History of the ATOM

1. History of the ATOM AP Chemistry

2. History of the Atomic Theory • Democritus (460-370 BC) thought that matter must be made up of tiny particle called “atomos” which means invisible in Greek.

3. History of the Atomic Theory • Plato (428-348 BC) and later Aristotle (384-322 BC) believed there could be no “ultimate particle”. This view was preferred by most until the late 1700s.

4. Fundamental Chemical Laws • Law of Conservation of Mass-proved through Lavoisier’s careful measurements of masses of reactants and products that mass is neither created nor destroyed. • Law of Definite Proportions-Proust showed that a given compound always contains exactly the same proportion of elements by mass.

5. Fundamental Chemical Laws • Law of Multiple Proportions-Dalton found that when two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers. • These laws led to….

6. Dalton’s Atomic Theory • In the early 1800s John Dalton revived the “atoms” concept. • “Everything is made of atoms.” • His theory proposed that atoms were invisible and indestructible.

7. Dalton’s Theory Cont’d • He also stated that atoms of one element are all the same and different from those of other elements • Atoms combine in small whole number ratios to form compounds. • In chemical reactions atoms are just rearranged.

8. Gay-Lussac and Avogadro • Gay-Lussac first determined the formula for water by measuring the volumes of gases that combined to form a given volume of water. • Avogadro’s hypothesis-based on this work stated that equal volumes of gases at standard temperature and pressure contain equal numbers of particles.

9. J.J. Thomson and e- • Using cathode ray tubes, Englishman J.J. Thomson found the ratio between charge and mass for subatomic particles that we know as electrons. • This proved part of Dalton’s theory wrong.

10. Robert Millikan finds the mass of the electron. Plum-pudding • Using Thomson’s experiments and his own data he determined the mass of the electron and found it to be very small, about 9.1 x 10-28g. • With this information, Thomson proposed a model of the atom that had positive and negative charges in the atoms like plums in a pudding. (“plum pudding” model)

11. Rutherford’s Gold Foil Experiment. A look inside. • Ernest Rutherford tried to determine the internal arrangement of atoms by shooting a particles at VERY thin gold foil. • If the charges inside the atom were evenly spaced the particles would pass through unchanged. • That wasn’t the case, a few were deflected at large angles.

12. Rutherford’s Nuclear Atom • The results of the gold foil experiment led Rutherford to realize that there was a small positive mass in the center of atoms that contained most of the mass of the atoms, called the nucleus, and electrons must orbit the nucleus. • He called the positive particles that made up the nucleus “protons”.

13. 3 Subatomic Particles • Protons are found in the nucleus. They have a positive charge. They have a mass 1840 x that of the electron. • Neutrons are found in the nucleus. They have no charge. They have a mass about 1840 x that of the electron. • Electrons are found in the orbital and have a negative charge. Table 2-1

14. Atomic Number • All atoms of the same element have the same number of protons, this is their atomic number. • For neutral atoms, the number of protons is the same as the number of electrons. • Atomic number (Z) = # of protons= # electrons

15. Isotopes • Some atoms of an element have different numbers of neutrons. There weight is different from other atoms of the same element. • These are called isotopes. • Mass number (A) is the number of protons and the number of neutrons.

16. e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- Nonmetallic Atom Nonmetallic Atom Molecule Covalent Bonds form molecules. • Covalent bonds occur between atoms that result in the sharing of electrons between two nonmetallic atoms so that each achieves a noble gas configuration.

17. Forming covalent molecules • Only nonmetals are bound. • Enough electrons are shared so that each atom “thinks” that it has a full outer shell. • A molecule is formed that has an overall charge of zero.

18. Representations of Molecules Ball-and-Stick Model Space-filling model

19. e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- Nonmetallic Atom Nonmetallic Ion -1 Anion Nonmetallic Atom Metallic Ion +1 Cation Metallic Atom Metallic Atom Ionic Bonds form ions. • Ionic Bonds are formed when electrons are given by metallic atoms to nonmetallic atoms so that each achieves noble gas configuration.

20. Forming ionic compounds • The metal gives electrons to the nonmetal. • The number of electrons given and received must be equal, the sum of the charges = 0. • Sometimes more than one ion of each is necessary to equal zero

21. The Periodic Table

22. PT Organization • To the left of the staircase are the metals. • To the right are the nonmetals. • Metalloids are touched on two sides by the staircase (except boron and aluminum). • Groups or families are the columns (they are alike) • Periods are the rows.

23. PT by Group • Alkali metals – group 1, most reactive metals, form ions with a +1 charge • Alkaline earth metals – group 2, still pretty reactive, form ions with a +2 charge • Halogens- group 17, most reactive nonmetals, form ions with a -1 charge • Noble Gases- group 18, very stable