1. Atoms and the Periodic Table
Chapter 3
Atoms and the Periodic Table

2. Classification of Matter

3. Dalton’s Atomic Theory
All matter is composed of very tiny particles, which Dalton called atoms
Atoms of different elements are different
Compounds are atoms combined in whole number ratios
Ionic compounds
Molecular compounds
Chemical reactions only rearrange atoms in how they are combined.

4. Evidence for Dalton’s Theory
Law of Conservation of Mass
Law - mass can be neither created nor destroyed
Theory - if matter is made up of indestructible atoms, then any chemical reaction just changes the attachments among atoms, but does not destroy nor change the atoms identities

5. Classification of Matter
Element: a substance that consists of atoms with the same number of protons in the nucleus
there are 114 known elements
of these, 88 occur in nature; the others have been made by chemists and physicists
their symbols consist of one or two letters

6. Classification of Matter
Compound: a pure substance made up of two or more elements in a fixed mass ratio
Formula of a compound: tells us the counting number ratios of its constituent elements and identifies each element by its atomic symbol.
NaCl: the ratio of sodium atoms to chlorine atoms in sodium chloride is 1:1
H2O: the ratio of hydrogen atoms to oxygen atoms in water is 2:1

7. The Elements Monatomic elements: consist of single atoms
Diatomic elements: there are seven elements that occur as diatomic molecules
Polyatomic elements: some elements have three or more elements per molecule

8. Atoms exist … Now what? If atoms exist, how can we characterize them?
Study the particles they emit
R. A. Millikan, an American physicist, studied electrons separate from the atom as they became attached to oil droplets falling towards a magnet. He was able to calculate the charge of one electron to 5 sig figs as – x C
Link to Video

9. Atoms exist … Now what? If atoms exist, how can we characterize them?
Study the particles they emit
J. J. Thomson – bended the path of a stream of electrons in a cathode ray tube enabling the direct calculation of an electrons mass.

10. Atoms exist … Now what? If atoms exist, how can we characterize them?
Study the particles they emit
Ernest Rutherford -postulated the nuclear atom after experimenting with bombarding thin foils of metals with alpha rays.
Link to Video

11. A Typical Atom
Protons and neutrons are found in the nucleus, and electrons are found as a cloud outside the nucleus

12. Diameter of a nucleus is only about 10-15 m.
Diameter of an atom is only about m.

13. Mass and Atomic Numbers
Mass number: the number of protons plus neutrons in the nucleus of an atom
Atomic number: the number of protons in the nucleus of an atom
a carbon atom of this composition is referred to as carbon-12

14. Isotopes
Isotopes: atoms with the same number of protons but a different number of neutrons
carbon-12 has 6 protons and 6 neutrons;
carbon-13 has 6 protons and 7 neutrons;
carbon-14 has 6 protons and 8 neutrons;
Most elements found on Earth are mixtures of isotopes
chlorine is 75.77% chlorine-35 and 24.23% chlorine-37

15. Atomic Weight
Atomic weight: the weighted average of the masses in amu of the isotopes of an element found in nature
example: chlorine is 75.77% chlorine-35 and 24.23% chlorine-37

16. Periodic Table Dmitri Mendeleev (1834-1907)
arranged the known elements in order of increasing atomic weight
he observed certain sets of properties recur periodically
he then arranged elements with recurring sets of properties in the same column called groups or families
Rows are called periods

17. Periodic Table
Fluorine, chlorine, bromine, and iodine fall in the same column

18. Classification of Elements

19. Classification of Elements
Metals
are solids (except for Hg), shiny, conductors of electricity, ductile, and malleable
form alloys
tend to give up electrons
Nonmetals
except for hydrogen (H), lie on the right side of the Periodic Table
except for graphite, do not conduct electricity
tend to accept electrons

20. Classification of Elements
Metalloids
six elements are classified as metalloids: boron, silicon, germanium, arsenic, antimony, and tellurium
they have some of the properties of metals and some of nonmetals

21. Examples of Periodicity
The halogens, Group 7A elements

22. Examples of Periodicity
The alkali metals, Group 1A elements

23. Examples of Periodicity
The noble gases, Group 8A elements

24. Do Electrons orbit the Nucleus?
Do you Know that?
Do you think that?
Do you believe that?

25. Electrons are the key to chemistry
Early experiments where elements were studied lead to an understanding that each element emitted a specific light pattern when excited.
Hydrogen Linker
Helium Linker
Link to Flame Tests

26. “Fingerprint” Meaning Revealed
The energy of electrons in an atom is quantized
An electron in an atom can have only certain allowed energies.
The fingerprint we see is an electron relaxing from an excited state to a lower excited state or the ground state thus giving off a specific amount of energy.

27. “Fingerprint” Meaning Revealed
Neils Bohr - Interpreted fingerprint as transitions between allowed distances of an orbit from the nucleus. That si to say electrons orbit the nucleus!
Math works for 1 electron systems only
Link to Bohr Model Explanation

28. “Fingerprint” Meaning Revealed
Schrodinger – Interpreted fingerprint as transitions between allowed energies of waves.
Math works for all atoms spectra plus can be applied to molecules as well
Electron energy descriptions are called electronic configurations. These come from the math of the “Schrodinger Equation.”
Link to Idea … Play to give an idea of complexity.

29. Electron Configuration
Electron configuration: the arrangement of electronic energies in the extranuclear space
Ground state: the electron configuration of lowest energy
Excited state: all electronic configurations other than the ground state
The fingerprint we see is an electron relaxing from an excited state to a lower excited state or the ground state thus giving off a specific amount of energy.

30. Electron Configuration
Electron configurations are governed by three rules
Rule 1: orbitals fill in the order of increasing energy from lowest to highest
elements in the first, second, and third periods fill in the order 1s, 2s, 2p, 3s, and 3p

33. Electron Configuration
Rule 2: each orbital can hold up to two electrons with spins paired
with four electrons, the 1s and 2s orbitals are filled and are written 1s2 2s2
with an additional six electrons, the three 2p orbitals are filled and are written either 2px2 2py2 2pz2, or they may be written 2p6

34. Electron Configuration
Spin pairing means that electrons spin in opposite directions

35. Electron Configuration
Rule 3: when there is a set of orbitals of equal energy, one orbital becomes half filled before any of them becomes completely filled
example: after the 1s and 2s orbitals are filled, a 5th electron is put into the 2px, a 6th into the 2py, and a 7th into the 2pz. Only after each 2p orbital has one electron is a second added to any 2p orbital.

36. Electron Configuration
Orbital box diagrams
a box represents an orbital
an arrow represents an electron
a pair of arrows with heads in opposite directions represents a pair of electrons with paired spins
Example: carbon (atomic number 6)

37. Electron Configuration
Noble gas notation
the symbol of the noble gas immediately preceding the particular atom indicates the electron configuration of all filled shells
Example: carbon (atomic number 6)

38. Exercise for EC and OD Link to Video … some Chem 1A terms
Link to HTML … Fun exercise View full screen!

39. Electron Configuration
Valence shell: the outermost incomplete shell
Valence electron: an electron in the valence shell
Lewis dot structure:
dots represent valence electrons