Multiple Choice
An atom has 18 protons and 22 neutrons. Which element is it, and what is its mass number?
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4. Atoms and Elements
Isotopes
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Which element is described by an atom that has 19 protons and 20 neutrons?
A
Argon (Ar), specifically
B
Calcium (Ca), specifically
C
Sodium (Na), specifically
D
Potassium (K), specifically
Verified step by step guidance1
Identify the number of protons in the atom, as the number of protons defines the element's atomic number. Here, the atom has 19 protons.
Recall that the atomic number corresponds to the element's position on the periodic table. Find the element with atomic number 19, which is Potassium (K).
Determine the mass number of the atom by adding the number of protons and neutrons: \(\text{Mass number} = \text{protons} + \text{neutrons} = 19 + 20\).
Express the isotope notation for the element using the element symbol, atomic number (protons), and mass number: \(^{\text{mass number}}_{\text{atomic number}}\text{K}\).
Compare this isotope notation with the given options to confirm that the atom described matches Potassium-39, \(^{39}_{19}\text{K}\).
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