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Atomic structure for OCR GCSE Chemistry

This page covers the following topics:

1. Atomic models
2. Subatomic particles

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

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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