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Unformatted text preview: ATOMIC STABILITY
Ion and Molecule Formation 1 Electronegativity In a covalent bond, we have seen that electron In pairs are shared between two nonmetals pairs Rarely are these electrons shared equally as one of Rarely the atoms has a stronger “desire” to have those electrons electrons How can we measure which atoms wants the How electrons more? electrons Electronegativity (EN) It is a measure of an atoms ability to attract a pair It of electrons in a molecule of 2 Sounds much like electron affinity, but electron Sounds affinity is for anion formation and electronegativity measures the ability within an already formed covalent molecule covalent F is the most electronegative element and is given a is value of 4.0 and all elements E.N. values are in comparison to this comparison Left to right across a period = ↑ in EN Left Down a group ↓ in EN or stays about the same Down 3 The higher the EN value, the more the atom will The attract shared electrons to it attract Depending upon how great the difference in Depending electronegativity is between the atoms the bond can have highly positive and negative regions have This is called a polar bond This polar 4 The only bond that is purely 100% covalent where The the electrons are equally shared is one in which the ∆ EN = 0 This only occurs when the electrons are shared by This identical atoms, like H2, or any of the diatomic identical or molecules molecules Every other bond will have a % ionic character Every and a % covalent character based upon the ∆ EN EN A bond is considered to be non-polar covalent if bond the ∆ EN is 0 – 0.3 A bond is considered to be polar-covalent if the bond ∆ EN is 0.3 – 1.7 Any bond with ∆ EN > than 1.7 is ionic in Any EN character character
5 Covalent Character decreases Ionic Character increases Electronegativity Bond difference Type 0 – 0.3 0.3 –1.7 Covalent Polar Covalent Ionic > 1.7 Electronegativities of the Elements Using the chart of Electronegativities, determine the type or bond formed between the following pairs of atoms: C and O Fe and O N and Br C and H Na and F In a molecule of H2O, a pair of electrons are O, shared between each O and H shared The EN of O = 3.5 and H = 2.1 ∆ EN = 1.4 – therefore is a polar covalent bond This means that O attracts the electrons towards it This and so will become slightly negative while the electrons move away from each H atom and they become a bit positive become This means the electrons are not shared evenly This and that one area is slightly positive, the other negative. negative. This is called a polar molecule This polar Indicated using small delta (δ). 9 Dipole Moments Dipole A molecule with a center of negative charge and a molecule center of positive charge is dipolar (two poles), (two or has a dipole moment. or dipole Center of charge doesn’t have to be on an atom. Will line up in the presence of an electric field. How It is drawn How
δ+ δ- H-F δ+ δ- H-F δ+ δ+ δ- H δ+ δ- H-F δ- -F Hδ+ δ- F
+ δ δ+ H δ H+ -δ F- δ- F - H
δ+ δ- H- F δ F - H-F + δ+ δ+ δ- H δ+ δ- H-F δ- -F Hδ+ δ- F
+ δ - δ+ H δ H+ -δ F- δ- F - H
δ+ δ- H- F δ F - H-F - δ+ δ- H-F
δ+ δ- δ+ δ- δ+ δ- + H-F H-F H-F δ+ δ- H-F
δ+ δ- δ+ δ- δ+ δ- H-F H-F H-F Which Molecules Have Dipole Moments? Which Any two atom molecule with a polar bond. Eg. NaCl or FBr With three or more atoms there are two With considerations. considerations. 1. There must be a polar bond. 2. Geometry can’t cancel it out (more about Geometry geometry later) geometry Eg. CH4 Eg. CH CO2 SO2 Ionic vs. Molecular Compounds There are two types of forces involved in There chemistry Intermolecular forces are those between molecules Intermolecular and are responsible for holding these molecules together (inter = between) together Intramolecular forces are those between atoms Intramolecular inside the actual molecule and are responsible for holding the molecule together (intra = within) holding These two forces explain many of the properties of These ionic and covalent compounds ionic 17 17 Ionic compounds are formed of positive and negative Ionic ions and these forces are very strong ions Each ion is held in place by at least 6 other ions and Each so both the inter and the intra molecular forces are strong strong Covalent compounds have strong intramolecular Covalent forces holding the atoms together to form a molecule, but rather weak intermolecular forces holding the adjacent molecules together adjacent Because the intermolecular forces are weak, covalent Because compounds have low boiling and melting points (little energy is needed to move molecules apart from a solid to liquid to gas) solid Many are gases at room temp
18 18 Ionic compounds tend to have high melting points Ionic and boiling points as much energy is required to pull the ions apart pull Most are solids at room temperature Ionic compounds are also hard and brittle This is because that while it is difficult to break This the ions apart (hard) if the ions get moved slightly the ions line up to repel one another and force the solid to “break” solid It is dependent upon how the crystal is arranged Carbon can be arranged to form graphite (brittle) Carbon or diamonds (hard) and each had a different crystal arrangement using the same atoms crystal 19 19 Solid ionic compounds do not conduct electricity Solid as the ions are held tightly, but when in the liquid state (called molten) the ions are free to move and so can conduct electricity so Ionic compounds dissolve easily in water as water Ionic is a polar