Unit 1: KA2 Structure and bonding Flashcards
Elements are held together in different ways and the properties of chemical compounds are determined by the bonding between atoms and the attractive intermolecular forces between molecules.
What do atoms share in covalent bonds?
Pairs of electrons.
Describe COVALENT BONDING.
When two positive nuclei are held together by their common attraction for the shared pair of electrons.
*an example of an intramolecular force
When are POLAR COVALENT bonds formed?
When the attraction of the atoms for the pair of bonding electrons is different.
Delta positive (δ⁺ or 𝛿⁺) and delta negative (δ⁻ or 𝛿⁻) notation can be used to indicate…
The partial charges on atoms, which give rise to a dipole.
How can ionic formulae be written?
By giving the simplest ratio of each type of ion in the substance.
Describe IONIC BONDING.
The electrostatic attraction between positive and negative ions.
What structure do ionic compounds form?
Ionic compounds form lattice structures of oppositely charged ions.
PURE COVALENT bonding (non-polar bonding) and IONIC bonding can be considered as…
Opposite ends of a BONDING CONTINUUM.
What (type of bonding) lies in the middle of the BONDING CONTINUUM?
POLAR COVALENT bonding.
What does the difference in electronegativities between bonded atoms indicate?
The ionic character.
The smaller the difference in electronegativities between bonded atoms, the more polar the bond will be. True or False.
False, the BIGGER the difference in electronegativity, the more polar the bond will be.
If the difference in electronegativities is LARGE, then the movement of bonding electrons from the element of lower electronegativity to the element of higher electronegativity is complete, resulting in the formation of ions. True or False?
*Feel free to rephrase
True
*LARGE DIFFERENCE in electronegativities = bonding electrons from lower electronegativity go to higher electronegativity = forms ions.
Compounds formed between metals and non-metals are often, but not always —–.
IONIC.
Physical properties of a compound should be used to deduce the type of bonding and structure in the compound. Name the physical properties you should refer to. [5 marks]
State at room temperature [1],
Melting point [1],
Boiling point [1],
Solubility [1],
Electrical conductivity [1].
All molecular elements and compounds, and monatomic elements condense and freeze at sufficiently low temperatures. What must exist between the molecules or discrete atoms for this to occur?
At least one form of attractive force.
Intermolecular forces acting between molecules are known as…
van der Waals forces.
There are several different types of van der Waals forces. Name the 3 forces we focus on in Higher Chemistry. [3 marks]
London dispersion forces [1],
AND
permanent dipole-permanent dipole interactions [1] that include hydrogen bonding [1].
London dispersion forces are forces of attraction that can only operate between a few atoms and molecules. True or False?
False. They can operate between ALL atoms and molecules.
London dispersion forces are much STRONGER/WEAKER than all other types of bonding.
What does the strength of London dispersion forces relate to?
WEAKER.
The STRENGTH of London dispersion forces is related to the NUMBER OF ELECTRONS within an atom or molecule.
How are London dispersion forces formed?
As a result of electrostatic attraction between dipoles (both temporary and induced –> caused by the movement of electrons in atoms and molecules.)
A molecule is described as POLAR if it has a ——— ——.
The spatial arrangement of polar covalent bonds can result in a molecule being polar. True or False?
PERMANENT DIPOLE.
True.
Besides London dispersion forces, what are additional electrostatic forces of attraction between polar molecules?
Permanent dipole-permanent dipole interactions.
Which is the stronger force of attraction for molecules with similar numbers of electrons? Permanent dipole-permanent dipole interactions OR London dispersion forces.
Permanent dipole-permanent dipole interactions are stronger.
Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen are HIGHLY —–.
POLAR.
Describe the bond between a hydrogen atom and a strongly electronegative element (fluorine, oxygen or nitrogen).
It’s a HIGHLY POLAR bond.
What’s a HIGHLY POLAR bond?
