Transcript Density
Chapter 10 Periodic Trends Density Atomic and Ionic Radii Density Density • Iridium (Z = 77) has the highest density. • Why not Meitnerium (Z = 109?) • We can predict that an element is more dense if it is closer to Iridium. Density Effective Nuclear Charge (Zeff) • The effective nuclear charge (Zeff) of an atom is basically how well it is able to hold on to its most loosely held electron. Atomic radius (radii) • The atomic radius is essentially the size of an atom. • The largest atom is Francium (Z = 87). Atomic Radius Xylophone monkey Rank the atoms from smallest to largest K, S, Rb, Cl Cl < S < K < Rb How does effective nuclear charge relate to the sizes of these atoms? Atomic Radius and Effective Nuclear Charge •The atomic radius increases as the effective nuclear charge decreases. Effective Nuclear Charge (Zeff) • The effective nuclear charge of an atom is primarily determined by: 1. The nuclear charge – The number of protons. Rank the atoms from smallest to largest Cl < S < K < Rb Why is Cl smaller than S? Both atoms are in the 3rd row (period) of the periodic table. What does this tell us? The Nuclear Charge (Z) • The size of atoms in the same period (row) is determined by the nuclear charge (number of protons in the nucleus). Nuclear charge explains why atoms get smaller across a period Atomic Radius Xylophone monkey Rank the atoms from smallest to largest Cl < S < K < Rb Why is K smaller than Rb? Does the number of protons in each of these atoms explain their size? Effective Nuclear Charge (Zeff) • The effective nuclear charge of an atom is primarily determined by: 1. The nuclear charge – The number of protons 2. The shielding effect. The Shielding Effect College Lecture Class Effective Nuclear Charge (Zeff) • The effective nuclear charge of an atom is primarily determined by: 1. The nuclear charge – The number of protons 2. The shielding effect. – The number of energy levels. Shielding Effect. • The shielding effect is energy levels between the nucleus and the outermost electrons in an atom shield or lessen the hold of the nucleus on the outermost electrons. K vs. Rb Shielding Effect explains why atoms get larger down a group Atomic Radius Xylophone monkey The Nuclear Charge (Z) • Example: Carbon vs. Nitrogen • Which atom would you predict to be smaller? The Nuclear Charge (Z) Carbon Nitrogen Conclusion: Nitrogen has a greater effective nuclear charge than carbon because nitrogen has 7 protons to pull in its two energy levels whereas carbon only has 6 protons holding it’s two energy levels. Therefore nitrogen atoms are smaller than carbon atoms. Nitrogen Carbon Shielding Effect. Why is He smaller than Ne? He Ne Conclusion: Helium has a greater effective nuclear charge than neon because helium has a lower shielding effect from having only one energy level whereas neon has two. He Ne Chemical Reactivity Chemical Reactivity Chemical Reactivity • Metals tend to lose electrons when reacting. – Large metal atoms are more reactive. • Nonmetals tend to gain electrons when reacting. – Small nonmetal atoms are more reactive. Chemical Reactivity • Metals increase in reactivity left and down. • Nonmetals become more reactive up and to the right. • Most reactive metal is? Fr • Most reactive nonmetal is?F Francium • Francium is the least stable naturally occurring element with a half-life of only 22 minutes. • It has been calculated that there is at most 30 g of francium in the Earth's crust at any time. Ionic Radius • Ionic Radius is the size of an ion. Size Change in Ion Formation Rules for Ionic Radius • Anions (negative ions) are “always” larger than cations (positive ions). • Ionic Radius goes by the same rules as atomic radius (ions get larger as we move down and to the left). • However it is necessary to treat anions and cations separately. (Anions are bigger). Rank the ions from smallest to largest K+, N3-, Na+, O2- + Na + <K < 2O < 3N Homework • Worksheet: Density, Atomic and Ionic Radii (due tomorrow). • Lab Summary: Reactivity and the Periodic Table (due Monday). • Study Guide Chapter 10 (due Tuesday).