Periodic Trends College Chemistry

PERIODICITY To play the movies and simulations included, view the presentation in Slide Show Mode.

General Periodic Trends Atomic and ionic size Ionization energy Electron affinity Electronegativity Going across- Electrons held more tightly(effective nuclear charge) Going down-Larger orbitals. Electrons held less Tightly (effective nuclear charge).

Effective Nuclear Charge Shielding effect – we discussed how the electrons closer to the nucleus (2s) “shield” the outer electrons from the charge of the nucleus (2p) Consequence: reduces the electrostatic attraction between the protons in the nucleus and the electrons in further orbitals

Effective Nuclear Charge Effective nuclear charge, Zeff – charge “seen” by an electron due to the shielding effect let’s consider Helium which has 2 protons to give the nucleus a +2 charge The electrons get less of a charge because they are partially offset by electron-electron repulsion We say the charge felt by an electron is given by Zeff = Z – s Z = actual nuclear charge s = shielding constant (with He, 1.69)

Effective Nuclear Charge For atoms with three or more electrons, electrons in a given shell are shielded by electrons in the inner shell Effective nuclear charge increases across because protons are added to the nucleus Effective nuclear charge increases up because there are less atomic orbitals

Atomic Radius / Size Size goes UP on going down a group. Atomic radius is the distance from the nucleus to the valance electrons. Size goes UP on going down a group. Because electrons are added further from the nucleus, there is less attraction.

Atomic Radii Figure 8.9

Atomic Size Size decreases across a period owing to increase pulling of the nucleus on the electrons. Each added electron feels a greater and greater + charge. Large Small

Ionic Radius Size of the atom from its outmost electron to the center of the atom when the atom has lost or gained an electron Anion – gained an electron, more negative Ex: Cl- Cation – lost an electron, more positive Ex: Na+

Ion Sizes Forming a cation. Li + , 78 pm 2e and 3 p Forming a cation. Li,152 pm 3e and 3p CATIONS are SMALLER than the atoms from which they come. The electron/proton attraction has gone UP and so size DECREASES.

Ion Sizes Forming an anion. - , 133 pm 10 e and 9 p F, 71 pm 9e and 9p Forming an anion. ANIONS are LARGER than the atoms from which they come. The electron/proton attraction has gone DOWN and so size INCREASES. Trends in ion sizes are the same as atom sizes if we look at ISOELECTRIC ions (same amount of electrons)

Trends in Ion Sizes Figure 8.13

Ionic Size Ionic size is the distance from the nucleus to the valence electrons after an atom has lost or gained electrons. Cations form when an atom loses one or more electrons. Cations are smaller than the atoms from which they form

Ionic Size Anions form when an atom gains one or more electrons Larger than the atoms from which they come from

Ionization Energy IE = energy required to remove an electron from an atom in the gas phase. Mg (g) + 738 kJ ---> Mg+ (g) + e-

Trends in Ionization Energy

Trends in Ionization Energy IE increases across a period Metals lose electrons more easily than nonmetals. Metals are good reducing agents. Nonmetals lose electrons with difficulty.

Trends in Ionization Energy IE decreases down a group Because size increases.

Ionization Energy More energy is required to remove the second or third electron Energy + X+(g)  X+2(g) + e- Energy + X+2(g)  X+3(g) + e- Exception: between Group 2 and 3 (ex: Be and B), Group 3 has lower IE because Group 2 is already “psuedo-stable.” Second, third, etc. IE usually follow the same trend (watch out for psuedo-stability!)

Electron Affinity A few elements GAIN electrons to form anions. Electron affinity is the energy involved when an atom gains an electron to form an anion. A(g) + e- ---> A-(g) E.A. = ∆E

Trends in Electron Affinity Atom EA F +328 kJ Cl +349 kJ Br +325 kJ I +295 kJ Affinity for electron increases across a period (EA becomes more positive). Affinity decreases down a group (EA becomes less positive).

Trends in Electron Affinity

Electronegativity Electronegativity increases across a period and decreases down a group Electronegativity is the tendency of an atom to remove an electron from another atom when forming a compound.

Electronegativity Values See page 177 in text