Periodic Trends in Elements Chapter 8

8.1 Electronic Structure and the Periodic Table Objectives Correlate the position of an element in the periodic table with its electron configuration Write the ground state electron configuration and orbital diagrams of any element on the basis of its location in the periodic table

Fig. 2-8, p. 56

Fig. 7-20, p. 272

Fig. 7-21, p. 272

The experimental trends in group properties on which the periodic table was based can now be explained by the arrangements of electrons in atoms. Periodic Trends of the Elements

The periodic table can be divided into four blocks of elements: elements with highest energy electrons in s, p, d, or f subshells. The arrangement of the elements in the periodic table correlates with the subshell that holds the highest energy electron. Electron Configurations and the Periodic Table

Using only the periodic table, determine the electron configurations of Al, Ti, Br, and Sr. Electron Configurations

Answers: Al = [Ne]3s 2 3p 1 Ti = [Ar]4s 2 3d 2 Br = [Ar]4s 2 3d 10 4p 5 Sr = [Kr]5s 2

8.2 Electron Configurations of Ions Objectives Write the ground state electron configuration of ions Recognize the order in which electrons are lost or gained to make ions Identify an isoelectronic series

A little chemistry humor….. Why does hamburger have lower energy than steak? Because it's in the ground state.

8.2 Electron Configurations of Ions When writing the electron configurations of ions it is important to remember the definition of valence electrons Valence Electrons = electrons with the highest principal quantum number and any electrons in an unfilled subshell from a lower shell

For anions, the additional electrons fill orbitals following the same rules that applies to atoms. Cl: [Ne] 3s 2 3p 5 Cl - : [Ne] 3s 2 3p 6 As: [Ar] 4s 2 3d 10 4p 3 As 3- : [Ar] 4s 2 3d 10 4p 6 Many stable anions have the same electron configuration as a noble gas atom. Electron Configurations of Anions

For the electron configurations of cations, electrons of highest n value are removed first. For cases of the same n level, electrons are first removed from the subshell having highest. As: [Ar] 4s 2 3d 10 4p 3 As 3+ : [Ar] 4s 2 3d 10 Mn: [Ar] 4s 2 3d 5 Mn 2+ : [Ar] 3d 5 NOTE: For d-block atoms, the ns electrons are removed before the (n-1)d electrons. Electron Configurations of Cations

Test Your Skill Write the electron configurations of the following ions: (a) N 3- (b) Co 3+ (c) K + Answers: (a) 1s 2 2s 2 2p 6 (b) [Ar] 3d 6 (c) [Ar]

An isoelectronic series is a group of atoms and ions that contain the same number of electrons. The species S 2-, Cl -, Ar, K +, and Ca 2+ are isoelectronic – they all have 18 electrons. Isoelectronic Series

8.3 Sizes of Atoms and Ions Objectives Arrange atoms and ions according to size Correlate the trends in sizes with the effective nuclear charge experienced by the outer electrons

It is difficult to measure the absolute size of an atom or ion An atomic radius is one half the distance between adjacent atoms of the same element in a molecule. Most accurate for determining radii of non-metallic elements Radii of metals and ions determined experimentally 8.3 Sizes of Atoms and Ions 198/2 = /2 = 114 Sum = 215

8.3 Sizes of Atoms and Ions Trends in the Sizes of Atoms

Atomic Radii of Main Group Elements

The sizes of atoms increase going down a group. 8.3 Sizes of Atoms and Ions

The increase in effective nuclear charge causes a size decrease across the period. 8.3 Sizes of Atoms and Ions

The sizes of atoms are impacted by the effective nuclear charge felt by the outermost electrons. 8.3 Sizes of Atoms and Ions

Identify the larger species of each pair: (a) Mg or Na (b) Si or C Answers:(a) Na is larger. (b) Si is larger.

8.3 Sizes of Atoms and Ions Comparative Sizes of Atoms and Their Ions Atoms are always larger than their cations.

8.3 Sizes of Atoms and Ions If an atom makes more than one cation, the higher-charged ion has a smaller size.

Anions are always larger than their atoms. 8.3 Sizes of Atoms and Ions

Size Trends for an Isoelectronic Series

Table of Ion Sizes

Identify the larger species of each pair: (a) Mg or Mg 2+ (b) Se or Se 2- Answer:(a) Mg is larger. (b) Se 2- is larger. 8.3 Sizes of Atoms and Ions

8.4 Ionization Energy Objectives Define ionization energy Arrange atoms according to ionization energies Predict the relative energies needed for successive ionizations

The ionization energy is the energy required to remove an electron from a gaseous atom or ion in its electronic ground state. 8.4 Ionization Energy

An atom has as many ionization energies as it has electrons. Example: Mg(g) → Mg + (g) + e - I 1 = first ionization energy Mg + (g) → Mg 2+ (g) + e - I 2 = second ionization energy

General Trends in First Ionization Energies

Trends in 1 st Ionization Energies The increase in the effective nuclear charge across a period causes an increase in the ionization energy as you go across that period. 8.4 Ionization Energy

Trends in 1 st Ionization Energies

The slight dip in ionization energy for O is because the fourth p electron now pairs with another electron, slightly repelling each other. 8.4 Ionization Energy

Isoelectronic species with the greatest charge in the nucleus will have the largest ionization energy. For the isoelectronic series S 2-, Cl -, and Ar, Ar has the largest ionization energy because it has the most protons in its nucleus.

