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Early Models of the Atom: Democritus, Dalton, Thomson, Rutherford

Explore the early models of the atom by Democritus, Dalton, Thomson, and Rutherford, and learn about the structure of the nuclear atom, isotopes, and atomic mass calculations.

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Early Models of the Atom: Democritus, Dalton, Thomson, Rutherford

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  1. Chapter 4 Atomic Structure

  2. Early Models of the Atom • Democritus’s Atomic Philosophy • Greek philosopher (460 B.C.-370 B.C.) • 1st to suggest the existence of atoms • Said that atoms were indivisible and indestructible

  3. Early Models of the Atom • Dalton’s Atomic Theory • English chemist and school teacher (1766-1844). • 1. All elements are composed of indivisible ‘solid sphere’ atoms. • All atoms of a given element are identical. • Atoms are different elements differ in their masses. • Different atoms combine in simple whole number ratios to form compounds.

  4. Defining the Atom • All elements are composed of atoms • All atoms of the same element are identical • Atoms of different elements combine to form compounds • Atoms of one element can never be changed into another element

  5. Structure of the Nuclear Atom Atoms are divisible Three kinds of subatomic particles are: Electrons Protons Neutrons

  6. Electrons Orbit the nucleus of the atom Have a negative (-1) charge Mass ~0 atomic mass unit (amu) Shorthand symbol (e-) J.J. Thomson discovered the electron using the cathode-ray tube

  7. CATHODE RAY A cathode ray produced in a discharge tube travels from left to right. The ray itself is invisible, but the fluorescence of a zinc sulfide coating on the glass causes it to appear green

  8. CATHODE RAY When the polarity of the magnet is reversed, the ray bends in the opposite direction

  9. CATHODE RAY The cathode ray is bent downward when the south pole of the bar magnet is brought toward it

  10. JJ Thomson’s Atom The plum pudding model

  11. 2. Protons Charge of +1 (positive) Mass of 1 amu (atomic mass unit) Located inside the nucleus Shorthand symbol (p+)

  12. 3. Neutrons Consists of charge of 0 (neutral) Mass of 1 amu Located inside the nucleus Shorthand symbol (n0)

  13. Discovering the Atomic Nucleus When the particles were discovered, scientists wondered how these particles were put together. Ernest Rutherford made it possible to understand how the subatomic particles were assembled

  14. Rutherford’s Gold-Foil Experiment Proposed that the atom is made up of empty space The mass and positive charge of the atom is located in the nucleus (a dense region)

  15. The Nuclear Atom • Nucleus • Contains positive electrical charge • Makes up the most of the mass of the atom • There are two subatomic particles: protons and neutrons • The electrons are distributed around the nucleus and occupy almost all the volume of the atom

  16. Atomic Number Elements are different because they contain different number of protons Atomic number is the number of protons in the nucleus of an atom of that element Atomic Number = protons or electrons Is usually written as a subscript Ex: 12Mg or Mg12

  17. Sodium 19 What element has 11 protons? How many protons does potassium have?

  18. Complete the Table

  19. Mass Number Mass Number = Protons + Neutrons Mass number is also known as atomic mass The number of neutrons in an atom is the difference between the mass number and the atomic number # of neutrons = mass number – atomic number Usually located at the top of the element symbol Ex: 12C C12 C-12

  20. Sample Problems • How many protons, electrons, and neutrons are in each atom? Beryllium (Be) - Neon (Ne) - Sodium(Na) - 4, 4, 5 10, 10, 10 11, 11, 12

  21. 16 – 8 = 8 • How many neutrons are in each atom? 8O16 47Ag108 82Pb207 108 – 47 = 61 207 – 82 = 125

  22. Express the composition of each atom in shorthand form. Nitrogen -14 Sodium - 23 Phosphorus -31 p+ = 7, n0 = 7, e- = 7 p+ = 11, n0 = 12, e- = 11 p+ = 15 , n0 = 16, e- = 15

  23. Isotopes

  24. Sample Exercise • Determine the number of protons, neutrons and electrons in the following: 1H1 1H2 1H3 p+ = 1, n0 = 0, e- = 1 p+ = 1, n0 = 1, e- = 1 p+ = 1, n0 = 2, e- = 1

  25. Natural Abundance of Stable Isotopes of Some Elements

  26. Uncovering Atomic Mass 63 Si -28 Atomic mass # is an average of atom’s naturally occurring isotopes Copper has 2 isotopes Cu -63 and Cu -65. Which isotope is most abundant? There are 3 isotopes of Silicon with mass numbers of 28, 29, and 30. Which is more abundant?

  27. Practice Exercise Boron has two isotopes: boron – 10 and boron – 11. Which is more abundant, given that the atomic mass of boron is 10.81? Answer: B-11

  28. Calculating Atomic Mass Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

  29. Sample Exercise An element consists of two isotopes. Isotope A has an abundance of 75%, and its mass is 14.000 a.m.u. Isotope B has an abundance of 25% and its mass is 15.000 a.m.u. What is the atomic mass of the element? Ans: 14.25

  30. Sample Exercise Boron in nature has two isotopes. Boron -10 has a mass of 10.0130 a.m.u. and abundance is 19.9%. B – 11 has a mass of 11.0093 a.m.u., and abundance is 80.1%. Calculate the atomic mass of boron. Ans: 10.8109 a.m.u

  31. Sample Exercise 207.2118 There are four isotopes of Lead: Pb – 204, Pb – 206, Pb – 207, Pb – 208. Their weights are 1.37%, 26.26%, 20.82%, and 51.55% respectively. Using this data calculate the atomic mass of lead.

  32. Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu (50.69%) and 80.92 amu (49.31%).

  33. Ions • An atom with an electrical charge • Occurs 2 ways: • Oxidation – loss of an e- results in a (+) charge • Reduction – gain of an e- results in a (-) charge Mnemonic: ‘LEO says GER’ Loss of electrons = oxidation Gain of electrons = reduction

  34. Examples of Ions • H+ • Cl- • As3- • Mg2+ • Oxidation State – • Number found on the periodic table • Shows the number of e-gained or lost • Many elements have more than one number Ions & Subatomic Particles Lost 1e- Gained 1e- Gained 3e- Lost 2e-

  35. Review and Assessment Would you expect two electrons to attract or repel each other? What charge does the atom become when it loses/gains an electron? What were the results of Rutherford’s experiment? What is the charge – positive or negative – of the nucleus of every atom?

  36. Review and Assessment Which subatomic particles are located in the nucleus? Name two ways that isotopes of an element can differ? What is the atomic mass of an element? How are the elements arranged in the modern periodic table?

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