1. History of the Atom Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical. The atoms of one element are different from the atoms of all other elements. 2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. 3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions. 2.1

2. 8 X 2 Y 16 X8 Y + 2.1

3. HISTORY OF THE ATOM Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge 1904 like plums surrounded by pudding. PLUM PUDDING MODEL Thompson found that atoms could sometimes eject a far smaller negative particle which he called an Electron.

4. J.J. Thomson, measured mass/charge of e - (1906 Nobel Prize in Physics) 2.2 1.In 1897 JJ.Thomson devised an experiment with cathode rays that resulted in the discovery of the electron. 2.Up to this time it was thought that the hydrogen atom was the smallest particle in existence. Thomson demonstrated that electrons comprising cathode rays were nearly 2,000 times smaller in mas than the lightest known particle the hydrogen ion. 3.When a high voltage is placed across a pair of plates, they become charged relative to each other. The positively charged plate is the anode and the negatively charged plate the cathode. 4.Electrons pass from the surface of the cathode and accelerate toward the oppositely charged anode. The anode absorb many electrons, but is the anode has slits some electrons will pass through. 5.The electron travel into an evacuated tube, where they move in a straight line until striking a fluorescent screen. this screen is coated with a chemical that glows when electrons strike it.

5. HISTORY OF THE ATOM 1910 Ernest Rutherford Fired alpha particle at a piece of gold foil which was only a few atoms thick. they found that although most of them passed through. About 1 in 10,000 hitRutherford’s new evidence allowed him to propose a more detailed model with a central nucleus. He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction

6. 1.atoms positive charge is concentrated in the nucleus 2.proton (p) has opposite (+) charge of electron 3.mass of p is 1840 x mass of e - (1.67 x 10 -24 g)  particle velocity ~ 1.4 x 10 7 m/s (~5% speed of light) (1908 Nobel Prize in Chemistry) 2.2

7. HELIUM ATOM + N N + - - proton electron neutron Shell What do these particles consist of?

8. ATOMIC STRUCTURE Particle proton neutron electron Charge + charge - charge No charge 1 1 nil Mass

9. Atoms have no overall electrical charge so, an atom must have as many electrons as there are protons in its nucleus. The atomic number of an element also equals the number of electrons in a neutral atom of that element.

10. Atomic Number (Z): is the number of protons in the nucleus of the atom. Z=#p The number of protons (atomic number) determine the identity of an element. The atomic number of an element also equals the number of electrons in a neutral atom of that element.

11. Ex: Sodium What is the atomic number of Sodium? How many protons does sodium have? How many electrons does sodium have?

12. Mass Number (A): The sum of the protons and neutrons in the nucleus. A=#p + #n

13. ATOMIC STRUCTURE the number of protons in an atom the number of protons and neutrons in an atom He 2 4 Atomic mass Atomic number number of electrons = number of protons

14. 14

15. Atomic Mass Unit is a unit used to compare the masses of atoms and has the symbol u or amu. 1 AMU is approximately equal to the mass of a single proton or neutron.

16. Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they have the same atomic number.

17. Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus X A Z H 1 1 H (D) 2 1 H (T) 3 1 U 235 92 U 238 92 Mass Number Atomic Number Element Symbol 2.3

18. How many protons, neutrons, and electrons are in C 14 6 ? How many protons, neutrons, and electrons are in C 11 6 ? 6 protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons Do You Understand Isotopes? 2.3