Types and properties of solids

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1 Types and properties of solids

The order of the arrangement of the particles in a solid can be divided into crystalline and amorphous solids
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Crystals are usually solid substances composed of mass points (atoms, molecules or ions) periodically and repeatedly arranged in space according to a certain law
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The crystal has a regular shape and forms a regular polyhedron; the crystal has a fixed melting point.

Non-crystals are also called amorphous bodies.
The arrangement of particles is irregular and there is no regular shape
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Amorphous crystals do not have a fixed melting point and gradually soften when heated and finally become liquid, with a wide temperature range

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2 Types of crystals

According to the nature of the particles in the crystal, crystals can be divided into four basic types, namely molecular crystals, ionic crystals, atomic crystals and metal crystals
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Molecular crystals

The orderly arranged mass points in molecular crystals are molecules
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For example, the ordered particles in ice are all H ₂ O molecules, and the ordered particles in dry ice are all CO ₂ molecules

Molecular crystals have poor electrical conductivity because the molecules have poor ability to transfer electrons
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Ionic crystals

The orderly arrangement of particles in ionic crystals are positive ions and negative ions
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For example, the ordered mass points in MgO crystals are Mg ²⁺ and O ^2-

After the ionic crystal is melted, it can be electrolyzed to transfer electrons, so it can conduct electricity
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After being dissolved in water, ionic crystals can be electrolyzed or help balance the partial non-electrically neutral charges in the water when the water is electrolyzed, so the aqueous solution of ionic crystals conducts electricity

Please note that the essence of ionic crystal conduction is not the directional movement of ions, but the directional movement of electrons
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The directional movement of electrons must be achieved through electrolysis

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Atomic crystals

The orderly arranged mass points in atomic crystals are atoms
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For example, the ordered mass points in diamond are C atoms, and the ordered mass points in quartz are Si atoms and O atoms

In atomic crystals, atoms are connected to each other by covalent bonds
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Because the covalent bond is very strong, the melting point of the atomic crystal is high, the hardness is high, and the conductivity is poor

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Metal Crystal

The orderly arranged mass points in metal crystals are metal atoms
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Some properties of metal crystals are quite different.
For example, the melting point of sodium is very low and the texture is very soft; while the melting point of tungsten is very high and the hardness is high
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These differences are caused by the strength of the metal bond
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There is no clear boundary between metal crystals and atomic crystals.
Metal crystals can be regarded as special cases of atomic crystals, that is, when all the atoms in the atomic crystals are metal atoms, they become metal crystals
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Arsenic has the highest melting point among the same group of elements.
It is generally considered that arsenic is a metal crystal.
We think it is more appropriate to regard arsenic as an atomic crystal, but arsenic has a large atomic radius, and its hardness and melting point are significantly lower than other atomic crystals
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The above are the most common types of crystals, and this distinction is not absolute
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For example, graphite is a kind of mixed crystal, with intermolecular forces between layers and covalent bonds within the layers
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