Why ions need to make crystals
Answers
Answer:
They are not forced to make crystals they make crystals in order to satisfy the inter-ionic Coulombs force
Answer:
Why ions need to make crystals?
Ions bound together by electrostatic attraction form ionic crystals. Their arrangement varies depending on the ions' sizes or the radius ratio (the ratio of the radii of the positive to the negative ion). ... The properties of ionic crystals reflect the strong interactions that exist between the ions.
Types of Crystals
Ionic Crystals
Ions in ionic crystals are bound together by electrostatic attraction.
LEARNING OBJECTIVES
Describe how ions form ionic crystals.
KEY TAKEAWAYS
Key Points
Ions bound together by electrostatic attraction form ionic crystals. Their arrangement varies depending on the ions’ sizes or the radius ratio (the ratio of the radii of the positive to the negative ion).
potassium fluoride (KF)
potassium chloride (KCl)
potassium bromide (KBr)
potassium iodide (KI)
sodium fluoride (NaF)
A Case Study: NaCl
The properties of NaCl reflect the strong interactions that exist between the ions. It is a good conductor of electricity when molten (melted state), but very poor in the solid state. When melted, the mobile ions carry charge through the liquid. NaCl crystals are characterized by strong absorption of infrared (IR) radiation, and have planes along which they cleave easily. Structurally, each ion in sodium chloride is surrounded by six neighboring ions of opposite charge. The resulting crystal lattice is of a type known as “simple cubic,” meaning that the lattice points are equally spaced in all three dimensions and all cell angles are 90°.
image
NaCl crystal structure: Spheres represent the Na+ and Cl- ions. Each ion is surrounded by six other ions of opposite charge, therefore NaCl is described as having (6,6) coordination.
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Halite: Halite, or rock salt, is the mineral form of sodium chloride. Halite forms cubic crystals. It occurs in evaporite minerals that result from the drying up of enclosed lakes and seas. This picture was taken in Wieliczka, Poland, one of the world’s oldest salt mines.
Energy of Formation of Ionic Salts
Since ionic salts have a lower energetic configuration than their individual elements, reactions forming ionic solids tend to release energy. For example, when sodium and chlorine react to form sodium chloride:
Na(s) + ½Cl2(g) → NaCl(s) + 404 kJ
The release of 404 kJ of energy shows that the formation of solid sodium chloride is exothermic. Due to the Second Law of Thermodynamics, the released energy spreads out into the environment and is therefore unavailable to drive the reverse reaction. This irreversibility is the main reason that sodium chloride is more stable than its component elements.
Lattice Energy
When sodium and chloride ions react to form NaCl, 787 kJ/mol of energy is released:
Na+(g) +Cl–(g) → NaCl(s) + 787 kJ
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CsCl structure: In CsCl, metal ions are shifted into the center of each cubic element of the Cl–-ion lattice. Each cesium ion has eight nearest-neighbor chloride ions, while each chloride ion is also surrounded by eight cesium ions in (8,8) coordination.
The two kinds of lattice arrangements exemplified by NaCl and CsCl are found in a large number of other 1:1 ionic solids, and these names are used generically to describe the structures of these other compounds. There are many other fundamental lattice arrangements (not all cubic), but the two described here are sufficient to illustrate the point that the radius ratio (the ratio of the radii of the positive to the negative ion) plays an important role in the structures of simple ionic solids.
Covalent Crystals
Atoms in covalent solids are covalently bonded with their neighbors, creating, in effect, one giant molecule.
LEARNING OBJECTIVES
Discuss the properties of covalent crystals or network solids
KEY TAKEAWAYS
Key Points
Covalent (or network) solids are extended- lattice compounds, in which each atom is covalently bonded to its nearest neighbors. Because there are no delocalized electrons, covalent solids do not conduct electricity.
The rearranging or breaking of covalent bonds requires large amounts of energy; therefore, covalent solids have high melting points.
Covalent bonds are extremely strong, so covalent solids are very hard. Generally, covalent solids are insoluble due to the difficulty of solvating very large molecules
Key Terms
covalent network solid: A solid formed when the atoms are bonded covalently in a continuous, extended network.
covalent bond: A type of chemical bond where two atoms are connected to each other by the sharing of two or more electrons.
diamond: a glimmering glass-like mineral that is an allotrope of carbon, in which each carbon atom is bonded to four others with a tetrahedral geometry.
carbide: A compound composed of carbon and a less electronegative element.
allotrope: A different form of an element in its natural state. For instance, oxygen is found predominantly in two forms: O2
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