Isotopes

Chloride is an element in Group 17 of the Periodic Table. It has two stable isotopes.

1. What do you understand by the term isotope?
2. Explain why isotopes of the same element have the same chemical properties

1. Isotopes are atoms of the same element with different nucleon numbers.
2. Chemical properties depend on the number or amount of (outer) electrons. Thus, the same chemical properties mean that they have the same number of electrons

The symbols below describe two isotopes of the element uranium: `""_92^235U` and `""_92^238U`

a. i. In what ways are these two isotopes of uranium identical?

    ii. In what ways do they differ?

b. In a mass spectrometer uranium atoms can be converted to uranium ions, `U^(2+)`. State the number of electrons present in one `U^(2+` ion.

a. i. Both have 92 protons and 92 electrons

    ii. Uranium-235 has 143 neutrons whereas uranium-238 has 146 neutrons

b. The number of electrons present in one `U^(2+` ion is 90

The table below shows the relative abundance of each isotope in a mass spectrum of magnesium.

a. How many molecular ion peaks are we expecting in the mass spectrum?

b. From the table above, calculate the relative atomic mass of this sample.

c. What does this tell you about the relative abundance of isotopes of magnesium?

a. There are 3 peaks expected in the mass spectrum

b. A_r = `(24xx0.735) + (25xx0.101) + (26xx0.164) = 24.43`

c. The magnesium-24 isotope is more abundant because the weighted average is the nearest to 24.

A sample of sulfur contains only two isotopes `"^32S` and `"^34S`. The relative atomic mass of this sample is 32.09

Calculate the percentage abundance of the isotopes present in the sample

Put the percentage abundance of the `"^32S` is `x`, then the percentage abundance of the `"^34S` is `100-x`.

We have the relative atomic mass of this sample is 32.09. That means that

`x/100 xx 32.0 + (100-x)/100 xx 34.0 = 32.09`

Solving `x`, we have `x` = 95.5

Therefore, 

% abundance `"^32S` = 95.5 % and `"^34S` = 4.5 %

Why are the physical properties of isotopes different?

The mass of an isotope is the primary determinant of its physical characteristics. Being aware of the distinctions enables us to distinguish between different isotopes.

Use of the Data Booklet is relevant to this question.

The isotope `"^99Tc` is radioactive and has been found in lobsters and seaweed adjacent to nuclear fuel reprocessing plants.

1. It has 13 more neutrons than protons.

2. It has 43 protons.

3. It has 99 nucleons.

Which statements are correct about an atom of `"^99Tc`?

A. 1, 2, 3 are correct

B. 1 and 2 only are correct

C. 2 and 3 only are correct

D. 1 only is correct

The answer is A

`"^99Tc` = 43 protons, 43 electrons and 56 neutrons

Therefore, the difference between neutrons and protons is `56-43=13`

A radioactive isotope of thallium, `""_81^201Tl`, is used to assess damage in heart muscles after a heart attack. Which statement about `""_81^201Tl` is correct? 

A. This isotope has a nucleon number of 120.

B. The number of electrons in one atom of this isotope is 81. 

C. The number of neutrons in one atom of this isotope is 201. 

D. `""_82^201X` is an isotope of `""_81^201Tl`.

The answer is B.

A. The nucleon number is the total number of protons and neutrons. Therefore, this isotope has a nucleon number of 201

B. This is correct because the number of protons in this isotope is 81. They are equivalent.

C. The neutrons = `201 - 81 = 120`

D. The isotope is when we have an atom with the same number of protons but different nucleon numbers.

In 2011 an international group of scientists agreed to add two new elements to the Periodic Table. Both elements had been made artificially and were called ununquadium (Uuq) and ununhexium (Uuh). 

1. One atom of Uuh has one more neutron than one atom of Uuq. 

2. One `Uuq^(2-` ion has the same number of electrons as one atom of Uuh. 

3. The `Uuh^+` ion has the same number of electrons as one `Uuq^-`  ion. 

Which statements about these elements are correct? 

A. 1, 2, 3 are correct

B. 1 and 2 only are correct

C. 2 and 3 only are correct

D. 1 only is correct

The answer is A

1. The neutron of Uuq is 175 and the neutron of Uuh is 176 

2. The number of electrons in `Uuq^(2-)` is 116 which is equal to one atom of Uuh

3. The number of electrons of `Uuh^+` is 115, the same as `Uuq^-`ion which has 115 electrons

Which species contains the smallest number of electrons?

A. `B^(3+)`

B. `Be^(2+)`

C. `H^-`

D. `He^+`

The answer is A

A. `B^(3+)` has a number of electrons of 2

B. `Be^(2+)`has a number of electrons of 2

C. `H^-`has a number of electrons of 2

D. `He^+`has a number of electrons of 1

When nuclear reactions take place, the elements produced are different from the elements that reacted. Nuclear equations, such as the one below, are used to represent the changes that occur. 

`""_92^235U + ""_0^1n rightarrow ""_56^144Ba + ""_36^89Kr + 3 ""_0^1n`

The nucleon (mass) number total is constant at 236 and the proton number total is constant at 92. 

In another nuclear reaction, uranium-238 is reacted with deuterium atoms, `""_1^2H`. An isotope of a new element, J, is formed as well as two neutrons. 

`""_92^238U + ""_1^2H rightarrow J + 2 ""_0^1n`

What is isotope J?

A. `"^238Np`

B. `"^238Pu`

C. `"^240Np`

D. `"^240Pu`

The answer is A

The number of nucleons will be the same as before and after reactions. 

Thus, in the second reaction, there are 240 nucleon numbers before the reaction leading to J having a nucleon number of 238 (`238+2-2xx1=238`).

Same as the atomic number,  the number of protons of J is `92+1-0=93`

Based on the periodic table, we have the isotope J is: `""_93^238Np`