Periodic Properties of the Elements Chapter 7 Periodic Properties of the Elements

Periodic Properties of the Elements Periodic Trends In this chapter, we will rationalize observed trends in  Atomic radii.  Ionic radii.  Ionization energy.  Electronegativity. These trends can be explained using the concepts of:  Coulomb’s law (we have seen this already).  Shell model (we have seen this one too).  Shielding effect.  Effective nuclear charge.

Periodic Properties of the Elements Shielding Effect Successive shells are larger than the previous shells. Electrons in successive shells spend more of their time further away from the nucleus. Electrons in successive shells are partly “shielded” from the attractive forces (of protons) by the electrons in the previous shells. This shielding effect (repulsion from inner electrons) reduces electrostatic attractions between the outer electrons and the nucleus.

Periodic Properties of the Elements Effective Nuclear Charge In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons (shielding effect). The nuclear charge that an electron experiences depends on both factors.

Periodic Properties of the Elements Effective Nuclear Charge The effective nuclear charge, Z eff, is found this way: Z eff = Z − σ where Z is the atomic number and σ is a shielding constant, usually close to the number of inner electrons.

Periodic Properties of the Elements Atomic radii The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei.

Periodic Properties of the Elements Sizes of Atoms Bonding atomic radius tends to… …decrease from left to right across a row due to an increase Z eff that pulls electrons closer to the nucleus. …increase from top to bottom of a column due to increasing value of n

Periodic Properties of the Elements Atomic radius in a group Atomic radii increase while moving down a group, as new shells are added with larger orbitals, increasing the shielding effect on the outer electrons. A 2s orbital is much larger than a 1s orbital. Increasing the principal quantum number is like adding layers to an onion.

Periodic Properties of the Elements Atomic radius down a group According to Coulomb’s law, the force of attraction between two charges decreases as the distance between the charges (protons and electrons) increases. F = k q 1 q 2 d 2

Periodic Properties of the Elements Atomic radius in a period Atomic radii decreases while moving left to right across a period. Valence electrons are in the same shell. The shielding effect experienced by the valence electrons in the same subshell is similar.(σ) But the effective nuclear charge increases because the number of protons increases. Z eff = Z − σ Increasing the number of protons creates a greater force of attraction on the electrons within a subshell. The greater force pulls the electrons closer to the nucleus, causing the atomic radius to decrease.

Periodic Properties of the Elements Atomic radius in a period According to Coulomb’s law, the force of attraction between two charges increases as the charges protons and electrons) increases. F = k q 1 q 2 d 2

Periodic Properties of the Elements Recall Ion Formation Metals lose electrons to form positive ions (cations). Non-metals gain electrons to form negative ions (anions). Hydrogen (non metal) may lose or gain. It acts as an anion (hydride) when it gains 1 e- (MgH 2 ) and as a cation when it loses 1 e- (HCl). Elements gain or lose electrons to acquire the electron configuration of the noble gases.

Periodic Properties of the Elements Size of species in an Isoelectronic Series Isoelectronic species are atoms and ions that share the same electronic configurations, but have different radii. S 2- 1s 2 2s 2 2p 6 3s 2 3p 6 (Z=16) Cl - 1s 2 2s 2 2p 6 3s 2 3p 6 (Z=17) Ar1s 2 2s 2 2p 6 3s 2 3p 6 (Z=18) K + 1s 2 2s 2 2p 6 3s 2 3p 6 (Z=19) Ca 2+ 1s 2 2s 2 2p 6 3s 2 3p 6 (Z=20) As the number of protons increases (and the number of electrons remains constant) the force of attraction on those electrons increases reducing the radii. Coulomb’s law

Periodic Properties of the Elements Sizes of Ions Ionic size depends upon:  Nuclear charge.  Number of electrons. (electron repulsion)  Orbitals in which electrons reside.

Periodic Properties of the Elements Cations are smaller than their parent atoms  The outermost electrons are removed and electron repulsions are reduced.  The amount of positive charge pulling on each electron increases.  Many atoms lose their outermost shell when they lose their valence electrons to become cations. When an atom lose its outermost shell the radius decreases dramatically.

Periodic Properties of the Elements Anions are larger than their parent atoms  Electrons are added to he same subshell.  Electron repulsions are increased.  This causes the electrons to move further away from the nucleus.

Periodic Properties of the Elements Sizes of Ions in a group Ions increase in size as you go down a column.  New shells are added with larger orbitals.  Valence electrons experience a greater shielding effect.  Coulomb’s law  Same as atomic radius

Periodic Properties of the Elements Ionization Energy Amount of energy required to remove the least tightly held electron from the ground state of a gaseous atom or ion.  Endothermic  First ionization energy is that energy required to remove first electron.  Second ionization energy is that energy required to remove second electron, etc.  Ionization energy increases with increase effective nuclear charge and decrease atomic radius.

Periodic Properties of the Elements Recall Photoelectron Spectroscopy PES Ionization Energies in MJ/mol obtained from PES Element1s2s2p3s3p3d4s Na Mg Al Si P S Cl Ar K Ca Sc

Periodic Properties of the Elements Shielding effect within a shell The average distance between the nucleus and electrons decreases as subsequent subshells are added within a shell. d 3s > d 3p > d 3d From Coulomb’s law we would expect that the IE for an electron in 3d would be greater than that for an electron in 3p. But PES indicates that the IE 3s >IE 3p >IE 3d Due to an increase in the shielding effect.

