Chlorine has two naturally occurring isotopes: Cl-35 (abundance 75.77%, mass 34.969 u) and Cl-37 (abundance 24.23%, mass 36.966 u). Which value is closest to chlorine's atomic mass on the periodic table?
A35.00 u
B35.45 u
C36.00 u
D36.97 u
Atomic mass = (0.7577)(34.969) + (0.2423)(36.966) ≈ 26.50 + 8.96 ≈ 35.45 u. It is not 35.00 (the mass of the most abundant isotope alone) or 36.00 (a simple average of 35 and 37). The weighted average is pulled toward the more abundant Cl-35, giving approximately 35.45 — which is why the periodic table shows a decimal, not a whole number.
Question 2 True / False
Electrons in an atom travel in fixed circular orbits around the nucleus, like planets around the sun.
TTrue
FFalse
Answer: False
This describes the Bohr model, which is a useful historical approximation but physically incorrect. Electrons exist in orbitals — three-dimensional probability distributions described by quantum wavefunctions. Their exact positions are fundamentally uncertain; what quantum mechanics defines is the energy level and the probability of finding an electron in a given region of space.
Question 3 Short Answer
Two atoms of the same element have different mass numbers. What structural difference accounts for this?
Think about your answer, then reveal below.
Model answer: They are isotopes — they have the same number of protons (which defines the element) but different numbers of neutrons, resulting in different mass numbers (protons + neutrons).
Atomic number (proton count) defines which element an atom is; changing the proton count changes the element entirely. Neutrons contribute to mass but not to chemical identity, so atoms with the same proton count but different neutron counts are the same element with different masses. These are called isotopes.