The atomic number of oxygen is 8 because oxygen has?
8 protons in the nucleus.
electrons in 8 shells.
a second shell with 8 electrons.
an atomic mass of 8.
The correct answer and explanation is :
Correct Answer: 8 protons in the nucleus.
Explanation (300+ words):
The atomic number of an element is a fundamental property that defines the identity of that element. The atomic number is determined by the number of protons present in the nucleus of an atom. Each element has a unique number of protons, and this number is what gives the element its atomic number and place in the periodic table.
For oxygen, the atomic number is 8, which means that every atom of oxygen has 8 protons in its nucleus. This is the key characteristic that distinguishes oxygen from all other elements. If the number of protons were to change, the atom would no longer be oxygen; it would become a different element entirely.
Now, in a neutral atom, the number of electrons equals the number of protons. So a neutral oxygen atom also has 8 electrons. These electrons are arranged in energy levels or shells around the nucleus. The first shell can hold up to 2 electrons, and the second shell can hold up to 8 electrons. For oxygen, 2 electrons fill the first shell, and the remaining 6 go into the second shell—not 8. Therefore, the option that says “a second shell with 8 electrons” is incorrect.
Additionally, oxygen does not have electrons in 8 shells. It only has 2 occupied shells. Atoms with electrons in 8 shells belong to elements with much higher atomic numbers (well beyond 100).
Lastly, atomic mass is a separate concept from atomic number. The atomic mass of oxygen is approximately 16, which is the sum of the protons and neutrons in the nucleus, not 8.
In conclusion, oxygen has an atomic number of 8 because it contains 8 protons in its nucleus, and this is what defines it as the element oxygen. This makes “8 protons in the nucleus” the correct and scientifically accurate answer.