of your atom (or radical or ion), while electrons can be thought of as zipping around the nucleus [2].

An element is defined by the number of protons in the nucleus. The element oxygen has 8 protons in the nuclei of its atoms. Any atom (or radical or ion) that has exactly 8 protons is an oxygen atom, and all oxygen atoms (or radicals or ions) have exactly 8 protons. It doesn’t matter how many electrons there are zipping around the nucleus; that determines the net charge. It doesn’t matter how many neutrons there are in the nucleus; that determines the atomic mass (and which isotope of oxygen you have). The number of protons in the nucleus is all that counts when you’re determining the element you’re dealing with.

Lots of compounds (like water) are made up of more than one element (here, hydrogen atoms and oxygen atoms in a ratio of 2:1). Elements, however, have molecules that are made up of a single kind of atom — elemental hydrogen is H2, while elemental oxygen comes in two forms, O2 and O3 (ozone). Most textbooks will define an element as a substance that can’t be broken down into simpler substances. (This means that chemists must view protons, neutrons, and electrons not as substances, but as the building blocks from which substances are made.)
[1] The abbreviations “amu” stands for atomic mass unit. 1 amu = 1.66056 x 10 -27 kg.

[2] Strictly speaking, you really shouldn’t think of electrons as having a well-defined location until you go looking for them with a “measurement event”. But as far as anyone can tell, they probably don’t stray too far from the positive charge concentrated in the nucleus.

Posted in Basic concepts, Chemistry, Teaching and learning.


  1. Not a bad first draft. But here are some indications that you need to tighten the definition.
    Is a neutron an atom of atomic number 0, the element neutronium? If not, why not?
    A bound state of an electron and a positron is called “positronium” — a name that suggests an analogy to elements. It comes in ortho and para forms, with different half-lives. Is positronium an element? Atomic number 0, but not the element neutronium? If not, why not?
    An antiproton orbited by a positron is referred to as an atom of antihydrogen. Is antihydrogen an element with atomic number -1?
    A hyperon (such as the strange Lambda) in a nucleus changes the orbital radii of the electrons. Of course, the hyperons decay quickly. But why arent these strange atoms elements? If not, why not? If so, what is their atomic number.
    Just being nitpicky here, but hope it helps makes the definitions more precise on rewrite.

  2. Also, for those of us who haven’t had chem since 1970, it would be helpful to define the terms “ion” and “radical” since you use them throughout as a first-order type along with “atoms” (I’m OK on ions, but on radicals I’m a little hazy).

  3. Then I will nitpick the nitpick, to go the full yard: AFAIK there are also synthetic atoms where (a few) electrons are substituted for other generation ‘electrons’. So far muons have been used, I think.

  4. Nitpickers, what exactly is wrong with the given definition? Is it that it is not clear how it applies if there are no protons present? Well, that should tip you off that the case never occurs in the context of basic chemistry. How does throwing nuclear transformation, antimatter, and exotic subatomics into the mix help at all to clarify the basic concept of an element? Wankers.

  5. Quick and dirty way to tell whether chemists consider something an element: consult the Periodic Table of the Elements. It is conveniently arranged by “atomic number” (i.e., the number of protons), starting at 1 and ascending in integral steps.
    HDP, you’re right that I should explain:
    Ions are nuclei (or multinuclear assemblies) where the total number of protons does not equal the total number of electrons — meaning they have a net-positive or net-negative charge. For example, Cl- has one more electron zipping around the Cl nucleus than there are protons in that nucleus.
    A radical is a nucleus (or a multinuclear assembly) with an unpaired electron that’s “looking for action” (i.e., is generally highly reactive). For example Cl. has the same number of protons and electrons (i.e., a neutral charge), but one of its 17 electrons is not paired, and thus the radical is “looking” for an opportunity to react with something else that will provide an electron to pair with.
    Not to get too anthropomorphic or anything …

