Three-Dimensional Structure Flashcards
Ionic Crystals
What is an ionic crystal?
- Due to the attractive forces that exist between the positive and the negative charges in an ionic compound, the ions will actually pack together very tightly, forming a 3D figure known as a “crystal lattice”. The lattice is a pattern of alternating positive and negative ions. Each ion is attracted to all the adjacent ions to it, of the opposite charge. So, when sodium ions and chloride ions join together to form NaCl, the formula NaCl(s) actually is referring to the ratio of sodium to chloride ions in the crystal(so in this crystal, they would be in 1:1 ratios). In CaF2, we would have a ratio of one calcium ion to two fluoride ions. The smallest ratio of ions in a crystal is called a formula unit.
- Some crystals are cubed, some are needle-like, and some are hexagonal columns. The shape depends on the way the ions pack together. The sizes and charges of the ions will also affect the pattern of the packing. If the crystals form slowly, the packing of the ions is uniform.
Network Solids
What are network solids?
- In a network solid (covalent bonding)- millions of atoms of the same element or two different elements, are covalently bonded in a variety of patterns. Four examples of network solids are carbon, but they have different physical properties, so we call them allotropes.
- Some network solids consist of two different elements- the most common compound in Earth’s crust, silicon dioxide, is one example. Sand and quartz consist almost entirely of silicon forming double bonds with the oxygen atoms. But, this is not typical for silicon. Instead silicon bonds to four oxygen atoms to form a tetrahedral shape.
Exceptions to the Octet Rule
The rule you have been provided with in terms of lewis structures, apply to almost all molecules. However, some exceptions to these rules are inevitable.
Elements that we can count on to obey the octet rule are carbon, nitrogen, oxygen and fluorine. On the flip side, some atoms exceed the octet rule. This behaviour is observed only for those elements in period three of the periodic table, and beyond. Some atoms can be electron deficient, and some can have a hyper octet,(exceeding eight electrons).
Provide 3 examples of ionic crystals found at home or in nature.
- Crystal of sodium chloride- common table salt.
- Crystal of sucrose- common table sugar.
- Crystalline form of pure carbon- uncut diamond.
According to laboratory evidence, each ion is attracted to all…
adjacent ions of the opposite charge.
What is a unit cell?
The smallest set of ions in a crystal that’s repeated over and over.
The smallest ratio of ions in a crystal such as NaCl(s) is called a…
formula unit.
What are the two factors that affect the shape of an ionic crystal?
a) The size of each ion.
b) The relative charge of each ion.
Why do ionic compounds exist in crystals while molecular compounds exist in molecules?
Ionic compounds are attracted to the opposite changed ion adjacent to them, whereas covalent bonds exist between pairs of atoms.
In ionic compounds with polyatomic ions, the two types of bonds present are_____(within the polyatomic ion) and ________(between the metal and polyatomic ion). Why do these compounds often decompose before they melt?
-Intramolecular
-Intermolecular
The intramolecular bonds are stronger than the intermolecular, causing them to decompose.
Explain each of these ionic compound properties:
- Solid at SATP
- Hard, brittle, break along smooth planes
- The crystal structure of the ionic bonds creates solids with high melting points and boiling points.
- The ions are close together and electrostatic repulsion(external forces) causes them to shatter.
- Ionic compounds have stronger bonds and all of the bonds must be broken to melt or boil.
Explain each of the metallic crystal properties:
- Shiney, silvery
- Flexible
- Conductive
- Crystalline
- The free electrons come into contact when they move which increases their energy. When the energy is released the metal shines.
- The delocalized electrons move around with external forces which allows the atoms to shift, making them malleable and ductile.
- The electrons hold energy and flow through the metal to the output and release the energy.
- The bonds form in tightly packed forms.
Solids that result from______ bonding, where the valence electrons are not held strongly by their atoms so they are free to move(electron sea).
-Metallic crystal
Properties of molecular crystals Pick the correct one and explain it. a) hard, not very hard, soft b) relatively low m.p. or relatively low m.p. c) conductive or non-conductive
Soft-intermolecular bonds are very weak.
relatively low m.p.-bonds can be easily broken.
non-conductive-valence electrons are not delocalized.
What are the properties of covalent network solids?
- Hard brittle solids
- High melting points
- Don’t dissolve in water or conduct
