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Atomic structure for Edexcel A-level Chemistry

Atomic structure

This page covers the following topics:

1. Subatomic particles
2. Relative atomic mass
3. Mass spectrometry

Each element has a characteristic number of protons, called the atomic number, which can be found in the periodic table. In atoms the number of electrons is the same as the number of protons. Since they have opposite charges, this ensures that every atom is neutral.

In atoms:
number of protons = atomic number
number of electrons = atomic number
number of neutrons = mass number โˆ’ atomic number

For charged ions, the number of negatively charged electrons changes. Negative ions will have a higher number of electrons since they have a negative charge that comes from the negative electrons. Thus, positive ions will have the usual number of electrons that belongs to the atoms minus their charge which indicates the number of electrons lost.

Subatomic particles

Relative atomic mass is a weighted average mass of all the isotopes of an element. Weighted means that it takes into account the natural abundance of each isotope.

To find the relative atomic mass from given atomic masses of isotopes and their abundances, multiply corresponding atomic masses and their abundances in decimals and add the multiplication products together. If you use abundances as percentages, divide your final result by 100%.

Relative atomic mass

A mass spectra shows the proportion of sample ions with each m/z value. Thus, this peak height is proportional to the abundance of compounds or isotopes when determining the relative atomic mass. When analysing m/z graphs, the peak with the highest m/z value in it is the molecular peak. Sometimes there might another small peak after molecular one arising due to various isotopes present in a molecule. This small peak may be ignored for the determination of the molecular peak value due to its small abundance.

In mass spectrometry a sample is vaporised and ionised by bombarding a sample with electrons (electron impact, loss of 1 electron) or adding protons (electrospray ionisation, gain of 1 proton). The electrons break some of the weaker covalent bonds, while the protons attach themselves to the molecules without much splitting. Since the charge of mass spectrometry ions is usually +, the mass of the molecule analysed is usually the same as the m/z value at the molecular peak when using electron impact. If the electronspray ionisation is used instead, because of the additional attached proton the mass of the molecular peak is bigger than the molecular mass by 1 unit. The base peak in an m/z graph is the highest peak.

Mass spectrometry

1

Identify the element and ionic charge from the diagram representing an ion.

atomic number = number of protons = 4
Beryllium has an atomic number of 4 and as an atom has 4 electrons.
2 electrons are missing, thus, this is an ion with a charge of 2+.

beryllium, 2+

Identify the element and ionic charge from the diagram representing an ion.

2

What is the relative atomic mass of platinum correct to 3 decimal places? The data in the table has been adapted for the question.

abundance of ยนโนยฒPt (aโ‚) = 0.782% = 0.00782
abundance of ยนโนโดPt (aโ‚‚) = 32.864% = 0.32864
abundance of ยนโนโตPt (aโ‚ƒ) = 33.787% = 0.33787
abundance of ยนโนโถPt (aโ‚„) = 25.211% = 0.25211
abundance of ยนโนโธPt (aโ‚…) = 7.356% = 0.07356
RAM = mโ‚aโ‚ + mโ‚‚aโ‚‚ + mโ‚ƒaโ‚ƒ + mโ‚„aโ‚„ + mโ‚…aโ‚…
RAM = 192 ร— 0.00782 + 194 ร— 0.32864 + 195 ร— 0.33787 + 196 ร— 0.25211 + 198 ร— 0.07356 = 195.121

195.121

What is the relative atomic mass of platinum correct to 3 decimal places? The data in the table has been adapted for the question.

3

Boron has two main isotopes, ยนโฐB (20%) and ยนยนB (80%). What is the relative atomic mass of boron? The data provided has been adjusted for the question.

abundance of ยนโฐB (aโ‚) = 20% = 0.2
abundance of ยนยนB (aโ‚‚) = 80% = 0.8
RAM = mโ‚aโ‚ + mโ‚‚aโ‚‚ = 10 ร— 0.2 + 11 ร— 0.8 = 10.8

10.8

Boron has two main isotopes, ยนโฐB (20%) and ยนยนB (80%). What is the relative atomic mass of boron? The data provided has been adjusted for the question.

4

Bromine has two stable isotopes, โทโนBr and โธยนBr with their abundances respectively being 51% and 49%. What is the relative atomic mass of bromine?

abundance of โทโนBr (aโ‚) = 51% = 0.51
abundance of โธยนBr (aโ‚‚) = 49% = 0.49
RAM = mโ‚aโ‚ + mโ‚‚aโ‚‚ = 79 ร— 0.51 + 81 ร— 0.49 = 79.98

79.98

Bromine has two stable isotopes, โทโนBr and โธยนBr with their abundances respectively being 51% and 49%. What is the relative atomic mass of bromine?

5

A protein underwent electrospray ionisation mass spectrometry, what is the relative molecular mass of the protein? What is the smaller peak caused by?

During electrospray ionisation an additional Hโบ is attached to a molecule.
Thus, the molecular ion peak at 530.1 m/z corresponds to MHโบ.
Since the charge of the ion is +, the mass is 530.1 a. u.
Subtracting the mass of a proton from 530.1 a. u. gives a mass of 529.1 a. u.
531.1 m/z is due to the presence of an isotope (ยฒH or ยนยณC).

529.1, H and C isotopes

A protein underwent electrospray ionisation mass spectrometry, what is the relative molecular mass of the protein? What is the smaller peak caused by?

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