When are atoms most stable?

• A. When they have an equal number of protons and neutrons
• B. When their nucleus contains only protons
• C. When their outermost electron shell is completely filled
• D. When they have more electrons than protons

Answer: (C)

Verified step by step guidance

1. Understand that atomic stability is primarily related to the arrangement of electrons around the nucleus, not just the composition of the nucleus itself.
2. Recall that atoms tend to be most stable when their outermost electron shell (also called the valence shell) is completely filled with electrons, which minimizes their reactivity.
3. Recognize that having an equal number of protons and neutrons affects nuclear stability but does not directly determine chemical stability of the atom.
4. Note that a nucleus containing only protons is highly unstable due to repulsive forces between positively charged protons.
5. Understand that having more electrons than protons results in a negatively charged ion, which is not necessarily more stable than a neutral atom with a filled outer shell.