The atomic number of an atom is always equal to the total number of A. neutrons in the nucleus B. protons in the nucleus C. neutrons plus protons in the atom D. protons plus electrons in the atom
The correct answer and explanation is:
Correct Answer: B. protons in the nucleus
Explanation (300 words):
The atomic number of an atom is a fundamental concept in chemistry and physics. It is defined as the number of protons present in the nucleus of an atom. This number is unique for each element and determines the element’s identity. For instance, hydrogen has an atomic number of 1, meaning every hydrogen atom has 1 proton in its nucleus. Carbon has an atomic number of 6, indicating 6 protons.
The atomic number is denoted by the symbol Z. It serves multiple roles:
- It defines the element.
- It determines the atom’s position in the periodic table.
- In a neutral atom, it also equals the number of electrons, because the positive charge of protons is balanced by the negative charge of electrons.
Let’s examine the incorrect options:
- A. Neutrons in the nucleus: The number of neutrons can vary among atoms of the same element, resulting in different isotopes. Neutrons do not determine the atomic number.
- C. Neutrons plus protons in the atom: This total is known as the mass number (A), not the atomic number.
- D. Protons plus electrons in the atom: In a neutral atom, the number of protons equals the number of electrons, but the sum of the two is not the atomic number. Also, atoms can gain or lose electrons (becoming ions), but this doesn’t affect the atomic number.
Summary:
- Atomic number = number of protons
- Determines the identity of the element
- Equals the number of electrons in a neutral atom
Therefore, the correct and most precise answer is:
B. protons in the nucleus.