Atomic structure

These are a few notes about atoms for Chemistry revision, I hope it helps

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  • Created by: catrin
  • Created on: 17-05-10 15:49

Atomic structure

Atoms consist of electrons surrounding a nucleus that contains protons and neutrons.

Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1, while electrons have a relative charge of -1.

The number of protons in an atom is called its atomic number. In the periodic table atoms are arranged in atomic number order.

Electrons are arranged in energy levels or shells, and different energy levels can hold different numbers of electrons. The electronic structure of an atom is a description of how the electrons are arranged, which can be shown in a diagram or by numbers. There is a link between the position of an element in the periodic table and its electronic structure.

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The structure of an atom

Although the word 'atom' comes from the Greek for indivisible, we now know that atoms are not the smallest particles of matter. Atoms are made from smaller subatomic particles.

At the centre of an atom is a nucleus containing protons and neutrons. Electrons are arranged around the nucleus in energy levels or shells. Make sure you can label a simple diagram of an atom like this one.

Both protons and electrons have an electrical charge. Both have the same size of electrical charge, but the proton is positive and the electron negative. The neutron is neutral.The electrical charge of particles

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The electrical charge of particles

particlerelative charge proton +1 neutron 0 electron -1

The total number of electrons in an atom is always the same as the number of protons in the nucleus. This means atoms have no overall electrical charge.

The number of protons in an atom is called its atomic number - also called the proton number. Atoms are arranged in the periodic table in order of increasing atomic number. You may need to re-visit the section in AQA GCSE Science on the periodic table to check you recall it.

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Energy levels and shells

Electrons are arranged in different shells around the nucleus. The innermost shell - or lowest energy level - is filled first. Each succeeding shell can only hold a certain number of electrons before it becomes full. The innermost shell can hold a maximum of two electrons, the second shell a maximum of eight, and so on.

The table gives the maximum capacity of the first four shells - which is as much as you need to know at GCSE.

Maximum capacity of the first four shells

energy level or shellmaximum number of electrons first 2 second 8 third 8 fourth 18

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Electronic structure 1

The electronic structure of an atom is a description of how the electrons are arranged. For your exam, you need to be able to describe the electronic structure of the first 20 elements in the periodic table. You may need to re-visit the section in AQA GCSE Science on the periodic table for this.

The first 20 elements in the periodic table run from hydrogen to calcium. Their electronic structures can be shown either as diagrams or numbers. You need to know how to do both.

Take lithium, for example. The drawing shows each energy level as a circle around the nucleus, with each electron represented by a dot. In the exam, do not worry about colouring in the electrons. Just make them clear and ensure they are in the right place. Sometimes you will be asked to use a cross rather than a dot. The numerical method is to write the chemical symbol (Li ) followed by the number of electrons in each energy level, innermost first, Li 2,1

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Electronic structure of lithium

ElementNumeric formatElectronsPeriodic table group

Structure of a lithium atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has one red dot on it, representing the second energy level with one electron (http://www.bbc.co.uk/schools/gcsebitesize/science/images/atom_lithium.gif)

Li 2,1

Lithium atoms have three electrons. Two of these fit into the first energy level, with the third in the second energy level.

Group 1

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Electronic structure 2

ElementNumeric formatElectronsPeriodic table group

Structure of a fluorine atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has seven red dots on it, representing the second energy level with seven electrons (http://www.bbc.co.uk/schools/gcsebitesize/science/images/atom_fluorine.gif)

F 2,7

Fluorine atoms have nine electrons. Two of these fit into the first energy level. The remaining seven fit into the second energy level.

Group 7

Structure of a neon atom. A black dot represents the nucleus. The small circle around this has two red dots on it, representing the first energy level with two electrons. A larger outer circle has eight red dots on it, representing the second energy level with eight electrons (http://www.bbc.co.uk/schools/gcsebitesize/science/images/atom_neon.gif)

Ne 2,8

Neon atoms have ten electrons. Two of these fit into the first energy level. The remaining eight electrons fit into the second energy level. Because its highest occupied energy level is full, neon is stable and unreactive.

Group 0 - that is, the eighth group

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