UNIT 4
tructure 2.1 The ionic model
S
.1.1: Forming ions
2
- When metal atoms lose electrons, they form positive ions called cations.
- When non-metal atoms gain electrons, they form negative ions called anions.
- Predict the charge of an ion from the electron configuration of the atom.
- Eg. 1s22s22p5 -> 1s22 s22 p6 = F-
- The formation of ions with different charges from a transition element should be included.
- Zn2+
- Ag+
2.1.2: The ionic bond
- The ionic bond is formed by electrostatic attractions between oppositely charged ions.
- Deduce the formula and name of an ionic compound from its component ions, including polyatomic ions.
- Binary ionic compounds are named with the cation first, followed by the anion. The anion adopts the suffix “ide”. Roman numerals are
used to show which oxidation state the transition metal is in.
- Interconvert names and formulas of binary ionic compounds.
- Eg. NaF -> sodium fluoride, calcium phosphate -> Ca3(PO4)2
- The following polyatomic ions should be known by name and formula:
- ammonium: NH4+
- hydroxide OH–
- nitrate NO3–
- hydrogen carbonate HCO3-
- carbonate CO32–
- sulfate SO42 –
- phosphate PO43–
2.1.3: The ionic lattice
- Ionic compounds exist as three-dimensional lattice structures, represented by empirical formulas.
- Electrical conductivity: can conductwhen molten orin solutionas ions are free to move and thereforecarry charge. However, as a
solid, ions are fixed in place.
- Solubility: soluble in water because ‘like dissolveslike’ ascharged ions can form bonds with polar watermolecules
- Volatility:High melting and boiling pointas theelectrostatic attraction/ionic bond is strong so needs a lot of energy to pull the
atoms apart and therefore change the state.
- Property of material: Brittle as layers in the 3Dlattice can break apart easily.
- Lattice enthalpyis a measure of the strength of theionic bond in different compounds, influenced by ion radius and charge.
- Ions stack together in planes, where negative ions surround positive ones and then the planes stack together in a 3D arrangement
called a lattice
- A lattice is a symmetrical arrangement
- The coordination number is the number of other atoms one atom is bonded to.
- What experimental data demonstrate the physical properties of ionic compounds?
- Melting points, boiling points, and solubility data can be measured for various ionic compounds. This can demonstrate the
theoretical trends of the compounds.
, tructure 2.2 The covalent model
S
.2.1: The covalent bond
2
- A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei.
- The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons.
- When atoms share pairs of electrons, both nuclei will have a pull on the electrons, although sometimes one nucleus attracts the
electrons more strongly than the other.
- Atoms held together by covalent bonds are called molecules
- Deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom.
- Lewis structures must show all the valence electrons (bonding and nonbonding pairs) in a covalently bonded species.
- Molecules containing atoms with fewer than an octet of electrons should be covered.
- BeCl2 is a linear molecule
- BF3 is a triangular planar molecule
- H can achieve a stable arrangement by gaining an electron to become 1s2, the same structure as the noble gashelium
- Li does the same, but losing an electron and going from 1s22s1 to 1s2 to become a Li+ ion
- Be from group 2, has two valence electrons and forms stable compounds with just four electrons in the valence shell
- B and Al in group 13 have 3 valence electrons and can form stable compounds with only 6 valence electrons
2.2.2: Single, double and triple bonds
- Single, double and triple bonds involve one, two and three shared pairs of electrons respectively.
- The length of the bond shortens as the number of bonds increase
- The bond strength increases as the number of bonds increases
- Bond strength is a measure of the energy required to break a bond
- Multiple bonds involve a greater number of shared electrons so the electrostatic attraction between them and the positive nuclei is
stronger, hence a stronger bond.
2.2.3: Coordinate bonds
- A coordination bond is a covalent bond in which both the electrons of the shared pair originate from the same atom.
- The coordinate bond is represented by an arrow which shows the origin of the electrons in the bond.
- Examples include: CO, NH4+, H3O+ , and the aluminumchloride dimer Al2C
l6
2.2.4: VSEPR
- Theva lencesh ellelectronpairrepulsion (VSEPR)model enables the shapes of molecules to be predicted from the repulsion of
electron domains around a central atom.
- The shape of a molecule or ion is formed by the arrangement of the electron pairs around the central atom.
- Electron pairs will repel each other so that they are as far away as possible; this gives maximum stability
- T o determine the shape of a molecule, you need to know how many total ‘electron domains’ are presentaround the central atom,
including bonding and nonbonding (lone) pairs.
- A double or triple bond count for just a single electron domain.
- Lone pairs form orbitals that are shorter and rounder than the orbitals that the bonding pairs occupy and take up more space in a
molecule, causing a greater repulsion with other electron domains.
- There is more repulsion between a lone pair and a bonding pair than there is between two bonding pairs.
Greatest Repulsion
2 lone pairs → lone pair and bonding
pair → 2 bonding pairs Least repulsion
- Predict the electron domain geometry and the molecular geometry for species with up to four electron domains.
tructure 2.1 The ionic model
S
.1.1: Forming ions
2
- When metal atoms lose electrons, they form positive ions called cations.
