The mass number A of an atom is

Answer: Protons + neutrons in the nucleus A = Z + N (protons + neutrons). The number of nucleons.

Isotopes of an element have the same number of but different numbers of .

Answer: protons; neutrons Same Z (same element) but different N → different mass numbers.

The isotope ¹⁴C has

Answer: 6 protons, 8 neutrons Z = 6 (carbon), A = 14, so N = 14 − 6 = 8 neutrons.

AP Chemistry · Topic 1.2

Mass Spectroscopy of Elements Practice

Part of Atomic Structure and Properties.(SPQ-1.B)

Practice questions

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Sample questions

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Sample 1difficulty 1/5
$The mass number A of an atom is$
- A
  $Avogadro's number$
- B
  $Number of electrons$
- C
  $Protons + neutrons in the nucleus$
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- D
  $Atomic mass in grams$
Why
$A = Z + N (protons + neutrons). The number of nucleons.$
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Sample 2difficulty 2/5
$Isotopes of an element have the same number of _______ but different numbers of _______.$
- A
  $atoms; molecules$
- B
  $protons; neutrons$
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- C
  $neutrons; protons$
- D
  $electrons; protons$
Why
$Same Z (same element) but different N \to different mass numbers.$
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Sample 3difficulty 2/5
$The isotope ¹⁴C has$
- A
  $8 protons, 6 neutrons$
- B
  $6 protons, 8 neutrons$
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- C
  $14 protons, 6 neutrons$
- D
  $6 protons, 6 neutrons$
Why
$Z = 6 (carbon), A = 14, so N = 14 - 6 = 8 neutrons.$
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Sample 4difficulty 2/5
$The mass spectrum of a pure element shows three peaks at m/z = 24, 25, and 26 with relative abundances 78.99%, 10.00%, and 11.01%.$
$What is the average atomic mass of this element?$
- A
  $26.00 amu$
- B
  $24.00 amu$
- C
  $24.32 amu$
  check_circle
- D
  $25.00 amu$
Why
$(24)(0.7899) + (25)(0.1000) + (26)(0.1101) = 24.32 amu, matching magnesium.$
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Sample 5difficulty 3/5
$Chlorine has two isotopes: ³⁵Cl (75%, mass 34.97) and ³⁷Cl (25%, mass 36.97). The atomic mass of Cl is approximately$
- A
  $35.97$
- B
  $35.45$
  check_circle
- C
  $36.97$
- D
  $34.97$
Why
$\bar M = 0.75 \times 34.97 + 0.25 \times 36.97 = 26.23 + 9.24 = 35.47$
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