Atomic Interactions

States of matter Matter is made of particles that constant interact with each other, slipping and sliding past each other.

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In a solid, these particles are packed close together. The particles have a fixed position but can still vibrate independently.

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In a liquid, these particles can move more freely able to slip past each other. As a result, a liquid still has a consistent volume, but can change its shape to match that of the container which carries it.

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In a gas, these particles are widely separated and move much more freely than in a liquid. It allows them to spread out to occupy the entire container. Gases are able to change their volume and shape.

Physical states of the elements The physical states of the elements at room temperature are: 

Two elements are liquids – bromine and mercury

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Eleven elements are gases – oxygen, nitrogen, hydrogen, helium, neon, argon, krypton, xenon, radon, fluorine and chlorine. All of these are colourless except fluorine which is pale yellow and chlorine which is greenish yellow.

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The rest of the elements are solids.

Atomic and mass numbers Atomic number (Z) refers to the number of protons in the nucleus of the atom (not an ion) Mass number (A) refers to the number of protons and neutrons in the nucleus. Therefore to calculate the number of neutrons in a nucleus we subtract Z from A, that is: number of neutrons = A − Z Isotopes are atoms with the same number of protons i.e. same atomic number but different number of neutrons i.e. different mass number, e.g. 

12 13 14 6C, 6C, 6C

The atomic number is a specific quantity that distinguishes a carbon atom from an oxygen atom.

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The atomic number is also equal to the number of electrons (provided the atom has an net charge of 0)

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A nuclide is any atom of a specified atomic number and mass number, i.e. any atom that has the same number of protons and neutrons (collectively called nucleons).

Electron configurations 

The reactivity of atoms is determined by the way the electrons are arranged in the electron cloud.

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Electrons possess energy which is sufficient to resist the attraction towards the positive nucleus.

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The way electrons are arranged around the nucleus is called the electron configuration.

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For example: 

Neon has 10 electrons, its electron configuration is 2, 8

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Magnesium has 12 electrons, its electron configuration is 2, 8, 2

Energy levels There is a set of discrete energy levels available in an atom, called the first, second, third etc. energy levels.

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Electrons in the shell closest to the nucleus have the least energy, while the electrons in the shell furthest from the nucleus usually have more energy. The number of shells around a nucleus depends on the number of electrons. Each shell can only hold a certain

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number of electrons. The first shell can only hold 2, the second shell 8, the third shell 18 and the fourth shell 32. Not all atoms fill up their inner shells before starting to fill up their outer shells. Potassium with 19 electrons has

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the electron configuration 2, 8, 8, 1 while Calcium has the electron configuration 2, 8, 8, 2 Note the presence of the stable octet (8 electrons) in the outer shell (valence shell) of the noble gases. Shell

Maximum Number of Electrons

st

2 8

rd

18 32

1 nd 2 3 th 4

Orbitals Each electron shell (or energy level) is divided up into orbitals, which occupy different areas of the particular energy level. This allows for more complex electron configurations, which can be useful for explaining why not all atoms fill up their inner shells before filling their valence shell. The order of which the orbitals are filled is given by the diagram on the right. However even then it gets complicated requiring all sorts of rules to determine electron configurations; (and even with these rules, some elements, such as copper and gold, do not follow these rules). Element Name Boron Carbon Nitrogen Oxygen Fluorine Neon

Atomic Number 5 6 7 8 9 10

Electron Configuration 2.3 2.4 2.5 2.6 2.7 2.8

Electron Configuration (with orbitals) 2 2 1 1s 2s 2p 2 2 2 1s 2s 2p 2 2 3 1s 2s 2p 2 2 4 1s 2s 2p 2 2 5 1s 2s 2p 2 2 6 1s 2s 2p