- When non-metal atoms gain electrons, they form negative ions called anions.
- Predict the charge of an ion from the electron configuration of the atom.
- Eg. 1s22s22p5 -> 1s22 s22 p6 = F-
- The formation of ions with different charges from a transition element should be included.
- Zn2+
- Ag+
2.1.2: The ionic bond
- The ionic bond is formed by electrostatic attractions between oppositely charged ions.
- Deduce the formula and name of an ionic compound from its component ions, including polyatomic ions.
- Binary ionic compounds are named with the cation first, followed by the anion. The anion adopts the suffix “ide”. Roman numerals are
used to show which oxidation state the transition metal is in.
- Interconvert names and formulas of binary ionic compounds.
- Eg. NaF -> sodium fluoride, calcium phosphate -> Ca3(PO4)2
- The following polyatomic ions should be known by name and formula:
- ammonium: NH4+
- hydroxide OH–
- nitrate NO3–
- hydrogen carbonate HCO3-
- carbonate CO32–
- sulfate SO42 –
- phosphate PO43–
2.1.3: The ionic lattice
- Ionic compounds exist as three-dimensional lattice structures, represented by empirical formulas.
- Electrical conductivity: can conductwhen molten orin solutionas ions are free to move and thereforecarry charge. However, as a
solid, ions are fixed in place.
- Solubility: soluble in water because ‘like dissolveslike’ ascharged ions can form bonds with polar watermolecules
- Volatility:High melting and boiling pointas theelectrostatic attraction/ionic bond is strong so needs a lot of energy to pull the
atoms apart and therefore change the state.
- Property of material: Brittle as layers in the 3Dlattice can break apart easily.
- Lattice enthalpyis a measure of the strength of theionic bond in different compounds, influenced by ion radius and charge.
- Ions stack together in planes, where negative ions surround positive ones and then the planes stack together in a 3D arrangement
called a lattice
- A lattice is a symmetrical arrangement
- The coordination number is the number of other atoms one atom is bonded to.
- What experimental data demonstrate the physical properties of ionic compounds?
- Melting points, boiling points, and solubility data can be measured for various ionic compounds. This can demonstrate the
theoretical trends of the compounds.
, tructure 2.2 The covalent model
S
.2.1: The covalent bond
2
- A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei.
- The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons.
- When atoms share pairs of electrons, both nuclei will have a pull on the electrons, although sometimes one nucleus attracts the
electrons more strongly than the other.
- Atoms held together by covalent bonds are called molecules
- Deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom.
- Lewis structures must show all the valence electrons (bonding and nonbonding pairs) in a covalently bonded species.
- Molecules containing atoms with fewer than an octet of electrons should be covered.
- BeCl2 is a linear molecule
- BF3 is a triangular planar molecule
- H can achieve a stable arrangement by gaining an electron to become 1s2, the same structure as the noble gashelium
- Li does the same, but losing an electron and going from 1s22s1 to 1s2 to become a Li+ ion
- Be from group 2, has two valence electrons and forms stable compounds with just four electrons in the valence shell
- B and Al in group 13 have 3 valence electrons and can form stable compounds with only 6 valence electrons
2.2.2: Single, double and triple bonds
- Single, double and triple bonds involve one, two and three shared pairs of electrons respectively.
- The length of the bond shortens as the number of bonds increase
- The bond strength increases as the number of bonds increases
- Bond strength is a measure of the energy required to break a bond
- Multiple bonds involve a greater number of shared electrons so the electrostatic attraction between them and the positive nuclei is
stronger, hence a stronger bond.
2.2.3: Coordinate bonds
- A coordination bond is a covalent bond in which both the electrons of the shared pair originate from the same atom.
- The coordinate bond is represented by an arrow which shows the origin of the electrons in the bond.
- Examples include: CO, NH4+, H3O+ , and the aluminumchloride dimer Al2C
l6
2.2.4: VSEPR
- Theva lencesh ellelectronpairrepulsion (VSEPR)model enables the shapes of molecules to be predicted from the repulsion of
electron domains around a central atom.
- The shape of a molecule or ion is formed by the arrangement of the electron pairs around the central atom.
- Electron pairs will repel each other so that they are as far away as possible; this gives maximum stability
- T o determine the shape of a molecule, you need to know how many total ‘electron domains’ are presentaround the central atom,
including bonding and nonbonding (lone) pairs.
- A double or triple bond count for just a single electron domain.
- Lone pairs form orbitals that are shorter and rounder than the orbitals that the bonding pairs occupy and take up more space in a
molecule, causing a greater repulsion with other electron domains.
- There is more repulsion between a lone pair and a bonding pair than there is between two bonding pairs.
Greatest Repulsion
2 lone pairs → lone pair and bonding
pair → 2 bonding pairs Least repulsion
- Predict the electron domain geometry and the molecular geometry for species with up to four electron domains.
