The Atom: From Philosophical Idea to Theory

1. The Atom: From Philosophical Idea to Theory Chapter3 Atoms: The Building Blocks of Matter 3.1 The Atom: From Philosophical Idea to Theory

2. Democritus (400 BC) "The only existing things are atoms and empty space; all else is opinion” Coined the term “atomos” which means, indivisible.

3. Law of Conservation of Mass Mass is neither created nor destroyed during chemical or physical reactions. Antoine Lavoisier Total mass of reactants = Total mass of products

4. Law of Definite Proportions • A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the sample

5. Law of Multiple Proportions • If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers a. CO2and CO b. H2O and H2O2

6. Dalton’s Atomic Theory (1808) • His theory combined the above three laws all together. • All matter is composed of extremely small particles called atoms • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties John Dalton

7. Dalton’s Atomic Theory (1808) • Atoms cannot be subdivided, created, or destroyed • Atoms of different elements combine in simple whole-number ratios to form chemical compounds • In chemical reactions, atoms are combined, separated, or rearranged John Dalton

8. Modern Atomic Theory Several changes have been made to Dalton’s theory. Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element.

9. Modern Atomic Theory #2 Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

10. The Atom: From Philosophical Idea to Theory Chapter3 Atoms: The Building Blocks of Matter 3.2 The Structure of the Atom

11. A Modern Atom • Atoms are the smallest particle of an element that retains the chemical properties of that element. • All atoms consist of two regions: • The nucleus is at the center and has positive protons and neutral neutrons within it. • Surrounding the nucleus is an area of negative electrons.

12. Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

13. Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

14. Millikan-Mass of theElectron 1909 – Robert Millikan determines the mass of the electron. Mass of the electron is 9.109 x 10-31 kg The oil drop apparatus

15. Conclusions from the Study of the Electron • Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. • Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons • Electrons have so little mass that atoms must contain other particles that account for most of the mass

16. Rutherford’s Gold Foil Experiment • Alpha particles are helium nuclei • Particles were fired at a thin sheet of gold foil • Particle hits on the detecting screen (film) are recorded

17. Rutherford’s Findings • Most of the particles passed right through • A few particles were deflected • VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Cool Website! Conclusions: • The nucleus is small • The nucleus is dense • The nucleus is positively charged

18. Atomic Particles

19. The Atom: From Philosophical Idea to Theory Chapter3 Atoms: The Building Blocks of Matter 3.3 Counting Atoms

20. Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

21. Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 18 8 8 18 Arsenic 75 33 75 16 15 31 Phosphorus

22. Practice 6 protons, 6 electrons & 7 neutrons 1. How many protons, neutrons & electrons make up an atom of carbon-13? 2. How many neutrons in Oxygen -16? 3. Write the hyphen notation for the element whose atoms have 7 electrons & 9 neutrons. Nitrogen-16

23. Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons

24. Atomic Masses Atomic mass is the average of all the naturally occurring isotopes of that element. Carbon = 12.011 1 amu (atomic mass unit) = 1/12 the mass of a carbon-12 atom; also = 1.660540 x 10-27 kg

25. Calculating Average Atomic Mass or Weighted Average •Multiply the % by the mass then add up the totals Example: #1 The mass of Copper-63 is 62.94 amu for 69.17% and copper-65 is 69.93 amu for 30.83 %. Find the Weighted average.

26. Example:#2Calculate the average atomic mass of Gallium

27. Example: #3 Calculate the atomic mass of oxygen. Its’ composition is 99.76% of atoms with the mass of 15.99 amu, 0.038% with a mass of 17.00 amu, and 0.20% with a mass of 18.00 amu.

28. The Mole • In chemistry, we use a new unit to quantify an amount of a substance. • This unit is called the mole; abbreviated mol. • The same way the supermarket uses a dozen to quantify 12 objects, we use a mole to quantify 6.02 x 1023 objects.

29. The Mole 12 1 dozen = 1 gross = 144 1 ream = 500 1 mole = 6.02 x 1023 There are exactly 12 grams of carbon-12 in one mole of carbon-12.

30. Avogadro’s Number 6.02 x 1023is called “Avogadro’s Number” in honor of the Italian chemist Amadeo Avogadro (1776-1855). I didn’t discover it. Its just named after me! Amadeo Avogadro

31. Calculations with Moles:Converting moles to grams How many grams of lithium are in 3.50 moles of lithium? X

32. Calculations with Moles:Converting grams to moles How many moles of lithium are in 18.2 grams of lithium?

33. Calculations with Moles:Using Avogadro’s Number How many atoms of lithium are in 3.50 moles of lithium?

34. Calculations with Moles:Using Avogadro’s Number How many atoms of lithium are in 18.2 g of lithium?

35. More Practice 43 g 23.5 g • What is the mass is grams of 3.6 mol of carbon? • What is the mass in grams of 1.733 mol of sulfur? • How many moles of copper are in 3.22 g copper? • How many moles of lithium are in 2.72 x 10-4 g lithium? • How many moles of carbon are in 2.25 x 1022 atoms of carbon? 0.0507 mol 3.92 x 10-5 mol 0.0374 mol

36. Even More Practice! 3 x 10-18 mol 6. How many moles of oxygen, O, are in 2,000,000 atoms oxygen? 7. How many atoms of sodium are in 3.80 mol sodium? 8. What is the mass in grams of 5.0 x 109 atoms of neon? 9. How many atoms of carbon are in 0.020 g carbon? 10. What mass of silver contains the same number of atoms as 10.0 g of boron? 2.29 x 1024 atoms 1.7 x 10-13 g 1.0 x 1021 atoms 99.8 g