Chapter 2


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Atomic structure, periodic table

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  • Chapter 2

    1. 1. Chapter 2: Atoms and Elements
    2. 2. Radioactivity <ul><li>One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934). </li></ul><ul><li>She discovered radioactivity , the spontaneous disintegration of some elements into smaller pieces. </li></ul>
    3. 3. ATOMIC COMPOSITION <ul><li>Protons </li></ul><ul><ul><li>+ electrical charge </li></ul></ul><ul><ul><li>mass = 1.672623 x 10 -24 g </li></ul></ul><ul><ul><li>relative mass = 1.007 atomic mass units (amu) </li></ul></ul><ul><li>Electrons </li></ul><ul><ul><li>negative electrical charge </li></ul></ul><ul><ul><li>relative mass = 0.0005 amu </li></ul></ul><ul><li>Neutrons </li></ul><ul><ul><li>no electrical charge </li></ul></ul><ul><ul><li>mass = 1.009 amu </li></ul></ul>
    4. 4. A TOM COMPOSITION <ul><li>protons and neutrons in the nucleus. </li></ul><ul><li>the number of electrons is equal to the number of protons. </li></ul><ul><li>electrons in space around the nucleus. </li></ul><ul><li>extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water. </li></ul>The atom is mostly empty space
    5. 5. <ul><li>He proved that atoms of any element can be made to emit tiny negative particles </li></ul><ul><li>From this he concluded that ALL atoms must contain there negative particles </li></ul><ul><li>He knew that atoms did not have a net negative charge and so there must be “something” to balance the negative charge </li></ul>
    6. 6. <ul><li>In 1909, Millikan discovered the mass of the electron to be 9.10x10 -28 grams </li></ul><ul><li>He used his oil-drop experiment </li></ul>
    7. 7. <ul><li>Chadwick discovered the neutrons in 1932. </li></ul>
    8. 8. <ul><li>The modern view of the atom was developed by Ernest Rutherford (1871-1937). </li></ul>Screen 2.9
    9. 9. Atomic Number, Z <ul><li>All atoms of the same element have the same number of protons in the nucleus, Z </li></ul>13 Al 26.981 Atomic number Atom symbol Atomic weight
    10. 10. Atomic Weight <ul><li>This tells us the mass of one atom of an element relative to one atom of another element. </li></ul><ul><li>Define one element as the standard against which all others are measured </li></ul><ul><li>Standard = carbon </li></ul>
    11. 11. Mass Number, A <ul><li>C atom with 6 protons and 6 neutrons is the mass standard </li></ul><ul><li>= 12 atomic mass units </li></ul><ul><li>Mass Number (A) = # protons + # neutrons </li></ul><ul><li>A boron atom can have A = 5 p + 5 n = 10 amu </li></ul>
    12. 12. Isotopes <ul><li>Atoms of the same element (same Z) but different mass number (A). </li></ul><ul><li>Boron-10 ( 10 B) has 5 p and 5 n </li></ul><ul><li>Boron-11 ( 11 B) has 5 p and 6 n </li></ul>10 B 11 B
    13. 13. Isotopes <ul><li>Because of the existence of isotopes, the mass of a collection of atoms has an average value. </li></ul><ul><li>Average mass = ATOMIC WEIGHT </li></ul><ul><li>Boron is 20% 10 B and 80% 11 B. That is, 11 B is 80 percent abundant on earth. </li></ul><ul><li>For boron atomic weight </li></ul><ul><li>= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu </li></ul>10 B 11 B
    14. 14. Isotopes & Atomic Weight <ul><li>Because of the existence of isotopes, the mass of a collection of atoms has an average value. </li></ul><ul><li>6 Li = 7.5% abundant and 7 Li = 92.5% </li></ul><ul><ul><li>Atomic weight of Li = ______________ </li></ul></ul><ul><li>28 Si = 92.23%, 29 Si = 4.67%, 30 Si = 3.10% </li></ul><ul><ul><li>Atomic weight of Si = ______________ </li></ul></ul>
    15. 15. Counting Atoms <ul><li>Chemistry is a quantitative science—we need a “counting unit.” </li></ul>1 mole is the amount of substance that contains as many particles (atoms, molecules) as there are in 12.0 g of 12 C. MOLE
    16. 16. Particles in a Mole <ul><li>6.02214199 x 10 23 </li></ul>Avogadro’s Number There is Avogadro’s number of particles in a mole of any substance. Amedeo Avogadro 1776-1856
    17. 17. Molar Mass <ul><li>1 mol of 12 C = 12.00 g of C = 6.022 x 10 23 atoms of C </li></ul><ul><li>12.00 g of 12 C is its MOLAR MASS </li></ul><ul><li>Taking into account all of the isotopes of C, the molar mass of C is 12.011 g/mol </li></ul>
    18. 18. PROBLEM: What amount of Mg is represented by 0.200 g? How many atoms? <ul><li>Mg has a molar mass of 24.3050 g/mol. </li></ul>= 4.95 x 10 21 atoms Mg How many atoms in this piece of Mg?
    19. 19. Periodic Table <ul><li>Dmitri Mendeleev developed the modern periodic table. Argued that element properties are periodic functions of their atomic weights. </li></ul><ul><li>We now know that element properties are periodic functions of their ATOMIC NUMBERS . </li></ul>
    20. 20. Regions of the Periodic Table