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Atomic Structure and Isotopes: Question 2

Syllabus 2.2, 2.3

Structured Core 6 marks

A hydroponics company grows lettuce indoors and tests a nutrient solution before it is pumped to the plants. One test measures the potassium content of the solution using the isotope of potassium shown below.

1939K{}^{39}_{19}\text{K}

(a) State what the numbers 39 and 19 represent in this symbol. [2]

(b) For one atom of this isotope, state the number of protons, the number of neutrons and the number of electrons. [3]

(c) In the nutrient solution, potassium is present as K+\text{K}^+ ions rather than as atoms. State the number of electrons in one K+\text{K}^+ ion of this isotope. [1]

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Worked solution

Part (a): Reading the nuclide symbol

In the general nuclide notation ZAX{}^{A}_{Z}\text{X}, the top number AA is the nucleon number (also called the mass number), the total number of protons and neutrons in the nucleus, and the bottom number ZZ is the proton number (also called the atomic number), the number of protons in the nucleus.

For 1939K{}^{39}_{19}\text{K}: A=39A = 39 (nucleon number) and Z=19Z = 19 (proton number).

Part (b): Counting particles in the atom

  • Protons =Z=19= Z = 19
  • Neutrons =A−Z=39−19=20= A - Z = 39 - 19 = 20
  • Electrons: in a neutral atom, the number of electrons equals the number of protons, so electrons =19= 19

Part (c): Counting electrons in the K+\text{K}^+ ion

A K+\text{K}^+ ion forms when a potassium atom loses one electron, giving it an overall charge of +1+1. The number of protons and neutrons in the nucleus does not change when an ion forms. Only the electron count changes.

electrons in K+=19−1=18\text{electrons in } \text{K}^+ = 19 - 1 = 18

Final answers

  • (a) 3939 is the nucleon number; 1919 is the proton number.
  • (b) Protons =19= 19, neutrons =20= 20, electrons =19= 19.
  • (c) Electrons in K+=18\text{K}^+ = \boxed{18}.