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The Structure of the Atom: From Democritus to Rutherford

Explore the journey of atomic theory from Democritus to Rutherford, learning about the structure of atoms, the discovery of electrons, and the role of protons and neutrons. Discover how changing the number of protons and electrons can result in different elements.

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The Structure of the Atom: From Democritus to Rutherford

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  1. CHAPTER 5 The Structure of the Atom 5.1 The Atom has a Structure

  2. What we’ve seen so far… - Chemistry is about discovering and understanding natural laws using the scientific method - Energy cannot be created nor destroyed Chapter 1 The Science of Chemistry

  3. What we’ve seen so far… - Matter can be mixtures or substances. The smallest unit of a pure substance is an element. - Each element is a type of atom, and they can be arranged in a periodic table. -Atoms come together to form compounds. Atoms with an electric charge are known as ions. Chapter 1 The Science of Chemistry Chapter 2 Matter and Atoms

  4. What we’ve seen so far… Chapter 1 The Science of Chemistry Chapter 2 Matter and Atoms Chapter 3 Temperature, Energy, and Heat - Temperature is a measure of kinetic energy. - Heat is thermal energy. - Heat gain and heat loss can cause phase changes, as molecules are rearranged.

  5. What we’ve seen so far… Chapter 1 The Science of Chemistry Chapter 2 Matter and Atoms Chapter 3 Temperature, Energy, and Heat Chapter 4 Physical and Chemical Change - In a physical change molecules are rearranged. In a chemical change atoms are rearranged to make new substances. - Chemical reactions can be endothermic or exothermic. - Many reactions occur in aqueous solution, and some elements are more reactive than others.

  6. Chapter 1 The Science of Chemistry Chapter 2 Matter and Atoms Chapter 3 Temperature, Energy, and Heat Chapter 4 Physical and Chemical Change How can we explain the properties and behaviors observed so far?

  7. Answers can be found inside the atom Chapter 1 The Science of Chemistry Chapter 2 Matter and Atoms Chapter 3 Temperature, Energy, and Heat Chapter 4 Physical and Chemical Change How can we explain the properties and behaviors observed so far?

  8. 92 different types of atoms!

  9. Atoms are made from smaller particles! protons neutrons electrons Each of the elements is a unique type of atom.

  10. Atoms are made from smaller particles! protons neutrons electrons How did scientists come up with this?

  11. It started a long time ago… 460 – 370 BC Today Democritus Beginning of Atomism You cannot divide something in half forever. The smallest piece of matter is called an atom.

  12. Dalton’s Postulates 1808 Today 460 – 370 BC 1.All elements are made of tiny indivisible particles called atoms. Democritus Atomism Over 2,000 years later John Dalton comes up with the first “modern” atomic theory.

  13. Dalton’s Postulates 1808 Today 460 – 370 BC 1.All elements are made of tiny indivisible particles called atoms. Democritus Atomism 2. All atoms of the same element are the same, but different from atoms of every other element. Over 2,000 years later John Dalton comes up with the first “modern” atomic theory.

  14. Dalton’s Postulates 1808 Today 460 – 370 BC 1.All elements are made of tiny indivisible particles called atoms. Democritus Atomism 2. All atoms of the same element are the same, but different from atoms of every other element. Over 2,000 years later John Dalton comes up with the first “modern” atomic theory. 3.Chemical reactions rearrange atoms but do not create, destroy, or convert atoms from one element to another.

  15. Dalton’s Postulates 1808 Today 460 – 370 BC 1.All elements are made of tiny indivisible particles called atoms. Democritus Atomism 2. All atoms of the same element are the same, but different from atoms of every other element. Over 2,000 years later John Dalton comes up with the first “modern” atomic theory. 3.Chemical reactions rearrange atoms but do not create, destroy, or convert atoms from one element to another. 4. Compounds are made from combining atoms in simple whole number ratios.

  16. Another kind of light? Today 1870 460 – 370 BC 1808 Democritus Atomism Dalton “Modern” atomic theory William Crookes invents a tube in which virtually all the gas has been removed. Under high voltage, a ray was emitted from the cathode end of the tube.

  17. It started a long time ago… 1897 Today 460 – 370 BC 1808 1870 Democritus Atomism Crookes Cathode rays Dalton “Modern” atomic theory J.J. Thomson discovers the electron Cathode rays must be negative.

