Atoms, Molecules, and Ions Chapter 2 Atoms, Molecules, and Ions

Early Atomic Theory History Democritus – “Atomos” (has idea of atoms) John Dalton & His Laws J.J. Thompson – Discovered electron Rutherford – Discovered Nucleus in atom

Dalton’s Atomic Theory Elements are made up of atoms Atoms of each element are identical. Atoms of different elements are different. Compounds are formed when atoms combine. Each compound has a specific number and kinds of atom. Chemical reactions are rearrangement of atoms. Atoms are not created or destroyed.

Dalton’s Laws Conservation of Mass – Matter cannot be created or destroyed Law of Definite Proportion- compounds have a constant composition. The react in specific ratios by mass. Water always has 2 hydrogen atoms and 1 oxygen atom.

Law of Multiple Proportions- When two elements form more than one compound, the ratios of the masses of the second element that combine with one gram of the first can be reduced to small whole numbers.

What?! Water (H2O)has 8 g of oxygen per 1.0 g of hydrogen. Hydrogen peroxide (H2O2) has 16 g of oxygen per 1.0 g of hydrogen. 16/8 = 2/1 The ratios of the masses of oxygen is small whole number.

Example 2: Sulfur and oxygen react to form both SO2 and SO3. In SO2 there are 32.02g of sulfur and 32.00g of oxygen. In SO3, 32.06g of sulfur are combined with 48.00grams of oxygen. What is the ratio of the weights of oxygen that combine with 32.06g of sulfur? b) How does this illustrate the law of multiple proportions?

SO2 32.00 g Oxygen = 0.999g O 32.02 g Sulfur SO3 48.00 g Oxygen = 1.497 g 32.06 g Sulfur Shows law of multiple proportions because ratio of 1.497g/0.999g = 1.5 ratio Which is 3/2 ratio!

- + Thomson’s Experiment J. J. Thomson- used Cathode ray tubes 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, he found that the moving pieces were negative

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

Millikan’s Experiment Atomizer Oil droplets - + Oil Telescope

Millikan’s Experiment X-rays X-rays give some electrons a charge.

Millikan’s Experiment Some drops would hover From the mass of the drop and the charge on the plates, he calculated the mass of an electron

Rutherford’s Experiment Used uranium to produce alpha particles. Aimed alpha particles at gold foil by drilling hole in lead block. Since the mass is evenly distributed in gold atoms alpha particles should go straight through. Used gold foil because it could be made atoms thin.

Florescent Screen Lead block Uranium Gold Foil

What he expected

Because

Because, he thought the mass was evenly distributed in the atom.

What he got

How he explained it Atom is mostly empty Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough. +

+

Modern View The atom is mostly empty space. Two regions Nucleus- protons and neutrons. Electron cloud- region where you might find an electron.

Sub-atomic Particles Z - atomic number = number of protons determines type of atom. A - mass number = number of protons + neutrons. Number of protons = number of electrons if neutral.

Symbols A X Z 23 Na 11

Using Atomic Number and Mass Numbers to determine the number of protons, neutrons and electrons 19 K Mass # 39 Element Symbol Atomic # Number of Protons = Atomic number = 19 protons Number of Neutrons = Mass # - Atomic # = 39 – 19 = 20 neutrons Number of electrons = # protons = 19 electrons (In a neutral atom!)

How Can Atoms of the Same Element be Different? Isotopes — Atoms of the same element that have different number of neutrons Since neutrons do not have a charge, adding more does not affect the atom. This only adds to the mass of the atom (increases mass number). Each particle in the nucleus has a mass of one. That means . . . The number of particles in the nucleus = Mass Number The number of electrons will still be the same as the protons.

Ions — Atoms of the same element that have a different number of electrons than protons. Having more or less electrons creates an imbalance in the charge of the atom. The number in the charge is how many more or less electrons there are. If there are more protons than electrons, then the charge is positive. (Loss of electrons) If there are more electrons than protons, the charge is negative. (Gain of electrons)

Ions Protons Neutrons Electrons 8 10 -2 8 O Charge 8 16 8 O Charge 8 Protons Neutrons Electrons 8 (-2 )charge means that there was a gain of 2 electrons, so you add 2 to the number of protons. 10

Practice – Finding the number of p+, no and e- 1.) 56 Ba 3.) 17 Cl * (-) charge means add to p+ 38 -1 142 56 86 56 17 21 18 p+ ___ no___ e-___ p+ ___ no_____ e-___ 2.) 13 Al +3 4.) 27 Co 27 60 13 14 10 p+ ___ no___ e-___ 27 33 p+ ___ no___ e-___ 27 * (+) charge means subtract from p+