Metallic bonding AS Bonding & Periodicity Test: Answers
Q1 Ti is a giant metallic la ce. There is strong electrosta c a rac on between posi ve metal ions and ‘sea’ of delocalised electrons. Layers of atoms slide over one another when hammered, but, in the new metallic shape, the same strong metallic bonding (and bond strength) is re- formed between metal ions and delocalised electrons. Q2 Na conducts and NaCl does not conduct. Delocalised electrons flow through the metal and transfer charge. Ions are rela vely fixed in solid NaCl, so it does not conduct. However, aqueous and molten NaCl can conduct as Na+ Cl- are mobile and able to transfer electrical charge. Ionic Bonding Q1 MgO is a giant ionic la ce. There is strong a rac on between oppositely charged Mg2+ and O2- ions.
Iodide ion has larger atomic radius. Thus, there is increased separa on between Na+ and I- nuclei, Q2 which decreases the strength of the ionic a rac ve force between ions, thus decreasing the overall strength of the ionically bonded giant la ce.
Covalent and da ve covalent bonding
PH + H+ à PH + 3 4 Q1 + PH4 formed by lone pair from P + PH3 is pyramidal atom being donated to H . This is a steric 4, 1 LP coordinate (da ve covalent) bond. bond angle 107° Steric 4, No LP, bond angle 109.5°
How bond shapes are formed: Q2 + Equal repulsion between H3O is pyramidal, bond angle 107°; steric 4, 1LP bonding electron pair ligands Lone pair from O donated to H+ −
Q3 + F− Lone pair from F− donated to B atom to form da ve covalent bond
− BF3 trigonal planar BF4 tetrahedral steric 3, No LP, bond angle 120° steric 4, No LP, bond angle 109.5°
Q4 M 267 = Al Cl + Cl− r 2 6
1 − − Lone pair from Cl donated to Al atom to form da ve covalent bond in AlCl4 Bond Polarity Electronega vity is the ability of an atom to a ract electron density from a pair of electrons Q1 contained in a covalent bond
Q2 CH4 intermolecular force= van der Waals ; NH3 intermolecular force= hydrogen bonding
Large electronega vity difference between N and H (3.0-2.1=0.9) Q3 This forms a dipole charge separa on Nδ− and Hδ+ within the N-H covalent bond + Lone pair of electrons on N atom is a racted to (hydrogen bonds with) Hδ of adjacent NH3 molecule
Q4 NO has covalent bonding as there is only a small electronega vity difference between N and O (difference is 0.5 units according to table of electronega vity values)
Forces between molecules
The hydrogen bonding intermolecular forces that exist between methanol molecules are Q1 much stronger than the van der Waals forces that exist between oxygen molecules, hence methanol has a higher b.p. oxygen.
Q2 Intermolecular forces: F2 is van der Waals, CH3F is dipole-dipole, HF is hydrogen bonding.
Large electronega vity difference between F and H This forms a dipole charge separa on Fδ− and Hδ+ within the H-F covalent bond Lone pair of electrons on F atom is a racted to (hydrogen bonds with) Hδ+ of adjacent HF molecule Q3 The molecular mass (size) of hydrogen halides increases as the group is descended, hence there is increasing strength of van der Waals forces of a rac on. Conversely, the difference in electronega vity between hydrogen and halogen decreases as the group is descended, hence there is decreased dipole-dipole intermolecular forces of a rac on. Overall, these trends culminate in an increased boiling point as the hydrogen halide group is descended.
Q4 Hydrogen bonding exists between NH3 and H2O or
Q5 Hydrogen bonding exists between HF molecules
The hydrogen bonding intermolecular forces (IMF) that exist between HF molecules are much stronger than the van der Waals forces that exist between F molecules. 2 2 Hence, lower energy is required to break IMF of liquid F2 so F2 has lower boiling point. Bonding and physical proper es and states of ma er Iodine is molecular crystal and graphite is giant covalent macromolecule Q1 Graphite: composed of layers of hexagonal covalently bound carbon atoms that are separated by weak van der Waals forces between the layers. Strong covalent bonds needs to be broken to melt graphite, hence it has a high mel ng point.
The hydrogen bonding intermolecular forces (IMF) that exist between H2O molecules are Q2 much stronger than the van der Waals and dipole-dipole forces that exist between H2S molecules. The increased molecular size of the compounds formed between hydrogen and group VI elements as the Group is descended means there are increased van der Waals a rac ve forces between molecules, hence a trend towards increased boiling point.
Shapes of simple molecules
Cl2O Q1 Steric 4, two LP Angular, 104.5° bond angles BeCl2 Steric 2, no LP Linear, 180° bond angle .. Q2 NHF2 Steric 4, one LP pyramidal, 107° bond angle
BF3 Steric 3, no LP Trigonal planar, 120° bond angles - Q3 − N NH2 Steric 4, two LP Angular, 104.5° bond angles
Q4 + AsCl4 ion has a bond angle of 109.5°, because the shape formed has equal repulsion between 4 bonding (electron pair) ligands.
