THE PERIODIC TRENDS Essential questions/concepts

Transcription

1 THE PERIODIC TRENDS Essential questions/concepts Define atomic radius, ionic radius, ionization energy, electron affinity, and electronegativity. What are the trends among the elements for atomic radii, ionic radii, ionization energy, and electronegavitiy?

2 HOW DO YOU FIND THE SIZE OF AN ATOM? Where do you start to measure? The electron cloud doesn t have a clearly defined edge. They get around this by measuring more than 1 atom at a time.

3 ATOMIC RADIUS } Radius Atomic Radius is half the distance between adjacent nuclei in metals and of identical atoms in nonmetals.

4 Atomic Radius Trend within a group Hydrogen (1s 1 ) Lithium (1s 2 2s 1 ) Atoms get larger going down a group Sodium (1s 2 2s 2 2p 6 3s 1 ) Shielding Effect

5 WHAT CAUSES THE ATOMIC RADIUS TO INCREASE AS YOU MOVE DOWN A GROUP? H Li As we increase the atomic number (or go down a group)... each atom has another energy level, so the atoms get bigger. Na K Rb

6 Atomic Radius-Trend within a period 1s 2 2s 2 2p 1 1s 2 2s 2 2p 2 1s 2 2s 2 2p 3 5p + 6p + 7p + Atoms get smaller as you move left-to-right across a period. Shrinking Effect

7 What causes the atomic radius to decrease as you move across a period left-to-right? Going from left to right across a period, the size gets smaller. Electrons are in the same energy level. But, there is more nuclear charge. Outermost electrons are pulled closer. Na Mg Al Si P S Cl Ar

8 Atomic Radii of Representative Elements (nm) 1A 2A 3A 4A 5A 6A 7A Li Be B C N O F Na Mg Al Si P S Cl K Ca Ga Ge As Se Br Rb Sr In Sn Sb Te I Cs Ba Tl Pb Bi Po At

9 Atomic Radius Atomic Radius decreases H Li Na K Fr F At

10 CHALLENGE 1. Arrange the following groups of atoms in order of decreasing atomic radius: a. B, Al, Ga b. Sn, Sb, Te c. Cd, Si, Ga d. As, P, Cl e. O, Cl, F

11 ANSWERS 1. a. Ga, Al, B b. Sn, Sb, Te c. Cd, Ga, Si d. As, P, Cl e. Cl, O, F

12 WHAT ARE IONS? Atoms can gain or lose electrons. When they do, they form ions. An ion is an atom (or group of atoms) that has a positive or negative charge.

13 IONS Metals tend to LOSE electrons, from their outer energy level. Removal of the electron form positive ions (atoms with a + charge) are called cations. (pronounced cat-eye-on ) Nonmetals tend to GAIN one or more electrons. Adding electrons form negative ions (atoms with a - charge) are called anions. (pronounced an-eye-on )

14 IONS Sodium loses one: there are now more protons (11) than electrons (10). The charge is written as a number followed by a plus sign: Na 1+ Now named a sodium ion Chlorine will gain one electron Protons (17) no longer equals the electrons (18), so a charge of -1 Cl 1- is re-named a chloride ion

15 WRITE THE SYMBOLS AND CHARGES FOR THE FOLLOWING ELEMENTS: Element Calcium Chemical Symbol Charge Bromine Lithium

16 WRITE THE SYMBOLS AND CHARGES FOR THE FOLLOWING ELEMENTS: Element Chemical Symbol Charge Calcium Ca Ca 2+ Bromine Br Br 1- Lithium Li Li 1+

17 IONIC RADIUS When atoms lose electrons, and form cations, they will have a smaller radius. When atoms gain electrons, and form anions, they will have a larger radius.

18 IONIC RADIUS TRENDS Left-to-right across a period, the size of the positive ion decrease and size of the negative ion increase. As you move down a group, the ionic radii of both positive and negative ions increase.

19 CHALLENGE 1. Arrange the following groups of atoms in order of increasing ionic radius: a. F, Li, B, b. K, Se, Ga c. Cs, Li, K

20 ANSWERS 1. a. Li, B, F b. K, Ga, Se c. Li, K, Cs

21 Ionization Energy Energy needed to remove one of atom s electrons from its outermost shell A + E A + + e -

22 Ionization energy increases Ionization Energy H Li Na K Fr F At Ionization energy increases

23 Electron Affinity A + e - A - + E Measure of an atom s attraction, or affinity, for an extra electron. Electron affinity increases within a period from left to right. As one goes down a group, electron affinity decreases.

24 ELECTRONEGATIVITY: THE ABILITY OF AN ATOM IN A BOND TO PULL ON THE ELECTRON. (LINUS PAULING)

25 Electronegativity increases Electronegativity Ability of an atom to attract electrons in a chemical bond. H Li Na K Fr F At Electronegativity increases

26 ELECTRONEGATIVITIES OF SOME ELEMENTS Element Pauling scale F 4.0 Cl 3.0 O 3.5 N 3.0 S 2.5 C 2.5 H 2.1 Na 0.9 Cs 0.7

27 NOTE Most electronegative element is F (EN 4.0) Least electronegative stable element is Cs (EN 0.7)

28 Electronegativity increases Electron affinity increases Ionization energy increases Summary of Periodic Trends Atomic Radius increases/ Ionic size H Li Na K Fr F At Electronegativity increases Ionization energy increases Electron Affinity increases

29 CHALLENGE 2. For each of the following pairs, predict which element has (1) the larger radius, and (2) the larger ionization energy: a. Na & Cl b. C & O c. Li & Rb d. As & F e. Ne & Xe f. N & Sb g. Sr & Si h. Fe & Br

30 ANSWERS a. (1) Na, (2) Cl b. (1) C, (2) O c. (1) Rb, (2) Li d. (1) As, (2) F e. (1) Xe, (2) Ne f. (1) Sb, (2) N g. (1) Sr, (2) Si h. (1) Fe, (2) Br

31 CHALLENGE 3. List the following ions in order of increasing ionic radius: N 3-, Na +, F -, Mg 2+, O 2-4. Indicate which one of the two species in each of the following pairs is smaller: a. Cl or Cl - b. Na or Na + c. O 2- or S 2- d. Mg 2+ or Al 3+ e. Au + or Au 3+