IONS, ATOMIC RADUIS, IONIZATION

Periodic Properties Periodic law = elements arranged by atomic number gives physical and chemical properties varying periodically. We will study the following periodic trends: Atomic radii Ionization energy

Ions are atoms with charges The charges can be positive or negative Positive charge is a Cation Negative charge is Anion

Ions Here is a simple way to remember which is the cation and which the anion: This is a cat-ion. This is Ann Ion. He’s a “plussy” cat! She’s unhappy and negative. +

Atomic Size Size goes UP on going down a group. Size goes UP on going down a group. Because electrons are added farther from the nucleus, there is less attraction. Because electrons are added farther from the nucleus, there is less attraction. Size goes DOWN on going across a period. Size goes DOWN on going across a period. Size goes UP on going down a group. Size goes UP on going down a group. Because electrons are added farther from the nucleus, there is less attraction. Because electrons are added farther from the nucleus, there is less attraction. Size goes DOWN on going across a period. Size goes DOWN on going across a period.

Ion Sizes Does the size go up or down when losing an electron to form a cation? Does the size go up or down when losing an electron to form a cation?

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

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

Ion Sizes Does the size go up or down when gaining an electron to form an anion?

Atomic Radius Atomic radii actually decrease across a row in the periodic table. Due to an increase in the effective nuclear charge. Within each group (vertical column), the atomic radius tends to increase with the period number. John A. Schreifels Chemistry 211Chapter 8-10 Fig Atomic Radii for Main Group Elements

Atomic Radii Figure 8.9

Atomic Radius The effect is that the more positive nucleus has a greater pull on the electron cloud. The nucleus is more positive and the electron cloud is more negative. The increased attraction pulls the cloud in, making atoms smaller as we move from left to right across a period.

Atomic Radius The trend across a horizontal period is less obvious. What happens to atomic structure as we step from left to right? Each step adds a proton and an electron (and 1 or 2 neutrons). Electrons are added to existing PELs or sublevels.

Ionization Energy This is the second important periodic trend. If an electron is given enough energy (in the form of a photon) to overcome the effective nuclear charge holding the electron in the cloud, it can leave the atom completely. The atom has been “ionized” or charged. The number of protons and electrons is no longer equal.

Ionization Energy The energy required to remove an electron from an atom is ionization energy. (measured in kilojoules, kJ) The larger the atom is, the easier its electrons are to remove. Ionization energy and atomic radius are inversely proportional. Ionization energy is always endothermic, that is energy is added to the atom to remove the electron.

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

Trends in Ionization Energy IE decreases down a group IE decreases down a group Because size increases. Because size increases. Reducing ability generally increases down the periodic table. Reducing ability generally increases down the periodic table. See reactions of Li, Na, K See reactions of Li, Na, K

Trends in Ionization Energy

Atomic Radius 2 If positively charged the radius decreases while if the charge is negatively the radius increases (relative to the atom). When substances have the same number of electrons (isoelectronic), the radius will depend upon which has the largest number of protons. E.g. Predict which of the following substances has the largest radius: P 3 , S 2 , Cl , Ar, K +, Ca 2+. John A. Schreifels Chemistry 211 Chapter 8- 19

IONIZATION ENERGY Ionization energy, E i : minimum energy required to remove an electron from the ground state of atom (molecule) in the gas phase. M(g) + h  M + + e . E i related to electron configuration. Higher energies = stable ground states. Sign of the ionization energy is always positive, i.e. it requires energy for ionization to occur. The ionization energy is inversely proportional to the radius and directly related to Z eff. Exceptions to trend: B, Al, Ga, etc.: their ionization energies are slightly less than the ionization energy of the element preceding them in their period. Before ionization ns 2 np 1. After ionization is ns 2. Higher energy  smaller radius. Group 6A elements. Before ionization ns 2 np 4. After ionization ns2np 3 where each p electron in different orbital (Hund’s rule). Electron-electron repulsion by two electrons in same orbital increases the energy (lowers E I ). John A. Schreifels Chemistry 211 Chapter 8- 20

Ionization Energy: Periodic table

HIGHER IONIZATION ENERGIES The energies for the subsequent loss of more electrons are increasingly higher. For the second ionization reaction written as M + (g) + h  M 2+ + e  E i2. Large increases in the ionization energies vary in a zig-zag way across the periodic table. States with higher ionization energies have: 1s 2 2s 2 2p 6 (stable). John A. Schreifels Chemistry 211Chapter 8-22