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11 The Atomic Nature of Matter

• atoms• Molecules• Chemicals & matter

• Homework:• RQ: 6, 8, 11, 13, 14, 18, 22, 23, 24.• Ex: 2, 3, 8, 9, 17, 19, 27.

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The Atomic HypothesisConsider the following process:

Smash a rock

Smash the pieces

Continue till rough powder

Grind the powder indefinitely

When does the process stop?

Is it still rock when you are done?

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Atomic History

• Hypothesized in 5th Century B.C. (Greece)• Aristotle 4th Century B.C. did not believe in

atoms. Taught 4-element idea (earth, air, water, fire)

• Dalton 1800s England revived atomic idea• Brown 1827 hypothesized atoms caused

oscillation of microscopic particles• Today we know 88 naturally occurring

atoms and about 30 more lab created

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Unicellular Algae

Light Microscope

(SEM) Electron Microscope

SEM, about 100 times better resolution

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Resolution is Related to Wavelength of the Waves

Objects larger than distance between waves leaves a clear “shadow”.

Objects smaller than distance between waves do not.

Shorter waves give us more “resolution”

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Atoms

• years of experimentation led to a rough model: our own solar system, which is mostly empty space.

• “middle” of the atom is nucleus• “outside” of atom is an electron shell

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The Nucleus and the Atom

• nucleus: “protons” and “neutrons”• each of similar size and mass• proton has positive charge• neutron has no charge• electrons have negative charge and

are much less massive

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element• substance in which each atom has the

same number of protons (neutrons may vary)

• atomic number = # protons in atom• mass number = # protons + # neutrons• isotopes have same # protons but with

varying # neutrons. Example: • Hydrogen: 1 proton, 0 neutron, 1 electron• isotope: 1 proton, 1 neutron, 1 electron

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chemical properties

• depends on number of electrons which must equal number of protons for uncharged atom.

• atoms with more (or less) electrons than protons are called “ions” and have different chemical behavior.

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Periodic Table

• elements in same column have similar chemical properties due to similar electronic structure.

• major groups: metals, metalloid, non-metal• symbols are used to label the different

atoms. • some symbols come from different

languages e.g. gold = Au, silver = Ag, etc.

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The Periodic Table of The Elements (the distinct atoms)

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Molecules

Molecules are groups of atoms. Here are a few examples.

oxygen ammonia methane water

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Terminology• Element – atoms with the same number of protons.

• Molecule – smallest particle of chemical substance that has the same composition and chemical properties, e.g. water molecule.

• Compound – chemical substance formed from 2 or more elements and which has a definite chemical formula. Note that crystals such as NaCl are not molecular.

• Mixture – substance with components that have not combined chemically.

• Chemical Substance - an element, compound, or mixture.

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A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.

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Summary• atoms make up matter, atoms are: protons,

neutrons, electrons, type of atom determined by # protons.

• molecules are formed from atoms• atomic and molecular structures determined by

electron microscopy• definitions: element, mixture, isotope,

compound, substance.• periodic table: columns have similar chemical

properties.