CHM 1045 STUDGY GUIDE CHPTS 8, 9 and 10CHAPTER 8 PERIODIC RELATIONSHIPS AMONG ELEMENTSValence electrons Outermost electrons; reactivity of elements depend on this; in the highestoccupied n shell; all elements in a given group have the same number andtype of valence e-Core electrons Non-valence electrons in an atomNoble gas Filled orbit and chemically inertTransition metal characteristicsResemble one another and are set apart from other elements because they all have incompletely filled d sub-shellsIsoelectronic Have the same number of electrons and therefore the same ground-state electron configurationEx. H-- and He Cations derived from transition metalsWhen a cation is formed from an atom of a transition metals, electrons are always removed first from the ns orbital and then from the (n-1)d orbitalsEffective nuclear charge (Zeff)The nuclear charge felt by an electron when both the actual nuclear charge (Z) and the repulsive effects (shielding) of the other electrons are taken into accountZeff = Z – σSigma is the shielding constant which is greater than 0 but smaller than ZShielding Core electrons are on average closer to the nucleus than valence electrons therefore core electrons shield valence e- much more than valence e- shield one anotherEffective nuclear charge TRENDIncrease in effective nuclear charge from left to right across a period and from top to bottom in a group Atomic radius We define the size of an atom in terms of this which is one half the distance between the two nuclei in two adjacent metal atoms or in a diatomic moleculeAtomic radius trend Because the effective nuclear charge increases from left to right, the added valence e- at each step is more strongly attracted by the nucleus than the one before, therefore the atomic radius decreases from Li to Ne or INCREASES FROM RIGHT TO LEFT and INCREASES FROM TOP TO BOTTOMCHM 1045 STUDGY GUIDE CHPTS 8, 9 and 10Ionic radius The radius of a cation or anion - Atom to anion (size)Size (radius) increases-Because the nuclear charge is the same but the repulsion resulting from the additional e- enlarges the domain of the e- cloud- Atom to cation (size)Radius decreases-Removing an e- reduces electron-electron repulsion but nuclear charge remains the same so e- cloud shrinks - Isoelectronic ions (size)Cations are smaller than anions Ex. Na+ is smaller than F-- - Isoelectronic cationsRadii of tripositive ions (3 positive charges) are smaller than dipositive ions which are smaller than unipositive ionsEx. Al3+ < Mg2+ < Na+- Isoelectronic anionsRadius increases as we go from ions with uninegative charge (-) to those with dinegative charge (2-) and so onVariation of physical properties-Metallic propertyFrom left to right across a period there is a transition from metals to metalloids to nonmetals Metallic character decreases from left to right and increases from top to bottom Ionization energy The minimum energy (in KJ/mol required to remove an electron from a gaseous atom in ground state-the amount of energy in KJ needed to strip 1 mole of electrons from 1 mole of gaseous atoms -measured by how tightly the electron is held in the atom Higher the IE, the more difficult it is to remove the e-When an e- is removed from an atom the repulsion among the remaining electrons decreases. Because the nuclear charge remains constant, more energy is needed to remove another e- from the positively charged ion- thus IE’s increase in this order: IE1<IE2<IE3-endothermic process- always positiveFirst ionization energy The amount of energy (in a many electron atom) required to remove the first electron from the atom in its ground stateEnergy + X (g)→ X+ (g) + e- X is an atom of any element Second + third IE Energy + X+ (g)→ X2+ (g) + e- second ionizationEnergy + X2+(g)→ X3+ (g) + e- third ionizationIonization energy TREND Increase from left to right; increase from bottom to top The trend is due to the increase in effective nuclear charge from left to right –a larger effective nuclear charge means a more tightly held valenceCHM 1045 STUDGY GUIDE CHPTS 8, 9 and 10e- and thus a higher first IEElectron affinity Ability to accept one or more electrons The negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion X (g) + e- → X—(g)Nobel gases have extremely low EA’sElectron affinity TRENDSame as ionization energyIncreases from left to right and from bottom to topGeneral trends in chemical properties:The first member in each group (the element in the second period from Li to F) differs from the rest of the members of the same groupEx. Li exhibits many but not all the properties characteristic of the alkali metals -this is because of the unusually small size of the 1st element-Diagonal relationships-Greater variations in properties among group 3A to 6A because elementsare changing from non metals to metals or from nonmetals to metalloids Diagonal relationship Similarities between pairs of elements in different groups and periods of the periodic table -first three members of second period (Li, Be, B) exhibit many similarities to those elements located diagonally below them - Hydrogen (1s1)-trendShown in !A but can be a class of itselfLike the alkali metals is has a single s valence e- and forms a unipositive ion H+ but can also form the hydride ion H- in ionic compounds In this respect H resembles the halogens - Group 1A elements (ns1, n≥2)Alkali metalsLow ionization energies = great tendency to lose the single valence e-Very reactive, never found in pure stateCHM 1045 STUDGY GUIDE CHPTS 8, 9 and 10Create metal hydroxides, oxides, peroxides, or superoxides (difference has to do with stability of the oxides in solid state)- Group 2A elements (ns2, n≥2)Alkaline earth Less reactive than 1AForm M2+ ions- Group 3A elements (ns1np1 n≥2)Boron is a metalloid the rest are metals B doesn’t form binary ionic compounds and is unreactive toward oxygen gas and water Other group 3A form both unipositive and tripositive ions - Group 4A elements (ns2np2, n≥2)Carbon is a nonmetal and the next two members (Si and Ge) are metalloidsThe metallic elements Sn and Pb don’t react with water but do react with acids Form compounds in both 2+ and 4+ oxidation states C and Si = 4+ more stable as you go down 2+ becomes more stable (ex in lead)- Group 5A elements (ns2np3, n≥2)Nitrogen and phosphorus are nonmetals, arsenic and antimony are metalloids and bismuth is a metal
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