Final answer:
The number of electrons in an atomic shell when n=3 is 18, using the formula 2n². The notation nl is used for shells and subshells, and the principal quantum number n indicates the average distance of an electron from the nucleus.
Step-by-step explanation:
If n equals a number (for example, n=3), then to find the number of electrons in a given shell, we use the formula 2n². Here, for n=3, we would calculate 2(3)² which equals 18. This implies that the n=3 shell can hold a maximum of 18 electrons. It's important to note that this is different from the number of electrons in a charged atom, which is the number of protons minus the charge of the atom. In a neutral atom, the number of electrons is equal to the number of protons.
The notation nl helps to denote shells and subshells. For example, the spectroscopic notation 1s² represents two electrons in the n=1 shell, while 2p³ indicates three electrons in the n=2 shell with a p subshell. Additionally, the principal quantum number n indicates the relative distance of an electron from the nucleus, and its increase implies a greater distance hence reduced attraction to the nucleus.