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.
The atomic number of an element is defined by the number of protons in the nucleus of one of its atoms. In the case of oxygen, its atomic number is 8, which means that each oxygen atom contains 8 protons in its nucleus. This is a fundamental property that defines the element; if an atom had more or fewer protons, it would no longer be oxygen but a different element entirely.
Let’s break down why the other answer choices are incorrect:
- Electrons in 8 shells: This is not true for oxygen. Oxygen only has two electron shells. The first shell can hold up to 2 electrons, and the second can hold up to 8. Oxygen, with 8 electrons, fills the first shell with 2 electrons and places the remaining 6 in the second shell. Therefore, it does not have electrons in 8 shells.
- A second shell with 8 electrons: This is also incorrect. Oxygen’s second shell holds 6 electrons, not 8. An atom with 8 electrons in its second shell would be neon, which has an atomic number of 10. Oxygen is two electrons short of a full second shell, which is why it tends to form bonds with other atoms—to fill that shell and become more stable.
- An atomic mass of 8: This is also incorrect. Oxygen has an atomic mass of approximately 16 atomic mass units (amu). This mass is due to the sum of its 8 protons and 8 neutrons (on average). The atomic number is not determined by the atomic mass.
To summarize, the atomic number of oxygen is 8 because it has 8 protons in its nucleus. The atomic number is a key identifier for elements on the periodic table and is always equal to the number of protons in the atom. This number also determines the number of electrons in a neutral atom, which in turn affects the atom’s chemical behavior.