Atoms and the Periodic Table Chapter 3 Atoms and the Periodic Table

Classification of Matter

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.

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

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

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

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

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 –1.6022 x 10-19 C Link to Video

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.

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

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

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

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

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

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

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

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

Classification of Elements

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

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

Examples of Periodicity The halogens, Group 7A elements

Examples of Periodicity The alkali metals, Group 1A elements

Examples of Periodicity The noble gases, Group 8A elements

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

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

“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.

“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

“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.

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.

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

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

Electron Configuration Spin pairing means that electrons spin in opposite directions

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.

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)

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)

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

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