John Dalton

1. John Dalton • English scientist • 1766-1844 • Dalton’s atomic theory

2. Dalton’s Theory: • 1. All elements are composed of tiny indivisible particles • 2.Atoms of the same element are identical. • The atoms of any one element are different from those of any other element.

3. Dalton’s Theory cont. • 3.Different elements can physically or chemically combine to form compounds • 4. Chemical reactions occur when elements are separated, joined or rearranged. • We cannot change an atom of one element into an atom of another element by chemical means

4. The atom: • The atom composed of the following particles: • Electrons • Protons • Neutrons

5. Electrons: • Negatively charged • J.J. Thomson discovered electrons with cathode ray tube • Found in “electron cloud” surrounding nucleus of atom

6. Protons: • Positively charged sub atomic particles

7. Neutrons: • Neutrally charged subatomic particles • Have a mass nearly equal to protons

8. Distinguishing between the atoms • Atomic number = the number of protons • Mass number = the total number of protons + neutrons • Number of neutrons = the mass number – number of protons

9. Table 5.2 • Table 5.2 is a list of the first ten elements and some relative information • Properties such as atomic number and mass number are identifying features of an element

10. Examples: • Practice problems pg. 115 and 116 • Before we answer these questions lets look at the periodic table of elements.

11. PCA: • Pg 114 • ChemMath • Practice problems A-N

12. Isotopes: • What is an Isotope? • Hint: think back to the lab with the fictitious element vegium! • Isotope = same number of protons different number of neutrons • Give an example: • Carbon 14 (Carbon 12)

13. Hydrogen Isotopes: • Hydrogen has three isotopes • Most common- Hydrogen – 1 • 1 proton • No neutrons • Second isotope = Hydrogen –2 • 1 proton and 1 neutron • Also called deuterium

14. Hydrogen Isotopes: • Third isotope = Hydrogen-3 • 1 proton • 2 neutrons • Also called tritium

15. Practice Problems: • Try the practice problems on pg. 117

16. Atomic Mass: • Atomic mass vs. actual mass • Actual mass is measured with a mass spectrometer • Atomic mass is determined by a reference to a standard isotope • Carbon 12 was chosen to be the reference isotope

17. Atomic Mass: • Carbon was assigned 12 amu’s • Or atomic mass units • amu’s are defined as one – twelfth the mass of carbon • It was assigned this number because Carbon has 6 protons and 6 neutrons, of which almost all of the mass of the element is composed

19. How do we explain atomic mass: • Definition: atomic mass of an element is the weighted average mass of the atoms in a naturally occurring sample of an element. • This reflects both mass and relative abundance

20. Lets try again: • Naturally, elements occur as a mixture of two or more isotopes. • Each isotope having a fixed mass and percent abundance • The amu of an element is usually very close to the atomic mass number of the most frequently occurring isotope

21. Calculating atomic mass: • Need three numbers • 1. The number of stable isotopes of the element • 2. The mass of each isotope • 3. The natural percent abundance of each isotope

22. Sample Problems: pg 121 • Review the sample problems then attempt the practice problems on page 121.