2General info.Periodicity is concerned with trends or patterns seen within elements on the periodic table.The patterns that will be covered are:Atomic radiusIonization energyElectronegativityMelting points (IB/SL only)Period 3 (IB/SL only
4Remember, the radius is determined directly by the electron cloud Atomic radiiGoing down a group (column)The radius increases going down a group.This increase is due to the addition of energy levels. Each e.l. increases the electron cloud significantly.
5Going across a period. The radius decreases going across a period. The nucleus increases, which means there is a greater positive charge present. This increase in positive charge pulls the electrons in closer to the nucleus, thus, reducing the radius.No energy levels are being added.
9Ionic radiiThe radius of an atom increases when an electron is added to make a (-) ion (anion).Anions have more e- than protons so the additonal e- is not pulled in close to the nucleus. This causes the e- cloud to expand.
10Ionic radii continuedThe radius of an atom decreases when an electron is lost to make a (+) ion (cation).Cations have more protons than electrons, so the electrons now have a stronger pull on them from the nucleus, thus shrinking the e- cloud.Also, usually when atoms lose any e-, it is all of the valence e- they lose, so the entire energy level is lost. This really decreases the e- cloud.Transitional metals do not lose an e.l.
14Ionization energy (IE) Ionization energy- the energy required to remove an electron from a gaseous atom.Since the electrons are attracted to the nucleus, it takes energy to pull the e- from the atom.Electrons in the last e.l. are the electrons that are being removed. These electrons are called valence electrons. D-block elements have their valence e- in the last two energy levels.First ionization energy. The energy required to remove the first valence electron from a neutral atom.
15Trends in the first ionization energy Going down a groupThe first ionization energy decreases.The radius is increasing and the further away an e- is from the nucleus, the less pull (attractive force) there is on the electron.The further away an e- is from the nucleus, there are more e- (core e-) between the nucleus and the valence e-, thus weakening the pull from nucleus. (this is called shielding.)With the addition of each new energy level the valence electrons become further away from the nucleus.
16Going across a period The first ionization energy increases The atomic radius decreases. The valence e- become closer to the nucleus going from left to right , therefore there is a stronger pull (attractive force) from the nucleus on the valence e-. So, more energy is required to remove an electron.
18First ionization energy trends across several periods.
19Successive ionization energies (IE) The energy required to remove the first e- from an atom is the 1st ionization energy.The energy required to remove a 2nd e- is the 2nd ionization energy and so on.The energy required to remove an e- increases with each successive e-. For example: Al: IE1= 577 IE2= 1815, IE3=2740 all in kJ/mole
20Succesive ionization energies cont. Successive ionization energies increase because.After each e- is removed, the radius becomes smaller making the remaining e- closer to the nucleus.There are more protons than e-, making the attractive force from the nucleus stronger on the core e-.
21Successive ionization energies cont. After all of the valence e- have been removed from an atom, there is a huge increase in the IE.IE for core e- are extremely high due to them being in a lower e.l. that is closer to the nucleus.There are many more protons than e-.
23ElectronegativityA measurement of an element’s ability to attract an electron from another atom within a bond. When 2 atoms of different elements bond, one atom is better attracting the electrons in the bond. This attraction is electronegativity.
24Electronegativity continued An atom is more electronegative if:The atom has a small e- cloud. The small e- cloud allows its nucleus to get closer to another atom’s electrons and pull off e-Larger nucleus: more attractive force on another atom’s electrons.In summary: volume/mass ratio should be small.
25Electronegativity continued. The most electronegative element is Fluorine.Fluorine is given an electronegative rating of 4.0 which is the highest rating. All other elements are compared to it and are given a relative rating.The trends in electronegativity are the same as the ionization energy: increases across a period, decreases down a group.
28Applying periodic trends to chemical reactivity Metals are reactive if:(lose e- in reactions)Larger radiusLower ionization energyFewer valence e-Nonmetals are reactive if:(gain e- in reactions.)Smaller radiusHigh electronegativityMore valence e-
29Melting point trendsDown a group: The melting points tend to decrease. The large radius decreases the attraction between atoms, thus, making a substance easier to melt.Across a period: the melting points increase until group 14. Then they decrease.
32Melting and boiling point trends for elements 1-95
33Chemical and physicial properties Alkali metals:Very soft metalsNot very dense and so float on waterLow melting points: Li=454K to Cs=302 KOne valence e- with a low ionization energy.Readily react with nonmetalsReact with water to form a strong base. Example:2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
34Continued: Halogens Halogen Fluorine Chlorine Bromine Iodine Color Pale yellowYellow greenRed brownBlack or purpleState at room temp.GasLiquidsolid
35Continued: HalogensAll exist as diatomic molecules in their pure state. F2, Cl2 Br2 I2Are nonpolar and have only van der waal forces when in their pure form.Slightly soluble in water.When in water they dissociate slightly:X2 + H2O H+ + X- + HOX,The HOX is a weak acid but a strong oxidant due to the oxygen which reacts with other materials and bleaches them. Example is HOCl is in bleach. These acids are also toxic to microbes, and so make disinfectants and are used in water treatment.
36Continued: halogensThey combine with metals to produce ionically bonded salts called hallides. Ex. NaCl, Most are soluble in water, exception, AgCl, PbCl2The reactivity decreases going down the group. (This is the opposite of the Alkali metals.)The radius increases which reduces the ability of the elements to attract electrons.
37Trends in period 3Left hand side: groups 1 and 2 alkali and alkaline metals.Large radii, low IE, and so react as metals, losing e-React with nonmetals to make solid ionic compounds.These solids are crystaline solids with high melting and boiling points. Ex. NaClThe solids easily dissolve in water to form ions.Oxides, such as MgO, K2O dissolve in water to form bases. Ex. MgO + H2O Mg(OH)2and can neutralize acids. ex MgO + 2HCl MgCl2 + H2O
38Middle of period 3Group 3:Aluminum is a metalIts oxide is amphoteric, that is it can dissolve in either an acid or a base. Al2O3 + 6HCl 2AlCl3 + 3H2O,Al2O3 + 2OH- + 3H2O 2Al(OH)4-Makes crystalline solids like AlCl3 with high melting/boiling points.Group4: Si, the ionization energy too great to behave as a metal and so the first nonmetal appears.Its oxide will react with water to produce a weak acid.Ex. CO2 + H2O H2CO3 (H+ + HCO3-)Make large crystalline solid networks with very high melting a boiling points. Ex. Graphite, diamond, silicon chips.
39Group 5: Phosphorus: Makes primarily covalently bonded molecules. Weak forces are between its molecules, with chlorine or anything else.Very low melting and boiling points.Oxides form acid in water (versions of phosphoric acid.) H3PO4
40Oxides dissolve to form acid in water: Group 6 Sulfur.Makes covalent compounds with weak intermolecular forces, low melting/boiling points.Oxides dissolve to form acid in water:SO2 + H2O H2SO3or SO3 + H2O H2SO4
41Group 7:HalogensAre nonmetals that exist as diatomic molecules.F2, Cl2, are gases, Br2 liquid, I2 solid. Only van der waal forces exist between molecules.These elements combine with metals to make water soluble ionic compounds that have high melting and boiling points. (Silver compounds are not water soluble.)Reactions are in previous slide.
42Summary of period 3: trends seen. Metal or nonmetal: Metallic nonmetallicPhysical state in pure form: solidgas.Pure element combines with water: bases to acidsOxides react with water to form: bases amphoteric acids, oxidants or bleaches.Compounds: crystalline solids (groups 1,2) strong networks (group 4) weak molecular solids (groups 5,6) gases (group7) exception is iodine