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Atoms and Elements

Atoms and Elements. CHM 109 Suroviec Spring 2014. I. Atoms and Atomic Theory. An element is composed of tiny particles called atoms All atoms of the same element have the same chemical properties Compounds are formed when two or more atoms of different element combine

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Atoms and Elements

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  1. Atoms and Elements CHM 109 Suroviec Spring 2014

  2. I. Atoms and Atomic Theory • An element is composed of tiny particles called atoms • All atoms of the same element have the same chemical properties • Compounds are formed when two or more atoms of different element combine • Chemical reactions involve only separation, combination or rearrangement or atoms

  3. A. Fundamental Laws of Matter • There are three fundamental laws of matter • Law of conservation of mass • Law of constant composition • Law of multiple proportions

  4. A. Law of conservation of mass • In a chemical reaction matter is neither created or destroyed

  5. B. Law of Definite Proportions • In 1797 Joseph Proust noticed that elements of a given compound always combined in definite proportions in all samples of a compound.

  6. C. Law of Multiple Proportions • This law asserts that when 2 elements (A&B) form two different compounds, the masses of element B that combine with 1 gram of element A can be expressed as a ratio of small whole numbers.

  7. D. Atomic Theory • John Dalton explained the 3 previous laws along with his Atomic Theory which states:

  8. II. Structure of an atom • Atom: basic unit of an element that can enter into chemical combination • Atom possesses structure: • Electron • Proton • Neutron

  9. A. Electrons • First evidence for subatomic particles came from the study of the conduction of electricity by gases at low pressures • J.J. Thomson, 1897 • Rays emitted were called cathode rays • Rays are composed of negatively charged particles called electrons • Electrons carry unit negative charge (-1) and have a very small mass (1/2000 the lightest atomic mass)

  10. Cathode Ray Apparatus

  11. 02_05.JPG

  12. III. Structure of the Atom • Since the overall atom is neutral, and we know that they contain electrons they must have a positive particle to balance the atom out.

  13. II. Structure of the Atoms

  14. A. Protons and Neutrons – The Nucleus • Ernest Rutherford, 1911 • Bombardment of gold foil with αparticles • Expected to see the particles pass through the foil

  15. Rutherford Backscattering • Found that some of the alpha particles were deflected by the foil

  16. B. Current Atomic Model • Most of the atom’s mass and all of its positive charge are contained in a small cone called the nucleus • Most of the volume of atoms is empty space which is where the electrons are • There are as many electrons as protons

  17. Rutherford’s Model of the Atom atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m If the atom is a football stadium, then the nucleus is a marble on the 50-yard line.

  18. VI. Proton, Neutron and Electron • Amu = 1/12 the mass of a carbon atom containing 6 protons and 6 neutrons. This makes the proton ~1amu. • The proton and electron have electrical charge

  19. A. Elements • What makes elements unique is the number of protons, neutrons and electrons in an atom of the element.

  20. B. Isotopes • All atoms of a given element have same number of protons, but the number of neutrons can change • This means that one element can have different masses.

  21. A X Mass Number Element Symbol Z Atomic Number B. Isotopes • Atomic number (Z) = number of protons in the nucleus • Mass number (A) = number of protons and neutrons in the nucleus • Isotope = atoms of the same element (X) with different numbers of neutrons in their nuclei

  22. 14 11 C C 6 6 How many protons, neutrons, and electrons are in How many protons, neutrons, and electrons are in ? ? Do You Understand Isotopes?

  23. H2 H2O NH3 CH4 C. Molecules and Ions • Molecules: aggregate of two or more atoms in a definite arrangement held together by chemical forces • Ion: atom or group of atoms with a net charge due to the loss or gain of electrons.

  24. C. Molecules and Ions • Cation: ion with a positive charge • Anion: ion with a negative charge

  25. How many protons and electrons are in ? 78 2- How many protons and electrons are in ? Se 34 27 3+ Al 13 Do You Understand Ions?

  26. Polyatomic ions

  27. V. Periodic Law and Table • Mendeleev constructed a periodic table based on the elements known at the time. • He noticed that certain groups of elements had similar properties

  28. A. Ions and Periodic Table • Metals tend to lose electrons • Nonmetals tend to gain electrons

  29. VI. Atomic Mass • Since isotopes have different masses for the same element an atomic mass is a weighted average of the isotopes

  30. VII. Molar Mass • We would like to know the number of atoms in a given mass of atoms. • Since atoms are so small we count them by weight. • The Mole. We use a mole to count the amount of material containing 6.022 x 1023

  31. B. Mole conversion • Using conversion factors we can convert between moles and atoms.

  32. C. Moles and Mass • The next question is what is the mass of that many atoms. • It has been determined that the mass of 1 mole of atoms is equal to its atomic mass in amu

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