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Atomic structure for OCR A-level Chemistry

This page covers the following topics:

1. Atomic models
2. Subatomic particles
3. Relative atomic mass
4. Mass spectrometry

According to Dalton atomic model, an atom is a smallest neutral particle that cannot be divided into smaller ones. This model has been disproven when negative particles electrons were discovered by Joseph John Thompson leading to a new plum pudding atomic model. According to this model an atom was considered to be a sphere consisting of negative electrons distributed in a positive dough.

Soon after the discovery of electrons, alpha scattering experiment has been performed by Ernest Rutherford that has suggested the existence of a nucleus at the centre of an atom. James Chadwick has then discovered that the nucleus exists of protons and neutrons. Niels Bohr has clarified that electrons in atoms are at specific distances from the nucleus called shells or energy levels.

Nowadays we use the nuclear model of an atom that combines many of the previous findings. According to the nuclear model an atom consists of a nucleus that is approximately 10,000 times smaller than the atom itself and shells of electrons surrounding the nucleus. The nucleus of an atom consists of protons and neutrons. Atoms have radii and bonds with an order of magnitude of 1 ร— 10โปยนโฐ m.

Atomic models

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

What discovery has led to Dalton atomic model being modified?

Dalton atomic model has been disproven when negative particles electrons were discovered by Joseph John Thompson leading to a new plum pudding atomic model.

discovery of an electron

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2

What was the discovery made by Ernest Rutherford using alpha particle scattering experiment?

Soon after the discovery of electrons, alpha scattering experiment has been performed by Ernest Rutherford that has suggested the existence of a nucleus at the centre of an atom.

nucleus of an atom

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3

Describe an atom according to Dalton model.

According to Dalton atomic model, an atom is a smallest neutral particle that cannot be divided into smaller ones.

a smallest neutral particle that cannot be divided into smaller ones

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