  6. Torbjörn Larsson is right. There are muonic atoms (for a short time). Also, as the heavier analogue of positronium, a negative and a positive muon can be electromagnetically bound in mutual orbit and form (for a while) muonium.
    We presume that there are taonic atoms and taonium, but I don’t think either has been observed.
    Note that radicals can be metastable. Some have been crystallized. So the lifetimes (or half-lives) of all the elements and atoms and pseudoatoms are relevent to the discussion.
    This does not get us to the “artificial atoms” and quantum dots, which have their own pseudochemistry.
    Nor does it go into isotopes and nuclear isomers.
    I wasn’t kidding, though, about neutronium. What is a neutron star? Roughly speaking, a crust of ordinary matter, and solid neutrons as the mantle. Maybe quarks or strange matter at the core. The atmosphere is mostly electrons. Is a neutron star an atom, with a humongous atomic number? It is held together by gravity and nuclear force and electromagnetic force. Best way to get into these might by thr late Dr. Robert Forward’s great novel: “Dragon’s Egg.”
    For the most part, biologists don’t care about these exotic atoms. Buit that’s because most of them study Life As We Know It. It is now respectable, academically, to study Life As We Don’t Know It. That adds lots of fun to Philosophers’ discussions of “multiple worlds.” That’s why Science Fiction, Science, and Philosophy are all in conversation with each other. Is there life on Mars right now? Did we posion and cook some with the 1976 Viking Lander?
    There are international treaties (CSIRO) to ethically protect the other plants against biocontamination from Earth. I’ve written reports for those guys, when I was Mission Planning Engineer for the Voyager Uranus mission. What were the odds of biocontaminating Uranus, Neptune, or their moons? They didn’t like my answer, so they took the report away from me, and “cooked the books.” Ethics, Adventures In.

  7. Wankers.

    I will protest (or is it nitpick? :-) to this characterization, since I reacted to a nitpick, not the original post.
    Besides, Jonathan and I enjoyed a pleasant tête-à-tête. Why do you intrude? Jealousness? :-)
    Btw, my background isn’t chemistry, but I do know some solid state physics. Did you see me complaining about the lack of constraint against crystals and crystal states (such pseudoparticles as holes and phonons) that can be sneaked into the original definition under “a single kind of atom”?

  8. I mostly agree with Dr. X. But I do have a question relating to last week’s posts.
    These atoms, of which you speak. Are they hypothesized, theory, a model, or “real”?

  9. Lab Lemming’s question: “These atoms, of which you speak. Are they hypothesized, theory, a model, or ‘real’?” is a VERY good question.
    In my educated opinion, positronium is real, but I’m hazy about muonium and consider tauonium mere hypothesis.
    Neutrons are real; neutron stars are fairly well supported by observation but not conclusively real; neutronium outside nuclei and neutron stars is (I think) never observed; antiprotons and antineutrons are real but antihydrogen is insufficiently experimented upon (the Director of Physics at NASA HQ is very eager to see antihydrogen dropped down in a vacuum to see if it falls exactly as fast as hydrogen).
    Muonic atoms are real. Hyperonic atoms likely but not conslusive. Quantum dots are real.
    Radioactive issues matter quite a lot in radiodating, nuclear chemistry, and in dealing with issues of radioactive waste, nuclear weapon effects, dirty bombs.
    In 1900 A.D. there were MANY scientists who considered atoms a useful theory, but privately (and even publically) doubted their reality. Brownian motion and the photoelectric effect and Boltzmann helped to turn the tide.
    Are quarks real? Depends. Are ‘strings” real? I am far from alone in seriously doubting so.
    Atoms are real. If all else were forgotten, that would be one of a handful of pearls of wisdom worh passing on to those who would rebuild civilization.
    Atoms and elements and periodic table, cells, DNA/RNA/protein, evolution by natural selection, Harvey’s heart as pump, brain for thinking, Gilbert’s Earth as magnet, stellar evolution and nucleosynthesis, Maxwell’s equations, Copernican theory, expanding universe… these are a few of my favorite things…

  10. guthrie: Now you’ve got to get the great author Charles Stross to post on this blog of Janet D. Stemwedel’s, or vice versa, and the cosmos will go into a recursive loop with bizarre consequences.
    — Prof. Jonathan Vos Post

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