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Atomic structure for Edexcel A-level Chemistry

This page covers the following topics:

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

Atoms consist of electrons, neutrons and protons that have charges of โˆ’1, 0, +1 and masses of approximately 0, 1, 1 respectively. These properties are used to create formulae for finding the numbers of electrons, neutrons and protons in various particles. For example, atoms have an equal number of electrons and protons as the negative charge of electrons with the positive charge of protons results in them being neutral.

Each element has its specific atomic number that is the same as the number of protons in an atom of the element. On the other hand, the number of electrons and neutrons can be different in particles, even if they are derived from the same element. The number of electrons in an atom is the same as the atomic number of the element. The number of electrons in an ion is the atomic number of the element minus the charge of the ion.

The number of neutrons in monoatomic particle is (mass number or relative atomic mass) minus the atomic number. When relative atomic mass is know, it is commonly rounded to an integer to be used for these calculations. Sometimes, instead of the relative atomic mass, the mass number of a specific particle is given and is usually displayed as a superscript, just before the chemical symbol.

Subatomic particles

Isotopes of an element are atoms with the same number of protons but a different number of neutrons or, alternatively, atoms with the same atomic number but a different mass number. Most elements have multiple different isotopes with different atomic masses. To simplify mass calculations for a generic sample of an element, relative atomic mass is used.

Relative atomic mass is a weighted average of atomic masses of naturally existing isotopes of an atom and is measured in g/mol. Relative atomic mass is found by multiplying atomic mass by the abundance for each isotope, adding the results and dividing the sum by the sum of abundances. The abundances should usually add up to 100% or 1, meaning all isotopes are considered in a calculation.

Relative atomic mass

One of the ways to obtain abundances of isotopes is time of flight (TOF) mass spectrometry. During this process atoms are ionised and accelerated in an electric field by providing an equal amount of kinetic energy as long as the charges of the particles are the same. This results in lighter ions moving faster and they can therefore be detected separately and visualised on a TOF mass spectra.

The abundance of an isotope in a mass spectra is proportional to the height of the column representing an isotope. This means that the heights of columns can be used as abundances in relative atomic mass calculations without converting them to percentages. A single relative atomic mass unit corresponds to 1/12 of ยนยฒC atomic mass.

Mass spectrometry

1

Becca has obtained a mass spectra of magnesium shown in the image. Use the spectra to find the relative atomic mass of magnesium. The data provided has been adapted to the question.

The abundance of an isotope in a mass spectra is proportional to the height of the column representing an isotope. This means that the heights of columns can be used as abundances in relative atomic mass calculations without converting them to percentages. Relative atomic mass is found by multiplying atomic mass by the abundance for each isotope, adding the results and dividing the sum by the sum of abundances.

M = (23.7 ร— 24 + 3.0 ร— 25 + 3.3 ร— 26) รท (23.7 + 3.0 + 3.3)
M = (568.6 + 75 + 85.8) รท 30
M = 24.3 g/mol

24.3 g/mol

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2

Define isotopes.

Isotopes of an element are atoms with the same number of protons but a different number of neutrons or, alternatively, atoms with the same atomic number but a different mass number.

atoms with the same number of protons but a different number of neutrons

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3

One of hydrogen isotopes has 1 proton in each of its atoms. How many protons does a twice heavier hydrogen isotope atom have?

Each element has its specific atomic number that is the same as the number of protons in an atom of the element. Since both of the mentioned atoms are of hydrogen element, they both have 1 proton in each of their atoms.

1

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