Multiple Choice
Which element is represented by an atom with 20 protons and 20 neutrons?
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4. Atoms and Elements
Isotopes
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An atom has protons and neutrons. Which element (and isotope) is it?
A
Chlorine-36 ()
B
Argon-35 ()
C
Chlorine-35 ()
D
Sulfur-35 ()
Verified step by step guidance1
Identify the element by its number of protons. The number of protons in an atom is called the atomic number, which uniquely defines the element. Here, the atom has 17 protons.
Look up the element with atomic number 17 on the periodic table. This element is chlorine (Cl).
Determine the isotope by calculating the mass number. The mass number is the sum of protons and neutrons: \(\text{mass number} = \text{protons} + \text{neutrons}\).
Calculate the mass number for this atom: \$17 + 18 = 35$.
Combine the element symbol with the mass number to name the isotope: Chlorine-35, written as \(\mathrm{^{35}Cl}\).
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