Question

PRACTICE
1 Chlorine has two stable isotopes, chlorine-35 and chlorine-37. The
atomic number of chlorine is 17. Calculate the numbers of protons,
electrons, and neutrons each isotope has.

Answer 1

Answer:

35.8 u

Explanation:

The atomic mass of Cl is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its percent abundance).

Atomic mass of Cl-35 = 17p + 18n = 17 × 1.007 u + 18 × 1.009 u

= 17.119 u + 18.162 u = 35.28 u

Atomic mass of Cl-37 = 17p + 20n = 17 × 1.007 u + 20 × 1.009 u

= 17.119 u + 20.180 u = 37.30 u

Set up a table for easy calculation.

0.755 × 35.28 u =  26.64  u

0.245 × 37.30 u =    9.138 u

             TOTAL = 35.8     u

Note: The actual atomic mass of Cl is 35.45 u.

The calculated value above is incorrect because

(a) the given isotopic percentages are incorrect and

(b) the protons and neutrons have less mass when they are in the nucleus than when they are free. Thus, the calculated masses of Cl-35 and Cl-37 are too high.

Answer 2

Answer:

1. For isotope A (chlorine 35)

Proton = 17

Electron = 17

Neutron = 18

2. For isotope B (chlorine 37)

Proton = 17

Electron = 17

Neutron = 20

Explanation:

Let isotope A be chlorine-35

Let isotope B be chlorine-37

1. For isotope A (chlorine-35)

Data obtained from the question include:

Atomic number = 17

Mass number = 35

Proton number = atomi number

Proton number = 17

Since the isotope is neutral,

Electron number = proton number

Electron number = 17

Mass number = Neutron + proton

Neutron = Mass number - proton

Neutron = 35 - 17

Neutron number = 18

2. For isotope B (chlorine-37)

Data obtained from the question include:

Atomic number = 17

Mass number = 37

Proton number = atomi number

Proton number = 17

Since the isotope is neutral,

Electron number = proton number

Electron number = 17

Mass number = Neutron + proton

Neutron = Mass number - proton

Neutron = 37 - 17

Neutron number = 20