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:
The correct answer is:
8 protons in the nucleus.
Explanation:
The atomic number of an element is defined as the number of protons in the nucleus of its atoms. For oxygen, the atomic number is 8 because every oxygen atom contains exactly 8 protons in its nucleus.
This number is fundamental because the number of protons determines the identity of the element. No matter how many electrons or neutrons an atom has, if it has 8 protons, it is oxygen. Changing the number of protons changes the element itself.
Let’s look at the other options and why they are incorrect:
- Electrons in 8 shells: This is incorrect because oxygen does not have electrons in 8 shells. Electrons in an atom are arranged in shells or energy levels around the nucleus, but oxygen, with 8 electrons, has only 2 shells occupied: the first shell has 2 electrons, and the second shell has 6 electrons. The number of shells depends on how many electrons the atom has and how they fill the shells, but it is not related to the atomic number.
- A second shell with 8 electrons: This is also incorrect for oxygen. Oxygen has 6 electrons in its second shell, not 8. The maximum capacity of the second shell is 8 electrons, but oxygen’s atomic number and electron configuration means it has 6 electrons in the second shell.
- An atomic mass of 8: This is incorrect. The atomic mass (or atomic weight) of oxygen is approximately 16 (more precisely about 15.999), which accounts for both protons and neutrons in the nucleus. The atomic mass is different from the atomic number. Atomic number is strictly the number of protons.
Summary:
- Atomic number = number of protons
- Oxygen has 8 protons, so its atomic number is 8.
- The number of electrons in a neutral oxygen atom is also 8, but that does not define the atomic number.
- Electron shells and atomic mass are related concepts but do not define the atomic number.
Thus, the atomic number of oxygen being 8 is directly due to the presence of 8 protons in its nucleus.