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:
The atomic number of an element is defined by the number of protons in the nucleus of one atom of that element. Therefore, oxygen has an atomic number of 8 because it has 8 protons in its nucleus. This is a fundamental characteristic of every element on the periodic table — each element has a unique number of protons that determines its identity.
For example:
- Hydrogen has 1 proton → Atomic number = 1
- Carbon has 6 protons → Atomic number = 6
- Oxygen has 8 protons → Atomic number = 8
If an atom gains or loses protons, it becomes a different element entirely.
Let’s break down the incorrect options:
- “Electrons in 8 shells” – This is incorrect. Oxygen does not have 8 electron shells. In fact, it only has 2 electron shells. The first shell holds 2 electrons and the second shell holds the remaining 6 electrons, making a total of 8 electrons in a neutral oxygen atom.
- “A second shell with 8 electrons” – This is incorrect because oxygen’s second shell only has 6 electrons, not 8. The second shell can hold up to 8 electrons, but oxygen has only 6 there, which is why it readily forms bonds (e.g., in H₂O) to fill this outer shell.
- “An atomic mass of 8” – This is also incorrect. Oxygen has an atomic mass of approximately 16 atomic mass units (amu). The mass is the sum of protons and neutrons, so with 8 protons and typically 8 neutrons, the mass rounds to 16.
In summary, the atomic number of oxygen is 8 because it has 8 protons in its nucleus, and this is the defining trait that classifies it as the element oxygen on the periodic table.