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## Q. 2.3

Atomic Numbers, Mass Numbers, and Isotope Symbols

(a) What are the atomic number (Z), mass number (A), and symbol of the chlorine isotope with 18 neutrons?

(b) How many protons, electrons, and neutrons are present in an atom of ${ }_{24}^{52} Cr$?

## Verified Solution

 (a) Look up the atomic number (Z) for chlorine on the periodic table. The atomic number specifies the number of protons. The mass number (A) for an isotope is the sum of the number of protons and the number of neutrons. The symbol for an isotope is its chemical symbol with the atomic number (Z) in the lower left corner and the mass number (A) in the upper left corner. Z = 17, so chlorine has 17 protons. A = number of protons + number of neutrons = 17 + 18 = 35 ${ }_{17}^{35} CI$ (b) For any isotope (in this case ${ }_{24}^{52} Cr$) the atomic number located at the lower left indicates the number of protons. Since this is a neutral atom, the number of electrons equals the number of protons. The number of neutrons is equal to the mass number (upper left) minus the atomic number (lower left). Number of protons = Z = 24 Number of electrons = 24 (neutral atom) Number of neutrons = 52 – 24 = 28