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The atom for AQA A-level Physics

The atom

This page covers the following topics:

1. Model of the atom
2. Relative mass and charge
3. Proton and nucleon numbers
4. Isotopes

The plum pudding model of the atom, suggested by J J Thompson, states that atoms are made up of a positive dough-like material with negative electrons dispersed throughout it. This was consistent with scientific knowledge at the time, since the atom being solid explained why it couldn't be squashed and the electrons throughout it could be transferred between solids, causing static charge. Rutherford tested the theory of this model by firing positively charged alpha particles at a thin gold leaf in a vacuum. The expected result of the experiment was the alpha particles going straight through the gold foil with no deflection. However, it was found that most alpha particles passed through, a small number of them were deflected by large angles and an even smaller number of them came straight back. It was thus concluded that since most alpha particles went straight through, most of the atom was empty space. Also, since some alpha particles are deflected, there must be a concentration of positive charge in the atom. Further, since a very small number of alpha particles come straight back, the positive charge must be concentrated in a tiny volume in the atom, which was called the nucleus. Rutherford's new model of the atom was thus established, which said that the atom was made up of a positively charged nucleus surrounded by empty space and electrons on the outside of it. Later on, Bohr added onto Rutherford's model that the electrons orbit the nucleus of the atom in different energy levels. This explained why burning chemicals resulted in specific coloured flames. Chadwick finalised the current model of the electron. He suggested that a proton-sized neutral particle called the neutron existed in the nucleus to help keep it stable and to make up for the difference between the atomic and mass number.

Model of the atom

Most of the atom is empty space, since the nucleus is very small compared to the atom, with a radius of up to 100000 times less than than of the atom, and electrons are even smaller. This shows that the masses of the subatomic particles (protons, neutrons and eelctrons) are tiny, therefore they are usually not given in the SI units of kilograms, but rather their relative masses are used. The mass of protons and neutrons are a lot more than that of eletcrons, which shows that most of the mass of an atom is concentrated in the nucleus.

Relative mass and charge

The mass number (A), also called the nucleon number, is the total number of protons and neutrons in an atom. The proton number (Z) is the number of protons in an atom. Atoms of the same element will always have the same proton number. The mass number can vary, however it can never be less than the proton number.

Proton and nucleon numbers

Isotopes are atoms of the same element with different mass numbers, due to the different number of neutrons in the nucleus. Isotopes have the same physical and chemical properties, since the number of electrons and protons is what governs these. However, tc

Isotopes

1

A bromine atom has 35 protons and 45 neutrons. State its atomic and mass number.

Atomic number = 35. Mass number = 35 + 45 = 80.

A bromine atom has 35 protons and 45 neutrons. State its atomic and mass number.

2

Define atomic number.

Atomic number is the total number of protons in an atom.

Define atomic number.

3

What was Bohr's addition to the model of the atom and what did it help explain?

Bohr added onto the model of the atom that the electrons orbit the nucleus of the atom in different energy levels. This explained why burning chemicals resulted in specific coloured flames.

What was Bohr's addition to the model of the atom and what did it help explain?

4

A carbon-12 isotope has 6 protons. State the atomic number of a carbon-14 isotope.

The number of protons in an atom of a specific element will always be the same, therefore the atomic number of the carbon-14 isotope is 6.

A carbon-12 isotope has 6 protons. State the atomic number of a carbon-14 isotope.

5

The atomic number of potassium is 19. Given that a potassium atom has 20 neutrons, calculate the mass number of the atom.

Mass number = 19 + 20 = 39.

The atomic number of potassium is 19. Given that a potassium atom has 20 neutrons, calculate the mass number of the atom.

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