S2.2 Covalent Flashcards
what is a covalent bond?
formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei
what does the theory of bonding propose?
that molecules are formed by overlapping regions of space allowing atoms to mutually share pairs of electrons
what is the linear combination of atomic orbital theory(LCAO)?
regions of space where electrons are found are called orbitals and the bond comes from direct overlap of atomic orbitals
what is the octet rule?
the tendency of atoms to gain a valence shell with 8 electrons
what is the expanding the octet rule?
being able to accommodate more than 8 electrons in the outer shell and can be done if they use d subshell
what is electron deficient?
accommodating less than 8 electrons in the outer shell
how do london forces(induced dipole dipole) form?
temporary dipole, electrons are constantly moving, electrons on an adjacent atom are repelled by the negative part of the dipole which results in an attraction
what does the strength of London(dispersion) forces depend on?
the number of electrons in the molecule, SA of the molecules
how does the no. of electrons affect the forces?
more electrons=greater likelihood of a distortion
how does the SA of a molecule affect the London forces?
larger SA=more contact with adjacent molecules
how do dipole-dipoles form?
molecules with permanent dipoles are attracted to each other by permanent dipole-dipole bonding
how do dipole-induced dipole attractions form?
when the permanent dipole of a polar molecule causes a temporary separation of charge on a non-polar molecule
how does hydrogen bonding form?
strongest intermolecular force-type of permanent dipole-permanent dipole bonding
what does the term Van der Waals’ forces include?
London dispersion, dipole-induced dipole, dipole-dipole
what is a coordinate bond and give an eg.
when one atom uses a lone pair of electrons to form a covalent bond eg with NH3 and a H+ ion to form ammonium
in an expanded octet what does the charge signify?
no. of single bonds
why does molecular geometry change when there are lone pairs on the central atom?
lone pairs are closer to the central nucleus so closer to bonding pairs, so lone pairs repel bonding pairs more strongly, so they move closer together and the bond angles decrease, 2 lone pairs cause greater repulsion than 1, so bond angle decreases as their numbers increase
describe structure of diamond
tetrahedral array, very hard, high m.p. and b.p., cant conduct electricity
describe structure of graphite
layered structure, hexagonal rings, delocalised electrons, high mp., can conduct within layers but not through them
describe structure of graphene
single layer of graphite, flexible, can conduct
describe structure of buckminsterfullerene
gigantic structure, 60 carbon atoms, alternating hexagons and pentagons, weak intermolecular forces, low mp, can’t conduct
what is average bond enthalpy?
the strength of the bond
how does the size of the + charge on the nucleus effect electronegativity?
across a period, no. of protons increases, so increased + charge increases attraction between nucleus and pair of electrons in covalent bond so elements on right are more electronegative
how does the atomic radius effect electronegativity?
smaller atomic radius=closer the bonding electrons are to nucleus, across of a period atomic radius decreases so on right elements more electronegative
how does shielding affect electronegativity?
more shielding=more electronegative
draw the 2 ways to show a polar bond
S+ and S- and the arrow pointing towards more electronegative atom with a line near the start
what is an overall polarity called?
a dipole moment
write up induced dipole-dipole; permanent dipole-dipole and hydrogen bonding
properties of water because of hydrogen bonding
high m.p. and b.p.
why is the solid form is less dense than liquid eg how ice floats on water?
in water, sometimes molecules are closer sometimes further apart so H bonds constantly being formed and broken, as water cools molecules move slower, reaching the freezing point(0), water molecules arrange themselves into ordered structure(ice) which is stabilised by network of H bonds, in ice water molecules further apart than in liquid making ice less dense than liquid water so floats on surface which insulates water below preventing from freezing under
properties of simple molecular substances
atoms are covalently bonded to each other, have small molecules w a fixed no. of atoms