molecule and water molecules surround the ions and pull them apart into the solution (process called solvation) (process 20 20 How are metal atoms held in place? Most metals have 1, 2 or 3 valence electrons The metal atoms are relatively close to each The other and their valence energy levels overlap overlap This allows the valence electrons to move This freely from one metal atom to those it overlaps with overlaps These electrons are not bonded to one These particular metal atom and are called delocalized electrons delocalized This is often referred to as the “Electron This Sea Model” of metallic bonding Sea Metallic Bonding Metallic 21 Ionic Bond, A Sea of Electrons Ionic 22 Because each metal atom is sort of positive and Because is surrounded by this moving “sea” of e-, the is the atoms are held in place very strongly atoms The bond strength is variable and depends The upon the number of electrons each atom allows to roam in the “sea” and thus the sea size is the determining factor determining Fe & Ti have large seas and so are strong Fe metals metals The sea of e- also explains the conductivity of metals as e- are free to move through the metal metals The ductility and malleability of metals is The explained by the fact that metal atoms are separated easily and in all directions separated 23 Sea of Electrons Sea Metals conduct electricity. Electrons are free to move through the Electrons solid. solid. + ++ + ++++ + ++ +
24 Metals are Malleable Metals
Hammered into shape (bend). Also ductile - drawn into wires. Also ductile Both malleability and ductility explained in Both terms of the mobility of the valence mobility electrons electrons 25 - Page 201 Due to the mobility of the valence electrons, metals have:
1) Ductility and 2) Malleability Notice that the ionic crystal breaks due to ion repulsion! 26 Malleable Malleable Force + ++ + ++++ + ++ + 27 Malleable Malleable Mobile electrons allow atoms to slide by, Mobile sort of like ball bearings in oil. sort Force + ++ + ++++ + ++ + 28 Ionic solids are brittle Ionic Force + + - + + + + - + +
29 Ionic solids are brittle Ionic Strong Repulsion breaks a crystal apart, due to similar ions being next to each other. due - Force + -+ +-+ -+ -+ +-+ 30 Hydrogen Bonding Hydrogen
This is a type of bonding involving hydrogen This and either F, O or N and When hydrogen bonds with either of these When elements there is a large ΔEN This results in a very polar molecule with large This dipoles dipoles This produces relatively high inter molecular This forces to adjacent molecules they are held together “tightly” together This accounts for the relatively high boiling This and melting point of H2O compared to other and compared covalent compounds (table of bp’s on page 31 31 Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.
A H…B or A & B are N, O, or F A H…A 32 11.2 This type of bonding is important in biology It is responsible for holding DNA molecules It together (adenine & thymine have triple hydrogen bonds; cytosine & guanine double) double) This is one reason that A-T and C-G pair This up (recall purines and pyrimidines?) These bonds can get messed up due to These chemicals, radiation and simple random occurrences, the result is often a mispairing of bases and the accompanying mutation of Missense, nonsense, silent mutations 33 Atom and Ion Size Atom
Size of atoms is based upon the radius of the Size atom atom Actually the distance between the nucleus Actually centre of 2 atoms is measured, so half of that would be the radius that Going down any group the size of the atoms Going increases increases This is because the atoms have more energy This levels levels Going across a period, the size tends to Going decrease due to the increase in nuclear charge pulling the electrons in charge 34 The Periodic Table and Atomic Radius Radius 35 The size of ions compared to their neutral The atom size depends upon whether the atom becomes a cation or anion becomes Cations are smaller in size than a neutral Cations atom as there are fewer electrons to hold in place as compared to the neutral atom and thus the “extra” protons pull the remaining electrons closer to the nucleus electrons Anions are the exact opposite, they increase Anions in size and so have larger radii than a neutral atom neutral This is due to the fact that the same number This of protons in the negative ion must hold more electrons in place and the electron cloud moves out cloud 36 Ionic Radii Ionic Cation loses electrons from Cation its valence shell. Electrons from other valence shells are closer to the nucleus. closer Cation also has more protons Cation than electrons which adds to the pull on the remaining electrons and decreases the radius. radius. Anion has more electrons than protons; the pull Anion of the nucleus is less per valence electron. of Also, the electron – electron repulsion is Also, greater. These lead to larger radius for an anion. Atomic Radii Ion Radius/Size
As atoms lose electrons to become cations, they decrease in size This is because as cations will have the same number or protons, but fewer electrons then the neutral atom This results in an increase in nuclear pull on the remaining electrons causing the cloud to be pull in Anions will become larger then their neutral atom This is because they will have more electrons to be held in place by the same number of protons, and so the nuclear pull on each electron becomes less and the cloud move out Relative sizes of some ions and their parent atoms. ...
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- Spring '10