Chapter 3

Atoms : The Building Blocks of Matter

Foundations of Atomic Theory Several basic laws were after the 1790’s (emphasis

on quantitative analysis):Lavosier:

Law of conservation of massLaw of conservation of mass: mass is neither created nor destroyed during ordinary chemical or physical processes. (Only re-arranged)

Mass of Reactants = Mass of Products

Law of conservation of energyLaw of conservation of energy: energy is neither created nor destroyed during ordinary chemical or physical processes. (Only changed from one form to another form)

Foundations of Atomic Theory Cont…

Proust: Law of definite proportions: chemical

compounds contain the same elements in exactly the same proportions by mass regardless of the size of the sample.

Ex. NaCl always is composed of 39.34% sodium and 60.66% chlorine by mass.

Foundations of Atomic Theory Cont…

Law of multiple proportionsLaw of multiple proportions: if two or more different compounds are composed of the same 2 elements, the ratio of mass of the second element combined with a certain mass of the first is always a ratio of small whole numbers.

Ex. CO and CO2: For the same mass of carbon, the mass of the O in CO to the mass of O in CO2 will be 1:2

Development of Atomic Models

John Dalton (1808):1. All matter is composed of extremely small particles

called atoms (cannot be subdivided, created, nor destroyed)

2. Atoms of the same element are identical; atoms of different elements are different

3. Atoms combine in simple whole number ratios to form compounds

4. In chemical reactions, atoms combine, separate, or are rearranged.

Dalton’s Model of the AtomBecause of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.

* small *indivisible*dense *uniform (same throughout)

Like a steel ball-bearing or marble.

J.J. Thomson (1897)

Used cathode rays to determine that atoms contained small negatively charged particles called electrons. These particles were attracted to the positive

plate and repelled by the negative plate. It didn’t matter which gas was in the tube they

all had the same results.

Cathode Ray Tube

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Electron beam

Beam deflected by Negative plate

Robert Millikan (1909)

Used the Millikan Oil Drop Experiment to prove the mass and charge of the electron

Thomson’s Model of the Atom The atom has electrons.

Electrons are embedded in a “positive goo” to keep the atom Neutral

CHOCOLATE CHIP COOKIE DOUGH MODEL (Raisin bun, Plum-pudding, blueberry Muffin)

Atoms must also contain positive charges to balance the negative electrons

Other particles must account for most of the mass of the atom

Issues Not Delt with by Thomson and Millikan

Ernest Rutherford (1911) Gold Foil Experiment:

Assumed mass and charge were evenly distributed throughout the atom (“plum-pudding” model)

Shot alpha particles (2 protons + 2 neutrons) at thin sheet of gold

Expected most of the particles to pass with only slight deflection

Most particles did, but some showed wide-angle deflections (some back to the source).

discovery of the NUCLEUS of the atom

Ernest Rutherford Cont…

causes proton-proton, proton-neutron, neutron-neutron attractions

small, dense, positively charged center of the atom

number of PROTONS in the nucleus determines the atom’s identity

Gold Foil Experiment

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Rutherford Activity

New Atomic Model

Positive Nucleus at center Negative

Electrons sitting in Empty Space

Problem with the New Model

Electrons would be pulled into the center of the atom by the very positive nucleus.

(Opposite charges attract)

We know that this doesn’t happen so how do the electrons keep from being pulled in ???

Forces in the Nucleus

Repulsive forces should exist between protons in the nucleus (like charges repel).

Strong (nuclear) force: Attractive force that acts over very small

distances in the nucleus. Overcomes the repulsive forces caused by like

charges, so the protons can all stay in the nucleus.

Atomic Dimensions

Atomic radii: 40 to 270 pm Nuclear radii: about 0.001 pm Nuclear density: about 2 x 108 metric

tons/cm3

1 amu (atomic mass unit) = 1.660540 x 10-27 kg