8.4 Ionization Energy Predict which species in each pair has the higher ionization energy. (a) Ca or As (b) K + or Ca 2+ (c) N or As Answers: (a) As (b) Ca 2+ (c) N

Successive ionization energies always increase because of the increasing hold the nucleus has on remaining electrons. I 1 I 2 I 3 I 4 Mg Al A much larger increase is seen when an electron comes from a lower-energy subshell. (all values in kJ/mol) 8.4 Ionization Energy

Which element, magnesium or sodium, has the greater second ionization energy? Answer: sodium

8.5 Electron Affinity Objectives Define electron affinity Relate trends in electron affinity to the periodic table State how electron affinity affects the formation of anions

The electron affinity of an element is the energy change that accompanies the addition of an electron to a gaseous atom to form an anion. A(g) + e - → A - (g) Electron affinities are generally favorable (exothermic) for elements on the right side of the periodic table. Group 7A elements have the most favorable electron affinities. Let’s explore why… Electron Affinity

Electron Affinities Think about how electron configuration is affected when an electron is added to an atom EA units = kJ/mol

8.6 Trends in the Chemistry of Elements in Groups 1A, 2A, and 7A Objectives Write equations for the common reactions of elements in Groups 1A, 2A, and 7A Identify reactivity trends within each of these groups

8.6 Trends in the Chemistry of Elements cont’d Elements within a group have the same number of valence electrons Elements within a group have similar chemical and physical properties Chemical properties of elements are strongly influenced by Size Ionization Energy Electron Affinity

8.6 Trends in the Chemistry of Elements cont’d Elements with low ionization energies are generally metals and form cations in their compounds Those elements with high ionization energies are non-metals and form anions

8.6 Trends in the Chemistry of Elements cont’d Group IA: The Alkali Metals Lithium, sodium, potassium, rubidium, cesium and francium Note the absence of hydrogen from the list!!!! Valence configuration = ns 1 Lose one electron to attain a noble gas configuration Often form cations with a +1 charge Appearance: Soft, silver colored metals (cesium is gold colored) Characteristic flame test colors

Flame Colors of the 1A Elements LithiumPotassiumSodium

8.6 Trends in the Chemistry of Elements cont’d Group IA: The Alkali Metals cont’d Highly reactive Reactivity increases as you move down the group due to decrease in ionization energy Always found in combination with other elements in nature b/c they are too reactive to exist in the environment uncombined

8.6 Trends in the Chemistry of Elements cont’d Group IA: The Alkali Metals cont’d React with water, but reactions can be dangerous due to reaction of hydrogen gas with oxygen gas in air 2 M (s) + 2H 2 O (l) → 2 MOH (aq) + H 2 (g) where M = Li, Na, K, Rb, Cs React with hydrogen 2 M (s) + 2H 2 (g) → 2 MH (s) where M = Li, Na, K, Rb, Cs

8.6 Trends in the Chemistry of Elements cont’d Group IA: The Alkali Metals cont’d React with halogens 2 M (s) + X 2 → 2 MX (s) where X = F, Cl, Br, I

8.6 Trends in the Chemistry of Elements cont’d Group IA: The Alkali Metals cont’d React with oxygen, but often form compounds other than oxides: 4 Li (s) + O 2 (g) → 2 Li 2 O (s) lithium oxide 2 Na (s) + O 2 (g) → 2 Na 2 O 2 (s) sodium peroxide K (s) + O 2 (g) → KO 2 (s) potassium superoxide Lithium also react with nitrogen 6 Li (s) + N (g) → 2 Li 3 N (s) lithium nitride

8.6 Trends in the Chemistry of Elements cont’d Group IIA: The Alkaline Earth Metals Beryllium, magnesium, calcium, strontium, barium and radium Valence configuration = ns 2 Lose two electrons to attain a noble gas configuration Often form cations with a +2 charge Appearance: Silvery white softer than most metals except for the alkali metals Characteristic flame test colors for Ca, Sr, Ba

Flame Colors of 2A Elements CalciumStrontiumBarium Flame Colors of 2A Elements

8.6 Trends in the Chemistry of Elements cont’d Group IIA: The Alkaline Earth Metals cont’d Very reactive, but not as reactive as alkali metals Reactivity increases as you move down the group due to the decrease in ionization energy Some alkaline earth metals react readily with water M (s) + 2H 2 O (l) → M(OH) 2 (aq) + H 2 (g) where M = Ca, Sr, Ba

8.6 Trends in the Chemistry of Elements cont’d Group IIA: The Alkaline Earth Metals cont’d Some alkaline earth metals will react with O 2 at high or room temperature 2 M(s) + 2 O 2 (g) → 2 MO (s) M = Be, Mg, Ca Heavier alkaline earth metals also form salts of the peroxide ion, O 2 2- Ba (s) + O 2 (g) → BaO 2 (s) Magnesium alloys are useful in aeronautical applications, where low density and high strength are important.

8.6 Trends in the Chemistry of Elements cont’d Group VIIA: The Halogens Fluorine, chlorine, bromine, iodine, and astatine Valence configuration = ns 2 np 5 Gain one electron to attain a noble gas configuration Form anions with a -1 charge Often found as diatomic molecules or combined with other elements in salts

8.6 Trends in the Chemistry of Elements cont’d Group VIIA: The Halogens cont’d Highly reactive Fluorine is the most reactive of all the non- metals Reactivity decreases as you move down the group X 2 (g) + H 2 (g) → 2HX (g) where X = F, Cl, Br, I hydrogen halides produce acidic solutions when dissolved in water