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Periodic Trends
- 1. Periodic Trends Elemental Propertiesand Patterns www.tmcleod.org/Level1/0809ch6per iodictrend.ppt
- 2. The Periodic Law •Dimitri Mendeleev was the first scientist to publish an organized periodic table of the known elements. • He was perpetually in trouble with the Russian government and the Russian Orthodox Church, but he was brilliant never-the-less.
- 3. The Periodic Law •Mendeleev even went out on a limb and predicted the properties of 2 at the time undiscovered elements. • He was very accurate in his predictions, which led the world to accept his ideas about periodicity and a logical periodic table.
- 4. The Periodic Law •Mendeleev understood the ‘Periodic Law’ which states: • When arranged by increasing atomic number, the chemical elements display a regular and repeating pattern of chemical and physical properties.
- 5. The Periodic Law •Atoms with similar properties appear in groups or families (vertical columns) on the periodic table. • They are similar because they all have the same number of valence (outer shell) electrons, which governs their chemical behavior.
- 6. Valence Electrons • Doyou remember how to tell the number of valence electrons for elements in the s- and p-blocks? • How many valence electrons will the atoms in the d-block (transition metals) and the f- block (inner transition metals) have? • Most have 2 valence e-, some only have 1.
- 7. A Different Typeof Grouping • Besides the 4 blocks of the table, there is another way of classifying element: • Metals • Nonmetals • Metalloids or Semi-metals. • The following slide shows where each group is found.
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- 9. Metals, Nonmetals, Metalloids •There is a zig-zag or staircase line that divides the table. • Metals are on the left of the line, in blue. • Nonmetals are on the right of the line, in orange.
- 10. Metals, Nonmetals, Metalloids •Elements that border the stair case, shown in purple are the metalloids or semi- metals. • There is one important exception. • Aluminum is more metallic than not.
- 11. Metals, Nonmetals, Metalloids •How can you identify a metal? • What are its properties? • What about the less common nonmetals? • What are their properties? • And what the heck is a metalloid?
- 12. Metals • Metals arelustrous (shiny), malleable, ductile, and are good conductors of heat and electricity. • They are mostly solids at room temp. • What is one exception?
- 13. Nonmetals • Nonmetals arethe opposite. • They are dull, brittle, nonconductors (insulators). • Some are solid, but many are gases, and Bromine is a liquid.
- 14. Metalloids • Metalloids, akasemi-metals are just that. • They have characteristics of both metals and nonmetals. • They are shiny but brittle. • And they are semiconductors. • What is our most important semiconductor?
- 15. Periodic Trends • Thereare several important atomic characteristics that show predictable trends that you should know. • The first and most important is atomic radius. • Radius is the distance from the center of the nucleus to the “edge” of the electron cloud.
- 16. AtomicRadius • Since acloud’s edge is difficult to define, scientists use define covalent radius, or half the distance between the nuclei of 2 bonded atoms.
- 17. Atomic Radius • Thetrend for atomic radius in a vertical column is to go from smaller at the top to larger at the bottom of the family. • Why? • With each step down the family, we add an entirely new PEL to the electron cloud, making the atoms larger with each step.
- 18. Atomic Radius • Thetrend 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.
- 19. Atomic Radius • Theeffect 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.
- 20. Atomic Radius • Theoverall trend in atomic radius looks like this.
- 21. Atomic Radius • Hereis an animation to explain the trend. • On your help sheet, draw arrows like this:
- 22. Ionization Energy • Thisis 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.
- 23. Ionization Energy • Theenergy 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.
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- 25. Ionization Energy (Potential) •Draw arrows on your help sheet like this:
- 26. Electron Affinity • Whatdoes the word ‘affinity’ mean? • Electron affinity is the energy change that occurs when an atom gains an electron (also measured in kJ). • Where ionization energy is always endothermic, electron affinity is usually exothermic, but not always.
- 27. Electron Affinity • Electronaffinity is exothermic if there is an empty or partially empty orbital for an electron to occupy. • If there are no empty spaces, a new orbital or PEL must be created, making the process endothermic. • This is true for the alkaline earth metals and the noble gases.
- 28. Electron Affinity • Yourhelp sheet should look like this: + +
- 29. Electronegativity • Electronegativity isa measure of an atom’s attraction for another atom’s electrons. • It is an arbitrary scale that ranges from 0 to 4. • The units of electronegativity are Paulings. • Generally, metals are electron givers and have low electronegativities. • Nonmetals are are electron takers and have high electronegativities. • What about the noble gases?
- 30. Electronegativity • Your helpsheet should look like this: 0
- 31. Overall Reactivity • Thisties all the previous trends together in one package. • However, we must treat metals and nonmetals separately. • The most reactive metals are the largest since they are the best electron givers. • The most reactive nonmetals are the smallest ones, the best electron takers.
- 32. Overall Reactivity • Yourhelp sheet will look like this: 0