modern chemistry ch 3 atoms: the building blocks of matter

MODERN CHEMISTRY CH 3

Atoms: The Building Blocks of Matter

3-1 The Atom: From Philosophical Idea to Scientific Theory

Democritus (400B.C.) – Greek philosopher who first used the term atom to describe a basic indivisible particle of matter.

Chemical Reaction – transformation of substances into one or more new substances

Law of Conservation of Mass – mass is neither created or destroyed during ordinary chemical reactions or physical changes

Law of Definite Proportions – chemical compounds contain the same elements in exactly the same proportions my mass (water is always 2-H and 1-O) EX: Every sample of pure salt (NaCl) contains 39.34% Na and 60.66% Cl by mass.


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.- carbon monoxide (CO) contains 1.33 g O and 1.0 g C- carbon dioxide (CO2) contains 2.66 g O and 1.0g C

- the ratio of masses of oxygen in the two compounds is

2.66 = 21.33 1


John Dalton’s Atomic Theory p. 661) All matter is composed of extremely small atoms.2) Atoms of a given element are identical in size mass, and other properties.3) Atoms cannot be subdivided, created, or destroyed.4) Atoms of different elements combine in simple whole number ratios to form compounds.5 ) In chemical reactions, atoms are combined, separated, or rearranged, but they are NOT created or destroyed.

Modern Atomic Theory – - Atoms can be subdivided into smaller particles. - All atoms of the same element are NOT identical.

3-2 The Structure of the Atom

Atom – smallest particle of an element that retains the chemical properties of that element

Nucleus – small region near the center of the atom (positively charged)

Subatomic Particles – positively charged protons, neutral neutrons, negatively charged electrons

Cathode Ray Tubes – glass tubes containing gases at low pressure exposed to electrical currents

J.J. Thomson – (1897, English Physicist) measured the charge to mass ratio of the cathode rays. The rays were composed of negatively charged particles later named electrons. He developed the Plum Pudding Model of the atom.

Robert Millikan – (1909, American Physicist) he used the Oil Drop Experiment to determine the exact charge and mass of the electron. Electron has a mass of 1/1837th of a single hydrogen atom (basically 1/1837th the mass of a proton) [p. 71]


From these two experiments we now know - atoms are electrically neutral – they have as many positive charges as they do negative charges- electron has so much less mass than atoms the atoms must contain other particles to account for their mass.

Ernest Rutherford – (1911, New Zealand) In the Gold Foil Experiment he used a radioactive source to fire positively charged alpha particles toward a sheet of gold foil. Protons must be contained in a dense nucleus. The atom is mostly empty space. (p. 72)

Niels Bohr – (1913, Denmark) his work showed that electrons follow specific paths or orbits around the nucleus of the atom in an electron cloud. (p. 96-97)


Subatomic ParticlesParticle Symbol Charge Mass # Relative Mass (a.m.u.) Actual Mass

(kg)

Electron e-, 0-1e -1 0 0.0005486 9.1x10-31

Proton p+, 11H +1 1 1.007276 1.64x10-27

Neutron n0, 10n 0 1 1.008665 1.68x10-27


Nuclear Forces – short range proton-neutron, proton-proton, and neutron-neutron forces holding the nuclear particles together (overcomes the repulsive forces of like charges)

Electron Cloud – region containing negative charge (electrons)

Atomic Radii – range from 40-270 pm (1 pm = 1x10-12)

3-3 Counting Atoms

Atomic Number (Z) – identifies the element and gives the number of protons in the nucleus of each atom of that element (whole number on the periodic table)

Neutral Atoms - # protons = #electrons Isotopes – atoms of the same element have the same #p+, but different

#n0 and, therefore different masses (p. 75-77)EX: Hydrogen isotopes:

Protium (Hydrogen -1) 1 proton, 1 electronDeuterium (Hydrogen -2) 1 proton, 1 electron, 1 neutronTritium (Hydrogen – 3) 1 proton, 1 electron, 2 neutrons

Mass Number – total number of protons and neutrons (#p+ + #n0 = mass#)

Nuclear Symbol – shows composition of isotope’s nucleus (superscript is the mass number and the subscript is the atomic number)EX: 235

92U (uranium – 235) and 21H (hydrogen – 2)

3-3 Counting Atoms

Nuclide – general term for any isotope of any element

EX #1: How many protons, electrons, and neutrons are there is an atom of chlorine – 37?

atomic number = 17#p+ = 17#e- = 17mass number = 37#n0 = mass number – atomic number = 37 -17 = 20

Pause for LOTS of practice with this! (Atomic Structure WS I and II)

3-3 Counting Atoms

Relative Atomic Mass – the mass of an atom compared to the carbon-12 isotope

Atomic Mass Unit – exactly 1/12 the mass of a carbon-12 atom Average Atomic Mass – weighted average of the atomic masses of

the naturally occurring isotopes of an elementEX: What is the average atomic mass of naturally occurring copper which consists of 69.17% copper-63 (mass of 62.929598 amu) and 30.83 % copper-65 (mass of 64.927793 amu)?

K: 69.17% Cu-63 30.83% Cu-65

unk: average atomic mass(0.6917)(62.929598amu) + (0.3083)(64.927793) = 63.55

amu

3-3 Counting Atoms

Composition Stoichiometry - this is basically doing dimensional analysis with atoms and mass

Mole (mol) – SI unit for the amount of substance- 1 mole contains as many particles as there are in exactly 12 g of carbon-12- Avogadro’s Number = 6.022x1023 the number of particles in exactly one mole of a pure substance- Molar Mass – the mass of one mole of a pure substance (g/mol); numerically equal to the atomic mass of the element in atomic mass units (periodic table)

EX: 1 mol Xe = 131.29 g Xe1 mol Na = 22.98977 g Na

3-3 Counting Atoms

1 mole = 6.022x1023 atoms (or particles)

1 mole = weight in grams from the P.T.

We will now work SEVERAL compositional stoichiometry practice problems from your book p. 83-85.

3-3 Counting Atoms

Compositional Stoichiometry Examples

modern chemistry ch 3 atoms: the building blocks of matter

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