Q5
As
+ AsF5 ClF2 Steric 5, no LP Steric 4, two LP 3 Trigonal Bipyramidal, 90° and 120° bond angles Angular, 104.5° bond angles Period 3: decreasing atomic radius, increasing 1st IE & trends in m.p.
Decreasing atomic radius: Q1 increasing nuclear charge, similar electron shielding ∴ increased effec ve nuclear charge density ∴ greater a rac on of valence electron(s) by nucleus ∴ decreased atomic radius
Q2 1st IE: the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous mono-posi ve ions. X(g) à X+(g) + e-
Q3 Mg+ (g) à Mg2+(g) + e- More energy is required to remove an electron from a posi vely charged ion than a neutral ion. Q4 Also, Mg+ has a smaller atomic radius than Mg, so greater energy is needed to overcome the increased nuclear a rac ve force that is holding the outer shell electron closer to the nucleus.
Increasing 1st IE: Q5 increasing nuclear charge, similar electron shielding ∴ increased effec ve nuclear charge density ∴ greater a rac on of valence electron(s) by nucleus and decreased atomic radius ∴ more energy needed to remove outershell (valence) electron
Sulfur: less than expected increase in 1st IE: Q6 Electron pair repulsion in the 3p suborbital means less energy is required to remove the valence e-
Aluminium: less than expected increase in 1st IE: Q7 The valence electron is located in a higher energy 3p orbital, which is further away from the nucleus and its nuclear a rac ve force, hence less energy is required to remove the valence e-
Q8 X is Aluminium (considerable increase in 1st IE between 3rd and 4th electron removal)
S and P exist as simple molecular structures with weak van der Waals intermolecular forces Q9 8 4 between molecules. However, as S8 is a larger sized molecule than P4, it has stronger vdW.
Thus, more energy is needed to break the IMF of S8 so it has a higher mel ng point.
Silicon is a giant covalent macromolecule. Each silicon atom is covalently bonded to 4 other Q10 silicon atoms in a diamond cubic crystal structure. Considerable energy is needed to break the strong covalent bonds within this giant structure arrangement, hence silicon has a high m.p.
Al and Na exist as giant la ces of metallic bonded atoms. However, Al, compared to Na, has a: Q11 • higher metal ion charge (+3 vs. +1) • more delocalised electrons per metal ion • smaller atomic radius than Na (closer distance between metal ion and delocalised e- sea). In combina on, these factors ensure Al achieves stronger electrosta c a rac on between the more posi vely charged metal ions and higher e- density delocalised e- sea, compared to Na. Hence, metallic bonding is stronger in Al and more energy is needed to overcome this a rac ve force and melt Al compared to Na. 4 Extended wri ng ques on: Bonding
• NaCl is an ionic compound. Oppositely charged ions are held together by electrosta c forces in a giant ionic cubic la ce. • Water is composed of covalent molecules that are held together by intermolecular hydrogen bonds. In each water molecule, a single oxygen atom is covalently bonded to two hydrogen atoms forming an angular shaped molecule. As ice, water molecules form a hexagonal crystal la ce where two hydrogen bonds form per molecule of water. • Overall, ionic bonding within the la ce of NaCl is stronger than the intermolecular forces holding water molecules together in ice. Hence, as more energy is needed to separate the separate the Na+ Cl- ions compared to breaking down hydrogen bonds in ice, NaCl has a higher m.p. than ice.
• SF4 is steric 5, 4 bonding ligands and one lone pair • Structure is based on equal repulsion between bonding ligands and lone pair • Hence, molecule structure is see-saw shape. The electron pair distribu on is trigonal bipyramidal with the lone pair located on the axial or equatorial loca on. • There is slight lone pair-bond pair repulsion that pushes the S-F bonds closer together, hence bond angles are slightly <120° and <180°, respec vely.
Extended wri ng: Periodicity
Elements in the p block have their outer electron(s) in p orbitals or levels or sub-shells e.g. Carbon 1s2 2s2 2p2
Periodicity: pa ern in the change in the proper es of a row of elements that is repeated in the next row (underneath or above the ‘index’ row).
Atomic radius decreases and electronega vity increases, across Period 3. Both trends share common reasons: increasing nuclear charge, similar electron shielding ∴ increased effec ve nuclear charge density ∴ greater a rac on of valence electron(s) by nucleus and decreased atomic radius.
Conduc vity: Metals Na, Mg, Al conduct electricity due to free delocalised electrons being able to transfer electrical charge through the metallic bonded structure. The electrical conduc vity increases from Na to Al due to increased e- density with the sea of delocalised e-. Non-metals (P, S, Cl) exist as covalent molecules and are unable to conduct electricity due to an absence of mobile delocalised high energy level electrons. Electrical conduc vity of silicon: silicon is a metalloid giant covalent macromolecule. At room temperature, few valence electrons are mobile (so poor conduc vity), however, at higher temperature, 5 more electrons are promoted to higher energy levels and electrical conduc vity increases.