If an atom contains 18 protons and 18 electrons, what is the atomic number of the element? A. 36 B. 18 C. 1 D. 8. The respective numbers of protons, electrons, and neutrons are: 18.
The correct answer and explanation is:
The correct answer is B. 18.
The atomic number of an element is determined by the number of protons in its nucleus. In this case, the atom contains 18 protons. Since the atomic number is equal to the number of protons, the atomic number of the element is 18. The number of protons also determines the identity of the element. Therefore, an atom with 18 protons is the element argon (Ar) on the periodic table, which has an atomic number of 18.
Electrons, which are negatively charged particles, typically equal the number of protons in a neutral atom, ensuring the atom has no overall electrical charge. Since this atom contains 18 electrons, it is electrically neutral because the number of positive charges (protons) is balanced by the number of negative charges (electrons).
Neutrons, which have no charge, are also part of an atom’s nucleus. The number of neutrons can be found using the formula:
Neutrons=Mass number−Atomic number\text{Neutrons} = \text{Mass number} – \text{Atomic number}
The mass number of an atom is the sum of protons and neutrons. If the mass number is not given, you cannot precisely calculate the number of neutrons without additional information. For example, the most common isotope of argon has a mass number of 40, so it would have:
Neutrons=40−18=22\text{Neutrons} = 40 – 18 = 22
To summarize, the atomic number of an element is determined by the number of protons in its nucleus. In this case, the atom has 18 protons, meaning the atomic number is 18.