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Dr. Christopher S. Baird

Are two atoms of the same element identical?

Category: Chemistry      Published: March 13, 2014

spilled salt
An atom of sodium from table salt behaves very differently from an atom of sodium from metallic sodium. In general, atoms of the same element are not identical as they can be in different states. Public Domain Image, source: Christopher S. Baird.

No. Two atoms of the same chemical element are typically not identical. First of all, there is a range of possible states that the electrons of an atom can occupy. Two atoms of the same element can be different if their electrons are in different states. If one copper atom has an electron in an excited state and another copper atom has all of its electrons in the ground state, then the two atoms are different. The excited copper atom will emit a bit of light when the electron relaxes back down to the ground state, and the copper atom already in the ground state will not. Since the states of the electrons in an atom are what determine the nature of the chemical bonding that the atom experiences, two atoms of the same element can react differently if they are in different states. For instance, a neutral sodium atom (say, from a chunk of sodium metal) reacts with water much more violently than an ionized sodium atom (say, from a bit of salt). Chemists know this very well. It's not enough to say what atoms are involved if you want to fully describe and predict a reaction. You have to also specify the ionization/excitation states of the electrons in the atoms. Even if left alone, an atom often does not come with an equal number of protons and electrons.

But what if two atoms of the same element both have their electrons in the same states. Then are they identical? No, they are still not identical. Two atoms of the same element and in the same electronic state could be traveling or rotating at different speeds, which affects their ability to chemically bond. Slower moving atoms (such as the atoms in solid iron) have time to form stable bonds, while faster moving atoms (such as the atoms in liquid iron) cannot form such stable bonds. A slow moving tin atom acts differently from a rapidly moving tin atom.

But what if two atoms of the same element both have their electrons in the same states, and the atoms are both traveling and rotating at the same speed. Then are they identical? No. Although two such atoms are essentially chemically identical (they will chemically react in the same way), they are not completely identical. There's more to the atom than the electrons. There's also the nucleus. The nucleus of an atom contains neutrons and protons bonded tightly together. The same chemical element can have a different number of neutrons and still be the same element. We refer to the atoms of the same element with different numbers of neutrons as "isotopes". While the particular isotope involved does not affect how an atom will react chemically, it does determine how the atom will behave in nuclear reactions. The most common nuclear reaction on earth is radioactive decay. Some isotopes decay very quickly into other elements and emit radiation, while other isotopes do not. If you are doing carbon dating, the fact that a carbon-12 atom is not identical to a carbon-14 atom is essential to the dating process. Simply counting the number of carbon atoms in a sample will not give you any information about the age of a sample. You will have to count the number of different isotopes of carbon instead.

But what if two atoms are the same element, have electrons in the same state, are traveling and rotating at the same speed, and have the same number of neutrons; then are they identical? No. Just like the electrons, the neutrons and protons in the nucleus can be in various excited states. In addition, the nucleus as a whole can rotate and vibrate at various speeds. Therefore, even if all else is identical, two gold atoms can have their nuclei in different excited states and behave differently in nuclear reactions.

To state the case succinctly, it is very hard to have two atoms of the same element be exactly identical. In fact, succeeding in coaxing a group of atoms to be very close to identical was worthy of a Nobel Prize. With that said, don't think that atoms have individual identities beyond what has been mentioned here. If two carbon atoms are in the exact same molecular, atomic, electronic and nuclear states, then those two carbon atoms are identical, no matter where they came from or what has happened to them in the past.

Topics: atom, atoms, carbon, chemical, electron, electronic state, element, isotope, nucleus