4 chemical bonding and structure Flashcards
4.1 what is an ion?
an ion is a charged particle. it forms from atoms or groups of atoms when they lose or gain one or more electrons
why do chemicals bond?
to achieve stability
definition of cations
atoms that lose electrons to form a noble gas electron configuration and have a positive net charge
definition of anions
atoms that gain electrons to form a noble gas electron configuration and have a negative net charge
what is an ionic bond
electrostatic attraction experienced between the electric charges of a cation and anion
what is a lattice?
structures that consists of three dimensional repeating units of positive and negative ions
state that transition metals
can form more than one ion
conditions for ionic bond
no of valence electrons
between a metal and non metal
between atoms with low and high electronegativity
exothermic reaction
big diff in electronegativity
what determines ionic character
- position of elements in the periodic table
- electronegativity
electronegativity of ionic compounds
electronegativity difference of about 1.8 is the point where a bond may be thought of as ionic, there are a few exceptions to this rule such as HF, in which the electronegativity difference is 1.9, but the molecular properties are decidedly covalent
what is lattice enthalpy?
strength of force between ions
what is the magnitude of lattice enthalpy affected by?
higher the charge and smaller the size of ions; larger the lattice enthalpy and more energetically stable the compound
volatility of ionic compounds
volatility is the tendency of a substance to vaporise
for ionic compounds the electrostatic forces of attraction are strong hence volatility is low
electrical conductivity of electrical compounds
electrons are not free to move in solid state however they are free to move in molten state and able to conduct electricity
solubility of ionic compounds
dissolve in polar solvents such as water since partial charges on water are attracted to ions in the lattice
4.2 what is covalent bond?
a covalent bond is formed by the electrostatic attraction between a shared pair of electrons and positively charged nuclei
what are lone pairs?
lone pairs are non bonding pairs of electrons
types of covalent bonds
single covalent bond (1 pair of e shared)
double covalent bond (2 pair of e shared)
triple covalent bond (3 pair of e shared)
what is a dative/coordinate covalent bond?
formed when both electrons of the shared
pair of electrons originate from the same
atom
symbol of a coordinate covalent bond
an arrow on the head of the bond is used to indicate the origin of the electron pair
what is a polar covalent bond?
covalent bond in which electrons are shared unequally
- due to unequal electronegativity
- thus bonded atoms acquire a partial positive/negative charge
trend in bond strength
triple bond > double bond > single bond
trend in bond length
single bond > double bond > triple bond
properties of covalent bonds
formed between non metals
covalent compounds consist of molecules
low mp and bp
may be volatile
typically insoluble in water
do not conduct electricity since there are no ions to carry charge
4.3 what is the octet rule?
atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons
exceptions on the octet rule
second-row elements B and Be often have
fewer than eight electrons around them in their
compounds (e deficient molecules)
second-row elements never exceed
the octet rule, since their valence orbitals
(2s and 2p) can accommodate only eight
electrons
third-row and heavier elements often
satisfy the octet rule but can exceed the
octet rule by using their empty valence d
orbitals (expand octet)
properties of electron deficient molecules
molecules with incomplete octets, which tend to accept an electron pair from a molecule with a lone pair
explanation of the order of electron repulsion among lone pair and bonding pair
electron repulsion trend
LP – LP > LP – BP > BP – BP
- orbitals that hold lone pairs are rounder and shorter and spread more easily
- orbitals that hold bonding pair are more elongated
definition of lewis structure
a diagram of molecules in which the valence e of the atom are represented by dots, and the sharing of e to form a covalent bond is shown
steps to draw a lewis structure
step 1
a. put the less electronegative atom at the center
b. use only “—” single bond to link the central atom to all the surrounding atoms
c. use only “..” lone pairs to complete the octet of each surrounding atom
step 2
a. calculate total no. of valence e used so far
b. calculate the total no. of valence e from all atoms
c. calculate difference, put the extra electrons as “x x”
lone pair or “x” single unpaired electron on
the central atom
step 3
a. calculate the Formal Charge (F.C.) for each atom
Formal Charge = valency – no. of electrons assigned to the atom
b. use the lone pairs on the surrounding atoms to make “=” or “≡” bonds to lower the Formal Charge on each atom as close to zero as possible
comment of formal charge
if a nonzero Formal Charges are present;
make sure the positive formal charge is on the less electronegative atom and the negative formal charge is on the more electronegative atom
VSPER theory
Valence Shell Electron Pair Repulsion Theory
total number of electron domains determines the shape of a covalent molecule
- repulsion applies to both bonding and non-bonding pairs (lone pairs) of electrons
- the total number of electron domain around the central
atom determines the electron domain geometry - lone pairs have a higher concentration of charge than a
bonding pair because they are not shared between two
atoms and so they cause more repulsion than bonding
pairs
difference between electron domain geometry and molecular geometry
The electron domain geometry is determined by the positions of all the electron domains
Molecular geometry (arrangement of atoms in space) depends on the positions of the bonded atoms
