If the atomic number is 18, then
· there are seven electrons in the outermost energy level.
· the atom has eighteen electrons in the nucleus.
· the outermost energy level is full.
· All of the above are true.
The Correct Answer and Explanation is :
The correct answer is:
“The outermost energy level is full.”
Explanation:
The atomic number of an element represents the number of protons in its nucleus, and in a neutral atom, it also equals the number of electrons. If an atom has an atomic number of 18, it means the atom has 18 protons and 18 electrons.
Electron Configuration:
Electrons are arranged in energy levels (shells) around the nucleus. The general electron configuration follows the 2-8-8 rule:
- First energy level (K-shell): Holds up to 2 electrons.
- Second energy level (L-shell): Holds up to 8 electrons.
- Third energy level (M-shell): Holds up to 8 electrons.
Thus, the electron configuration for an atom with atomic number 18 is:
2, 8, 8
This means the outermost energy level (third shell) has 8 electrons, which is the maximum number for this shell in a stable atom. Since the outermost shell is full, the atom is chemically stable and does not readily react with other elements.
Why Other Options Are Incorrect:
- “There are seven electrons in the outermost energy level.”
- Incorrect. The outermost energy level has 8 electrons, not 7.
- “The atom has eighteen electrons in the nucleus.”
- Incorrect. Electrons do not reside in the nucleus; they orbit around it. The nucleus contains protons and neutrons.
- “All of the above are true.”
- Incorrect, because two of the statements are false.
Thus, the only correct answer is: “The outermost energy level is full.”
This makes the element Argon (Ar), a noble gas, which is chemically inert because of its full valence shell.