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Atoms, Elements, and Atomic Theory

Explore the fundamental laws of matter, the structure of atoms, and the periodic table in this comprehensive guide. Learn about isotopes, molecules, ions, and more.

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Atoms, Elements, and Atomic Theory

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  1. Atoms and Elements CHM 109 Suroviec Fall 2015

  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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