2.1.1 (a) Isotopes

What does this mean?

Hopefully, you recall from previous years that Isotopes are different versions of the same element.

They have the same Chemistry (react in exactly the same way) but different masses.

How?

With any luck you'll also remember something about sub-atomic particles and bonding (if not, you will soon).

All atoms contain protons in their nucleus.

They contain an equal number of electrons in their shells.

These two particles have electric charge and so attract each other.

This is what causes their chemical behaviour - what they react with and how they "stick together".

But neutrons also exist.

They are neutral - they have no charge.

So having 1 or 2 extra neutrons doesn't change the way the atoms behave chemically.

But because neutrons have mass it still makes them heavier - this is a physical property.

Extra neutrons may also make an atom more or less stable - in other words, it may make it less radioactive or more.

This is also Physics - and therefore of no interest.

Exam-style questions

1. (a) Give the relative charge and relative mass of an electron.

Relative charge ...........................................................................................................

Relative mass .............................................................................................................. (2)

(b) Isotopes of chromium include 54Cr and 52Cr

(i) Give the number of protons present in an atom of 54Cr

...........................................................................................................................

(ii) Deduce the number of neutrons present in an atom of 52Cr

...........................................................................................................................

(iii) Apart from the relative mass of each isotope, what else would need to be known for the relative atomic mass of chromium to be calculated?

..............................................................................................................................................................................................................................................(3)

(2)

(b) An atom of element Q contains the same number of neutrons as are found in an atom of 27A1. An atom of Q also contains 14 protons.

(i) Give the number of protons in an atom of 27A1.

...........………………………………………………………………………….

(ii) Deduce the symbol, including mass number and atomic number, for this atom of element Q.

...........………………………………………………………………………….(3)

(c) Define the term relative atomic mass of an element.

.....................…................................................................................................................………………………………

.................................................................................................................................………………………………………(2)

(d) The table below gives the relative abundance of each isotope in a mass spectrum of a sample of magnesium.

Use the data above to calculate the relative atomic mass of this sample of magnesium.

Give your answer to one decimal place.

.....................…………………………………………….......................................................................................................

.....................…………………………………………….......................................................................................................

.....................…………………………………………….......................................................................................................(2)

1. (a)

Relative charge = –1 (1)

Relative mass £ 1/1800 or £ 5.55 x 10–4 (1)

Nb. zero / negligible has always been an acceptable answer but it is unclear yet whether this will still be the case with the new syllabus

(b)

(i) Protons = 24 (1)

(ii) Neutrons = 28 (1)

(iii) Need (relative) abundance /% / fraction of each isotope of the element (1)

2. (a)

Proton mass = 1 charge = +1 1

Electron mass £ 1/1800 Or £ 5.6 × 10–4 charge = –1

(Do not accept +1 for proton mass or ‘g’ units) 1

(b) (i) 13 1

(ii) Si 1

Mass number = 28 and atomic number = 14 (Do not accept 28.1 or 28.0 or ‘Silicon’) 1

(c)

Mean (average) mass of an atom / all the isotopes 1

1/12th mass of atom of 12C 1

Or Mass of 1 mole of atoms of an element (1)

1/12th mass of 1 mole of 12C (1)

Or Average mass of an atom / all the isotopes (1)

relative to the mass of a 12C atom taken as exactly 12 / 12.000 (1)

(Penalise ‘weight’ once only) (Ignore ‘average’ mass of 12C)

(Do not allow ‘mass of average atom’)

(d) Ar = (24 × 0.735) + (25 × 0.101) + (26 × 0.164) 1

= 24.4 1

(mark M2 conseq on transcription error or incorrect addition of %)

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