Describes the bond between a hydrogen atom (an atom with very few protons = low electronegativity) and a strongly electronegative element such as fluorine, oxygen or nitrogen.
Describe HYDROGEN BONDS.
The electrostatic forces of attraction between molecules that contain highly polar bonds.
Are hydrogen bonds weaker than forms of permanent dipole-permanent dipole interactions?
No, hydrogen bonds are STRONGER than other forms of permanent dipole-permanent dipole interaction but WEAKER than a covalent bond.
What can be rationalised in terms of the nature and strength of the intermolecular forces that exist between molecules? [3 marks]
Melting points,
Boiling points,
Viscosity
By considering the POLARITY and NUMBER OF ELECTRONS present in molecules, it’s possible to make ———– predictions of the ——– of the intermolecular forces.
qualitative,
strength.
The melting and boiling points of POLAR substances are HIGHER/LOWER than the melting and boiling points of NON-POLAR substances with similar numbers of electrons.
HIGHER
What are properties of substances that are affected by HYDROGEN BONDING?
Boiling points,
Melting points,
Viscosity,
Solubility/Miscibility in water
The anomalous boiling points of AMMONIA, WATER and HYDROGEN FLUORIDE are a result of…
HYDROGEN BONDING
What does hydrogen bonding between molecules in ice result in?
An expanded structure that causes the density of ice to be less than that of water at low temperatures.
Which compounds tend to be SOLUBLE in polar solvents such as water, and INSOLUBLE in non-polar solvents?
IONIC compounds and POLAR MOLECULAR compounds.
Which type of molecular substances tend to be INSOLUBLE in polar solvents such as water, and SOLUBLE in non-polar solvents?
NON-POLAR molecular substances.
To predict the solubility of a compound, key features to be considered are the:-
[2 marks]
presence of O-H or N-H bonds, which implies hydrogen bonding. [1]
OR
spatial arrangement of polar covalent bonds, which could result in a molecule possessing a permanent dipole. [1]
What is a COVALENT BOND?
A pair of shared electrons between two atoms.
Which type of atoms do covalent bonds usually form between?
Between non-metal atoms.
How are the atoms in a covalent bond held together?
The two positive nuclei and their common attraction for the shared pair of electrons, hold the atoms together.
What type of bond is formed when the ATTRACTION of the ATOMS for the pair of bonding electrons is DIFFERENT?
POLAR COVALENT BONDS
How can you distinguish between PURE covalent and POLAR covalent bonds?
By using the ELECTRONEGATIVITY VALUES
The POLARITY of a covalent bond depends on…
The difference in ELECTRONEGATIVITY within the bond
When there is a SMALL or NO difference in Electronegativity, what is the bond called?
PURE COVALENT bond
Give examples of where PURE COVALENT bonds can be found.
In ELEMENTS and HYDROCARBONS
When there is a MEDIUM difference in Electronegativity, what is the bond called?
POLAR COVALENT bond
Give examples of where POLAR COVALENT bonds can be found.
In:
H-Cl,
H₂O,
NH₃
When there is a LARGE difference in Electronegativity, what is the bond called?
IONIC bonding
Give examples of where IONIC bonds can be found.
In:
Na⁺Cl⁻,
Mg²⁺(Cl⁻)₂
What can you use to assign 𝛿⁺ and 𝛿⁻ partial charges on atoms?
The ELECTRONEGATIVITY VALUES in the data book.
*electronegativities of selected elements can be found on page 12 of the Higher Chemistry data booklet (Publication date: 2021)
The atom with the HIGHER ELECTRONEGATIVITY VALUE…
What partial charge will this atom have?
Has a GREATER SHARE of the bonding electrons.
This atom will have a 𝛿⁻ charge.
The atom with the LOWER ELECTRONEGATIVITY VALUE…
What partial charge will this atom have?
Has a SMALLER SHARE of the bonding electrons.
This atom will have a 𝛿⁺ charge.