Chapter 2The Atomic Nature of

Matter

Department of Chemistry and Biochemistry

Seton Hall University

2

Atomic Theory

• Bulk of atomic theory was described by Dalton

• Basic key is the definition of a compound (anybody remember?)

• We can “see” individual atoms via STM, and it confirms our expectations (consider: what if it didn’t?)

3

Dalton’s Theory

• All matter is composed of tiny particles called atoms

• All atoms of a given element have identical chemical properties that are characteristic of that element

• Atoms form chemical compounds by combining in whole-number ratios

• Atoms can change how they are combined, but they are neither created nor destroyed in chemical reactions (the Law of the Conservation of Mass)

4

Several Related Points

• Matter and energy are interconverted, the amount of mass involved is unmeasureable

• Atoms and molecules are continually in motion

• Diffusion - the passage of a particle

• Dynamic equilibrium - balanced motion of particles (consider evaporation, diffusion)

5

Atomic Architecture

• Matter is held together by forces– gravitational– electrical– magnetic

• Atoms consist of:– nucleus - contains protons and

neutrons– electrons

6

Atomic Architecture

• Nucleus contains essentially all of the mass of the atom, but essentially none of the volume

• Electrons have virtually none of the mass, but occupy basically all of the volume

• An atom the size of a baseball stadium will have a nucleus the size of a pea

7

Atomic Diversity

• An element is identified by the charge of its nucleus (the number of protons)

• Isotopes have the same number of protons, but different number of neutrons

• X - element symbol• A - atomic mass (number of protons and

neutrons)• Z - atomic number (number of protons)

XAZ

8

Isotopes

• Most elements occur naturally as two or more isotopes, hence the fractional atomic mass

• Isotopes of hydrogen– protium - one proton, one electron– deuterium - one proton, one

neutron, one electron– tritium - one proton, two

neutrons, one electron

• Mass spectrometry - see fig 53; measures mass very precisely

9

Charged species

• Charged chemical species are called ions– cations - positively charged– anions - negatively charged

• Ions can be atoms or molecules

• Compounds formed from two or more ions are called ionic compounds or salts

• Salts often dissolve in liquids to give separated ions

10

Forms of energy

• Kinetic - E=0.5mv2

• Potential

• Thermal

• Chemical

• Radiant

• Electrical

2

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11

Conservation of Energy

• Energy is neither created nor destroyed in a chemical process, but may be transferred from one body to another or changed from one form to another

• End of Chapter 2