Atoms and their structure Chapter 2 Atoms and their structure

History of the atom Original idea of the atom: Ancient Greece (400 B.C.) Greek philosophers Democritus and Leucippus Empty space and small particles

The Beginning of Atomic Theory Atomos - not to be cut, indivisible Atoms - smallest particles of matter, different types of atoms exist for every type of matter

Lavoisier French chemist (1743-1794) Observed chemical changes in sealed containers Mass of reactants = Mass of products Conservation of Matter

Who’s Next? John Dalton: A teacher in Late 1700’s England Summarized results of his experiments and those of other’s in: Dalton’s Atomic Theory Combined ideas of elements with that of atoms

Dalton’s Atomic Theory 1. All matter is made of tiny particles called atoms. 2. Atoms are indestructible and cannot be divided into smaller parts. 3. Atoms of the same element are identical, those of different atoms are different.

Theory or Law? A scientific law is a fact of nature Ex: the sun rises, conservation of matter A theory explains the law Dalton’s theory explains why matter in conserved

Parts of Atoms J. J. Thomson - English physicist, 1897 Cathode ray tube experiment. Vacuum tube - all air has been pumped out.

Thomson’s Experiment Voltage source - + Vacuum tube Metal Disks

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment Voltage source By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field

Thomson’s Experiment Voltage source + - By adding an electric field he found that the moving pieces were negative

Thomson’s Model Found the electron Couldn’t find positive charge(for a while) Said the atom was like plum pudding A bunch of positive stuff, with the electrons able to be removed

Other Subatomic Particles Proton - positively charged particles 1,840 times heavier than the electron Neutron - no charge, but the same mass as a proton. Where are the particles?

Rutherford’s Experiment Ernest Rutherford - English physicist, (1910) Used radioactivity Alpha particles - positively charged He nuclei given off by polonium Shot them at gold foil which can be made a few atoms thick

Radioactivity

Rutherford’s experiment When the alpha particles hit a fluorescent screen, it glows. Americium 241, just over 1cm from the screen.

Florescent Screen Lead block Uranium Gold Foil

He Expected… …The alpha particles to pass through without changing direction very much Because… The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

What he expected

Because…

Because he thought the mass was evenly distributed in the atom

Because he thought the mass was evenly distributed in the atom

What he got

How Rutherford explained it Atom is mostly empty Small, dense, positive particles at center Alpha particles are deflected by it if they get close enough (like repels like) +

+

Subatomic Particles Relative mass Actual mass (g) Name Symbol Charge Electron e- -1 1/1840 9.11 x 10-28g Proton p+ +1 1 1.67 x 10-24g Neutron n0 1 1.67 x 10-24g

Structure of the Atom There are two regions: 1. The nucleus - protons and neutrons - positive charge - almost all the mass 2. Electron cloud - Most of the volume of an atom - The region where the electron can be found

Counting the Pieces Atomic Number = number of protons # of protons determines kind of atom the same as the number of electrons in the neutral atom Mass Number = the number of protons PLUS the number of neutrons Includes all the things with mass

Isotopes Dalton was incorrect. Atoms of the same element can have different numbers of neutrons! different mass numbers called isotopes

Periodic Table Info Contains the symbol of the element, the atomic mass and the atomic number Symbol: letters in the middle Atomic mass: decimal number (usually at bottom) Atomic number: whole number (usually at top)

Periodic Table Info Label the following diagram:

Atomic Mass How heavy is an atom of oxygen? There are different kinds of oxygen atoms. Look at Average atomic mass. Based on abundance of each element in nature.

Atomic Mass Calculate the atomic mass of copper if copper has two isotopes. 69.1% of Cu atoms have a mass of 62.93 amu and the rest have a mass of 64.93 amu.

Bohr’s Model Why don’t electrons fall into the nucleus? Move like planets around the sun. Circular orbits at different levels. Amounts of energy separate one level from another.

Bohr’s Model Nucleus Electron Orbit Energy Levels

} Bohr’s Model Fifth Fourth Increasing energy Third Second First Further away from the nucleus means higher energy. There is no “in between” energy Energy Levels Fifth Fourth Third Increasing energy Second First Nucleus