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Periodic Trends

Periodic Trends. Atomic Size Ionization Potential Melting Point Boiling Point. Atomic Size. Atomic size or atomic radius can be measured in several different ways: The distance between the centers of diatomic molecules is divided by 2

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Periodic Trends

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  1. Periodic Trends Atomic Size Ionization Potential Melting Point Boiling Point

  2. Atomic Size • Atomic size or atomic radius can be measured in several different ways: • The distance between the centers of diatomic molecules is divided by 2 • Estimated radii values can be determined for elements that do not form diatomic molecules by determining distance between two unlike atoms and subtracting the known atom’s radius • Estimating radii values based on quantum mechanical value for the free (unbonded) atom

  3. Atomic radius trends Atomic radius decreases across a period. Why? As atomic number increases, the electrical attraction between electrons and protons increases – since electrons are added to sublevels in the same energy level, electrons are added to orbitals the same distance from the nucleus. The greater number of protons in the nucleus pulls these electrons closer to it.

  4. Atomic radius trend down a family increases. • Why? Electrons are added to orbitals with a higher PQN – meaning electrons fill orbitals that are further away from the nucleus. • Although the atomic number increases, the electrical attraction is overcome by high energy electrons in the outer orbitals.

  5. First Ionization Energy • First Ionization Energy (FIE) is the amount of energy required to remove the first valence electron from a neutral atom • What is the trend across a period? FIE increases across a period • Why? As the number of protons increases, the attraction for electrons increases – thus it is more difficult to pull an electron away.

  6. What is the FIE trend down a family? • FIE decreases as you move down a family • Why? As the electrons are added to orbitals further away from the nucleus, the attraction for them by the protons is not as great – thus it is easier to remove an electron. • Note that the FIE is indirectly related to the atomic radius. In other words, as atomic radius increases, FIE decreases.

  7. Boiling and Melting Pts. • Do melting pt and boiling pt following similar trends? • Let’s look at the graphs.

  8. No, these properties do not follow the same trends. • Both melting and boiling pts increase as one moves left to right for metals, but the points drop sharply for nonmetals. The points slowly increase for nonmetals left to right. • These physical properties and other chemical properties (especially bonding!) are determined by electron configurations and number of valence electrons.

  9. Why do these melting and boiling point trends exist? • See the following website for an explanation. http://www.creative-chemistry.org.uk/alevel/module1/trends8.htm

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