Mid.sem.ex. Flashcards
Type of covalent bond in which valence electrons are free to move from atom to atom in the crystal structure
Metallic bond (metals), relatively weak bonds, non-directional.
Electrons are fully exchanged between the two ions so that both obtain a noble gas electron configuration
Ionic bonds (NaCl), ions are packed closely together.
Sharing of electrons between two atoms by overlapping orbitals so that noble gas electron configurations is obtained
Covalent bonds, strongly directional and specific g.
Sigma bonds: strongest, end to end overlap.
Pi bonds: weaker, side to side overlap.
Does not involve valence electrons, but result from weak electrostatic attraction due to short-term polarization of molecules.
Van der Waals bonds, additional to valence electron bonds, weak.
Result from electrostatic attraction between a proton in one molecule and an electronegative atom in the other.
Hydrogen bonds.
What increases when more shells are filled with electrons, and decreases when sub shells are filled with electrons, and which way on the table?
Atomic radius, bigger on the left and smaller towards the right. Determines how atoms fit together in a crystal structure.
What decreases when more shells are filled with electrons, and increases when sub shells are filled with electrons? Which way on the periodic table?
Electronegativity, bigger on the right (F) and smaller towards the left. Determines what bonds are formed between atoms.
What is the oxidation state of an element determined by?
How many electrons must be gained or lost to reach the noble gas config.
What are valence electrons, and which types do we have?
Valence electrons are electrons that occupy sub shells in the outermost shells that are not entirely filled. Anions (- charge) and cations (+ charge).
What is the most important control on the crystal structure of a mineral?
The type of chemical bonds that holds the elements together.
How does metal atoms tend to pack together?
In a highly ordered atomic arrangement that minimizes void space: closest packing.
Hexagonal closest-packing will have a sequence of layers like_____
ABABAB, the third layer is placed in depressions directly above the spheres in the first layer.
Cubic closest packing will have a sequence of layers like_____
ABCABC, the third layer is placed in depressions above the gaps in the first layer.
How will structures with ionic bonds approximately be?
Close-packed, with anions as close-packed structures and cations in interstitial sites between atoms.
Which five rules can be used to predict the crystal structure for ionic compounds?
Pauling’s rules:
- The Coordination Principle
- The Electrostatic Valency Principle
- Sharing of Polyhedral Elements I
- Sharing of Polyhedral Elements II
- Principle of Parsimony
The Coordination Principle (Pauling’s Rule 1) tells us that____
Large cations only fit in large interstitial sites, and a large interstitial site requires a large number of anions.
The relative size of the cation and anion determines by how many anions a cation will be surrounded.
What is Coordination Numbers (C.N.)?
The numbers of anions that surround a cation.
Depends on the radius ratio = R_c/R_a
The C.N. determines the coordination polyhedron forming around the cation.
Cations with C.N. that will from a VI (octahedral) polyhedron are____
Mn2+, Fe2+, Mg2+, Fe3+, Ti4+, Al3+
Cations with C.N. that will form a IV (tetrahedral) polyhedron are____
Al3+, Si4+, P5+, S6+
Cations with C.N. that will form a III (triangular) polyhedron are____
C4+
Cations with a C.N. that will form a VI-VIII (octahedral to cubic) tetrahedron are____
Na+, Ca+
Cations with a C.N. of VIII-XII (cubic to close-packed) are___
K+
The Electrostatic Valency Principle tells us that___
The total strength of the valence bonds that reach an anion from all nearest neighbor cations is equal to the charge of an anion.
The strength of an evb = (charge on the cation, Z)/(Coordination Number fo the cation, C.N.).
Which three types of ionic bonds do we have?
Isodesmic, anisodesmic, and mesodesmic.
What is an isodesmic bond, and which minerals tend to have this type of bonding?
All the ionic bonds have the same strength that is less than half of the charge on the anion.
All ionic minerals with a single cation and a single anion (NaCl) tends to have a close-packed structure.
What is an anisodesmic bond, and which minerals tend to have anisodesmic bonding?
The bond strength is non-uniform and some bonds take more than half of the charge on the anion.
All ionic mineral with small and highly charged cations that form distinct anisodesmic anionic groups (CO3, PO4, SO4) tends to have decreased symmetry to accommodate groups.
What is a mesodesmic bond, and which minerals tend to have this type of bonding?
The bond strength is non-uniform and some bonds take exactly half of the charge of the anion.
Each oxygen has a - 1 charge leftover that is not shared, and can be used to bond to another cation or SiO4 tetrahedron: polymerization of silica tetrahedra.
Sharing of Polyhedra I tells us that____
Repulsion between cations is stronger when polyhedra share edges or faces, so it is most stable when sharing corners.
Silicon tetrahedra never share faces.
Sharing of Polyhedra II tells us that___
High-charges cations minimizes the number of anions that they share between polyhedra so that the cations can be kept apart at the maximum distance.
Principle of Parsimony tells us that___
Crystal structures have a limited set of distinctively different cation and anion sites.
Covalent bonds get structures that___
Are directional because a specific geometry is needed to make the orbitals overlap, so the structures are NOT close packed.
Isostructural minerals and isostructural groups are___
Isostructural minerals have atoms that are arranged in the same type of crystal structure.
Isostructural groups have isostructural minerals and are chemically related by a common anion or anionic group.
NaCl and PbS.