Multiple Choice
Arsenic has an atomic number of 33 and its most common isotope has a mass number of 75. How many neutrons are present in this isotope of arsenic?
2. Atoms & Elements
Isotopes
Multiple Choice
How many protons and neutrons are present in an atom of magnesium-25?
A
25 protons and 0 neutrons
B
13 protons and 12 neutrons
C
12 protons and 13 neutrons
D
12 protons and 12 neutrons
Verified step by step guidance1
Identify the atomic number of magnesium (Mg) from the periodic table. The atomic number represents the number of protons in the atom.
Recall that magnesium has an atomic number of 12, so it has 12 protons.
Understand that the number after the element name (25 in magnesium-25) is the mass number, which is the sum of protons and neutrons in the nucleus.
Use the formula to find the number of neutrons: \(\text{Number of neutrons} = \text{Mass number} - \text{Atomic number}\).
Calculate the number of neutrons by subtracting the atomic number (12) from the mass number (25), which gives the number of neutrons in magnesium-25.
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