Periodic Properties of the Elements First Ionization Energies in a group As one goes down a column, less energy is required to remove the first electron.  Valence electrons reside in a higher orbital, farther from the nucleus.  According to Coulomb’s law the force of attraction protons-electrons decreases as the distance between them increases.

Periodic Properties of the Elements First Ionization Energies in a period Generally, as one goes across a period, it gets harder to remove an electron. (IE increases)  As you go from left to right, Z eff increases.  According to Coulomb’s law the force of attraction protons-electrons increases as the # of protons and electrons increases.

Periodic Properties of the Elements Trends in First Ionization Energies However, there are apparent discontinuities in this trend.

Periodic Properties of the Elements Trends in First Ionization Energies The first occurs between Be and B and between Mg and Al. Electron removed from p-orbital rather than s-orbital.  Harder to remove an electron from a full subshell (2s or 3s) that it is to pull an electron out of a subshell with only one electron (2p or 3p) The second occurs between Zn and Ga and between Cd and In.  Harder to remove an electron from a full subshell (3d or 4d) that it is to pull an electron out of a subshell with only one electron (4p or 5p)

Periodic Properties of the Elements Trends in First Ionization Energies The third occurs between Groups VA (p 3 ) and VIA (p 4 ).  Electron removed comes from doubly occupied orbital.  Repulsion between electrons is reduced when the electron is removed helping stabilizing the ion.

Periodic Properties of the Elements Ionization Energy It requires more energy to remove each successive electron.  The radius is reduced upon removal of an electron and the effective nuclear charge increases. When all valence electrons have been removed, the ionization energy takes a quantum leap.  Electron configuration drops a quantum number and the radius decreases

Periodic Properties of the Elements Electronegativity An element ‘s ability to attract electrons in a chemical bond. Noble gases do not tend to form chemical bonds, thus, they do not have electronegativity values. It increases as the atomic radius decreases. F, O, N are the most electronegative elements.

Periodic Properties of the Elements Properties of Metal, Nonmetals, and Metalloids

Periodic Properties of the Elements Metals versus Nonmetals Differences between metals and nonmetals tend to revolve around these properties.

Periodic Properties of the Elements Metals versus Nonmetals Metals tend to form cations. Transition metals form colored compounds. Nonmetals tend to form anions.

Periodic Properties of the Elements Metals Compounds formed between metals and nonmetals tend to be ionic. Metal oxides tend to be basic. CaO + H 2 O  Ca OH -

Periodic Properties of the Elements Nonmetals Substances containing only nonmetals are molecular compounds. Most nonmetal oxides are acidic. SO 3 + H 2 O  2H + + SO 4 2-

Periodic Properties of the Elements Metalloids Have some characteristics of metals, some of nonmetals. For instance, silicon looks shiny, but is brittle and fairly poor conductor.

Periodic Properties of the Elements Group Trends

Periodic Properties of the Elements Alkali Metals Soft, metallic solids. Name comes from Arabic word for ashes.

Periodic Properties of the Elements Alkali Metals Found only as compounds in nature. Have low densities and melting points. Also have low ionization energies.

Periodic Properties of the Elements Alkali Metals Their reactions with water are famously exothermic. See video

Periodic Properties of the Elements Alkali Metals Flame test

Periodic Properties of the Elements Alkaline Earth Metals Be does not react with water, Mg reacts only with steam, but others react readily with water. Reactivity tends to increase as go down group. See video

Periodic Properties of the Elements Group 6A: Calcogens Oxygen, sulfur, and selenium are nonmetals. Tellurium is a metalloid. The radioactive polonium is a metal.

Periodic Properties of the Elements Oxygen Two allotropes:  O 2  O 3, ozone Three anions:  O 2−, oxide  O 2 2−, peroxide  O 2 1−, superoxide Tends to take electrons from other elements (oxidation)

Periodic Properties of the Elements Sulfur Weaker oxidizing agent than oxygen. Most stable allotrope is S 8, a ringed molecule. See video.See video

Periodic Properties of the Elements Group VIIA: Halogens Some physical properties.Some physical properties Name comes from the Greek halos and gennao: “salt formers”

Periodic Properties of the Elements Group VIIA: Halogens Large, negative electron affinities  Therefore, tend to oxidize other elements easily React directly with metals to form metal halides Chlorine added to water supplies to serve as disinfectant. Fluorine is the most reactiveFluorine Reactivity decreases as the atomic number increases.Reactivity decreases

Periodic Properties of the Elements Group VIIIA: Noble Gases Astronomical ionization energies Positive electron affinities  Therefore, relatively unreactive Monatomic gases

Periodic Properties of the Elements Group VIIIA: Noble Gases Xe forms three compounds:  XeF 2  XeF 4 (at right)  XeF 6 Kr forms only one stable compound:  KrF 2 The unstable HArF was synthesized in 2000.

Periodic Properties of the Elements Periodicity and the design of new molecules Since elements in the same group have similar physical and chemical properties, the prediction of properties of elements and compounds is possible. Formulas of compounds can also be predicted. Example: SiO 2 can be a ceramic, so SnO 2 is expected to be as well.