# Q & A: Arsenic Shells and subatomic particles

Q:
What is a subatomic partical, and how many shells are in arsenic and how many electrons are in each shell?
- Mike (age 13)
Nazareth Middle School, Rochester, N.Y.
A:
A subatomic particle is a particle which can be found in an atom. For example, electrons, protons, and neutrons are all subatomic particles. Over the course of the years, many particles have been discovered in high-energy collisions. Some of these, like pions, were routinely called "subatomic" because they act to carry the strong nuclear force between protons and neutrons, much as photons carry the electromagnetic force. Other particles were found, heavy cousins of the electron, and parts of protons and neutrons, called quarks. At some point we stopped calling the new ones "subatomic" because they really aren't found in atoms, they have to be made special in high-energy collisions and they decay quickly into more ordinary particles. Some of the ones recently discovered are heavier than most kinds of atoms. A top quark weighs about as much as a gold atom. But aside from its weight, and the fact that it decays quickly into lighter quarks, it acts a lot like the quarks that make up protons and neutrons. Is it "subatomic"? We tend not to use that word much.

Arsenic's electron structure is:

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3

I guess you could say it has four shells, with the fourth one not yet full. The first number is the "principle quantum number" (or the "shell" in old terminology), while "s" "p" and "d" refer to angular momentum states. The last number in each group above says how many electrons are in those states. The principle quantum number nearly determines the energy when its value is 1 or 2, but in atoms with more electrons, some with principle quantum numbers of 3 or 4, the energy can also depend a lot on whether the electron is in an s, p, or d state. Back in the old old days when people used the shell model, they were trying to explain the energy dependence on the principle quantum number, and understanding the differences between the energies of the s, p, and d levels came later.

Tom (w mike)

(published on 10/22/2007)