  18. It started a long time ago… 1910 Today 460 – 370 BC 1808 1897 1870 Democritus Atomism Crookes Cathode rays Thomson Discovery of the electron Dalton “Modern” atomic theory Ernest Rutherford discovers the nucleus

  19. Size and mass The size of the atom comes mostly from the space occupied by the electrons The mass of the atom comes mostly from the nucleus

  20. Charges in the atom neutrons protons electrons

  21. Charges in the atom A complete atom has a charge of zero

  22. Charges in the atom

  23. neutrons protons electrons What happens when you change the number of protons?

  24. 6 protons in carbon 7 protons in nitrogen 8 protons in oxygen You obtain a different element! The number of protons is also called the atomic number for that element.

  25. neutrons protons electrons What happens when you change the number of electrons?

  26. 11 protons A neutral sodium atom Na The protons and electrons cancel each other out Balanced charges A positive sodium ion Na1+ One proton is not neutralized by an electron, making this a +1 charged atom One electron short

  27. 8 protons A negative oxygen ion O2- Two electrons are not neutralized by protons, making this a –2 charged atom Two extra electrons

  28. The electron cloud Electrons are very light and fast. They are not organized along orbits around the nucleus. Except for mass, virtually every property of atoms is determined by electrons, including size and chemical bonding

  29. neutrons protons electrons What happens when you change the number of neutrons?

  30. Atomic number Neutrons act as “glue.” They hold protons together in the nucleus.

  31. Mass number = 6 p + 6 n = 12 12C “carbon-12” mass number: total number of protons and neutrons in a nucleus.

  32. Name Mass number # protons # neutrons

  33. Carbon-12 Carbon-13 Carbon-14 Name Mass number # protons # neutrons

  34. Carbon-12 Carbon-13 Carbon-14 Name Mass number 12 13 14 # protons # neutrons

  35. Carbon-12 Carbon-13 Carbon-14 Name Mass number 12 13 14 6 6 6 # protons # neutrons

  36. Carbon-12 Carbon-13 Carbon-14 Name Mass number 12 13 14 6 6 6 # protons 6 7 8 # neutrons

  37. isotopes: atoms or elements that have the same number of protons in the nucleus but different number of neutrons.

  38. Two lithium isotopes in nature Atomic mass How can lithium have 0.941 neutrons?

  39. Number of neutrons for each of 100 lithium atoms randomly sampled from nature It’s an AVERAGE mass!

  40. Isotope periodic table (first 4 rows)

  41. Radioactivity Lithium-8 is unstable and decays: radioactivity: a process by which the nucleus of an atom spontaneously changes itself by emitting particles or energy. decay: the process during which a nucleus undergoes spontaneous change.

  42. Forces in the atom There must be a force that keeps the nucleus together

  43. Forces in the atom In the nucleus Electrons The strong nuclear force attracts protons to protons, neutrons to neutrons, and protons to protons Electrons repel each other, but don’t “fall into” the nucleus because they are in constant motion. Electrons form bonds between atoms.

  44. neutrons protons electrons Today 460 – 370 BC 1808 1897 1910 1870 Democritus Atomism Crookes Cathode rays Thomson Discovery of the electron Rutherford Discovery of the nucleus Dalton “Modern” atomic theory The sizeof the atom comes mostly from the electrons The massof the atom comes mostly from the nucleus

  45. 5.1 Practice Problems • Vocabulary: # 1-9 • Conceptual (Idea): #28, 29, 33, 34, 35, 38, 39 (Book: 25, 28, 38, 39, 40, and 2 more) • Quantitative (Calculation): #60, 61, 62, 64, 65 (Book: 67, 71, 72, 74, 76)

  46. Calculating Atomic Mass from Relative Abundance • Convert the abundance % to a decimal • 90.4% = 0.904 • Multiply the mass of the isotope by the decimal • Add up all of the masses to get the average atomic mass • Oxygen: • Oxygen – 16: 99.759% • Oxygen – 17: 0.037% • Oxygen – 18: 0.204%

  47. Finding the Number of Particles • Protons = Atomic Number of the element • Electrons = Atomic Number – atom’s charge • Neutrons = Atomic Mass – Atomic number

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