Questions: Atomic Structure: Protons, Neutrons, and Electrons

Question 1

An atom has 6 protons, 6 neutrons, and 6 electrons. Another atom has 6 protons, 8 neutrons, and 6 electrons. What is the relationship between these two atoms?

• A. They are different elements
• B. They are ions of the same element
• C. They are isotopes of the same element
• D. They are identical atoms

Answer: (C)

Both atoms have 6 protons, so they are both carbon (the atomic number defines the element). They differ in neutron count (6 vs. 8), which changes the mass number but not the element's identity. Atoms of the same element with different numbers of neutrons are called isotopes. They are not ions because the electron count equals the proton count in both cases.

Question 2

Electrons travel around the nucleus in fixed circular orbits, like planets orbiting the sun.

• T. True
• F. False

Answer: False

This is the Bohr model, which is a useful first approximation but physically incorrect. Quantum mechanics shows that electrons occupy orbitals — regions of space described by wave functions where an electron has a certain probability of being found. Electrons do not have well-defined trajectories; their positions are fundamentally probabilistic until measured.

Question 3

What is the difference between atomic number and mass number, and which one determines an element's chemical identity?

Model answer: Atomic number is the number of protons in the nucleus; mass number is the total number of protons plus neutrons. Atomic number determines chemical identity.

Because electrons in chemical bonding are determined by the number of protons (through charge balance), the atomic number dictates how an atom bonds and which element it is. Mass number varies among isotopes of the same element without changing chemical behavior. For example, carbon-12 and carbon-14 both have atomic number 6 and behave chemically the same way, but have mass numbers 12 and 14 respectively.