1.1 ATOMIC STRUCTURE

Atomic number, Mass number and Isotopes

Section A

For each of the questions only one of the lettered responses (A-D) is correct.

Select the correct response in each case and mark its code letter by connecting the dots as illustrated on the answer sheet.

1. Which one of the following statements is not true about the atom ⁸¹₃₅X?

A It has mass number 35

B It contains 35 electrons

C It contains 46 neutrons

D Its nucleus contains 81 particles

2. Which one of the following statements about electrons is incorrect?

A They are negatively charged.

B They have a mass equal to approximately 1/1840 x the mass of the proton.

C They display both wave and particle properties.

D They are unaffected by a magnetic field.

3. Hydrogen has three isotopes, protium with no neutrons, deuterium with one neutron and tritium with two neutrons. In a sample trapped in a meteorite, they are found to occur in the ratio 10: 1: 1. What is the relative atomic mass of the hydrogen in the sample?

A 1.00

B 1.25

C 1.50

D 2.00

4. The relative atomic mass of an element is

A the mass of 6 x 10²³ atoms of the element

B the mass of one atom of the element on the scale ¹²₆C=12

C the average of the relative isotopic masses on the scale ¹²₆C=12

D the weighted average of the relative isotopic masses on the scale ¹²₆C=12

5. Which one of the following particles has seven protons, eight neutrons and nine electrons?

A ⁸₇N ^-

B ¹⁴₇N ^2-

C ¹⁵₈O ^-

D ¹⁵₇N ^2-

6. Atoms with different atomic numbers must have different

A numbers of protons

B numbers of neutrons

C numbers of electrons

D mass numbers

7. The atomic number of an element is the number of

A atoms in one mole

B electrons in the neutral atom

C protons plus neutrons in the atom

D protons plus electrons in the atom

8. An atom is electrically neutral because

A it contains neutrons

B the electrons are in a state of continuous motion about the nucleus

C it contains equal numbers of electrons and protons

D the nucleus contains both protons and neutrons

9. An atom which has lost an electron will be

A negatively charged

B neutral

C positively charged

D doubly positively charged

10. The nucleus of ²³₁₁Na contains

A 11 protons and 12 electrons

B 11 protons and 12 neutrons

C 12 neutrons and 11 electrons

D 23 protons and 11 electrons

11. The hydrogen atom ¹₁H and the deuterium atom ²₁H have

A a different number of electrons

B the same relative atomic mass

C a different number of protons

D a different number of neutrons

12. Neon occurs as two isotopes of mass numbers 20 and 22. Its relative atomic mass is 20.2. What is the percentage of ²⁰Ne in naturally occurring neon?

A 10

B 20

C 80

D 90

13. The relative atomic mass of boron is 10.81. The element consists of two isotopes, the nuclei of which contain five and six neutrons respectively. The ratio of the number of lighter atoms to the number of heavier atoms is approximately.

A 1 : 8

B 1 : 4

C 1 : 1

D 4 : 1

E 8 : 1

SECTION B

1.

(a) Explain what is meant by each of the following terms:

(i) electron

4^th electron removed from a shell nearer to the nucleus and at a lower energy level than the third. Therefore much more difficult to remove.

[1]

(ii) proton

4^th electron removed from a shell nearer to the nucleus and at a lower energy level than the third. Therefore much more difficult to remove.

[1]

(i) neutron

4^th electron removed from a shell nearer to the nucleus and at a lower energy level than the third. Therefore much more difficult to remove.

[1]

(ii) isotopes

4^th electron removed from a shell nearer to the nucleus and at a lower energy level than the third. Therefore much more difficult to remove.

[1]

(b) The atomic number provides three pieces of information about an element. What are they?

Number of protons / charge on nucleus / position in the periodic Table / number of electrons in a neutral atom

[3]

2.

(a) Explain what you understand by the terms:

(i) Mass number

[1]

(ii) Relative atomic mass

[1]

(b) Calculate the relative atomic mass of copper assuming it to contain 70% of ⁶³Cu and 30% of ⁶⁵Cu.

A_r (Cu) = 70 x 63 + 30 x 65/100 = 4410 + 1950/100 = 6363/100 = 63.60

[2]

3. Complete the table below:

Atom	Number of protons	Number of neutrons	Number of electrons
^7332Ge	32	41	32
^8537Rb	37	48	37

[2]

4. Electrons, protons and neutrons are the constituent particles of the atom.

Their masses based on the carbon- 12 standard are:

proton 1.007580 amu

neutron 1.008980 amu

electron 0.000548 amu

(amu = atomic mass unit)

(a) Explain the carbon- 12 standard.

[2]

(b) Complete the table showing the relative charges of the particles.

particle	charge
proton
neutron
electron

[2]

(c) Deuterium, ²H, is an isotope of hydrogen.

(i) Define the term isotopes

[2]

(ii) Draw a labelled diagram showing the constituent particles in a deuterium atom.

[2]

(iii) Name the instrumental method used to determine accurate atomic masses.

[1]

(iv) Use the values of the masses of the sub-atomic particles to calculate the mass, in amu, of a deuterium atom.

[1]

(v) The actual mass of a deuterium atom is 2.014102 amu. Calculate the mass lost, in amu, when the sub-atomic particles fuse together.

[1]

(vi) The mass lost is converted into radiant energy according to Einstein's equation E = mc² (m is measured in kilograms and c is the speed of light in m s^-1). Using this equation, together with E = hv, calculate the frequency of the radiation emitted.

(c = 3 x 10⁸ ms^-1, 1 amu = 1.66 X 10^-27 kg)

[4]

5 (a) Explain why ²³⁵ ₉₂U and ²³⁸ ₉₂U are regarded as isotopes of uranium.

Same atomic number but different mass numbers/Same number of protons but different numbers of neutrons

[2]

(b) The natural abundance of the two isotopes of uranium is:

²³⁵ ₉₂U 0.72% ²³⁸ ₉₂U 99.28%

Use the values to calculate the relative atomic mass of uranium to two decimal places.

Ar(U) = [(235 x 0.72) + (238 x 99.28)]/100 = 237.98

[2]

(c) Sketch the mass spectrum (not to scale) of naturally occurring uranium. Label the axes. [3]

Relative abundance

235 238 m/z