A few elements, including copper, silver, and gold, have been known for thousands of years There were only 13 elements identified by the year 1700. Chemists suspected that other elements existed. As chemists began to use scientific methods to search for elements, the rate of discovery increased.
Early chemists attempted to organize the known elements Some used the properties of the elements Dobereiner was a German chemist who published his classification of the elements He organized the elements into triads
A triad is a set of three elements with similar properties. The elements shown here formed one triad. Chlorine, bromine, and iodine may look different, but they have very similar chemical properties. Dobereiner noted a pattern in his triads. One element in each triad tended to have properties with values that fell midway between those of the other two elements.
In 1869, a Russian chemist and teacher, Dmitri Mendeleev, published a table of the elements. The organization he chose was a periodic table. Elements in a periodic table are arranged into groups based on a set of repeating properties. This concept is known as periodicity.
Mendeleev left spaces in his table He predicted that elements would be discovered to fill those spaces, and he predicted what their properties would be based on their location in the table.
Mendeleev s table was so successful because chemists were able to make predictions from it. However his table had one error It was arranged by increasing atomic mass.
Henry Moseley modified Mendeleev s table He arranged the elements by increasing atomic number. This led to the Periodic Law The physical and chemical properties of elements are periodic functions of their atomic numbers.
There are seven horizontal rows or periods in the table Each period corresponds to the energy level There are eighteen vertical columns or groups in the table
Although Mendeleev s table only had 60 elements, today s table has 118 elements Many elements have been discovered since his original work The noble gases - due to their unreactivity The lanthanides and actinides series many are man-made
Periodicity can be observed in the periodic table Each group has similar properties The electron configuration tells an element s position in the periodic table
The elements can be grouped into three broad classes based on their general properties. Three classes of elements: Metals Nonmetals Metalloids
The majority of elements are metals Metals have three key properties Shiny or luster Flexible (malleable hammer into a sheet and ductile drawn into a wire) Good conductor of energy (electricity and heat)
Although there are fewer nonmetals, they are more abundant on Earth Nonmetals have three key properties Dull Brittle Poor conductor of energy
Metalloids have properties of metals and nonmetals Their properties can be changed by conditions They are found along the stair step of the periodic table Silicon is the most famous metalloid It is responsible for computer chips
Remember: s block groups 1 & 2 p block groups 13 18 d block groups 3 12 f block bottom two rows s & p block elements are called main group (representative) elements
s block Group 1 Alkali metals (s 1 ) Most reactive metals So reactive, not found in nature as elements 1 valence electron Group 2 Alkaline Earth metals (s 2 ) Less reactive than group 1 metals 2 valence electrons
p block Group 13 Mixed group (s 2 p 1 ) Even less reactive than groups 1 & 2 3 valence electrons Group 14 Mixed group (s 2 p 2 ) 4 valence electrons Group 15 Mixed group (s 2 p 3 ) 5 valence electrons
p block Group 16 Mixed Group (s 2 p 4 ) 6 valence electrons Group 17 Halogens (s 2 p 5 ) Most reactive nonmetals 7 valence electrons Group 18 Noble Gases (s 2 p 6 ) 8 valence electrons (with the exception of helium) Completely unreactive nonmetals
d block Groups 3 12 Transition metals Less reactive than groups 1 & 2 f block Bottom two rows Known as the lanthanides and actinides (inner transition metals)
There are several trends in the periodic table Atomic radii Valence electrons Ionic radii Ionization energy Electron affinity Electronegativity
Atomic radii is one half the distance between the nuclei of identical atoms that are bonded together
Atomic Radius Trend: Down a group atomic radii increases This happens because of the increased number of energy levels The energy levels shield the electrons from the attraction of protons in the nucleus Across a period atomic radii decreases This happens because as more electrons are added to the same energy level, those electrons are pulled closer due to the increased number of protons in the nucleus Largest atomic radii francium Smallest atomic radii fluorine
Valence electrons are the electrons found in the outermost energy level These are the electrons available to be gained, lost, or shared All atoms want 8 valence electrons or a full outer energy level Noble gas electron configuration Valence electrons determine the chemical properties of the atom
Group Number # of Valence Electrons 1 1 2 2 13 3 14 4 15 5 16 6 17 7 18 8 Valence electrons can be represented using Lewis Dot Diagrams
Atoms are neutral because there are equal numbers of both protons and electrons Sometimes atoms can gain or lose electrons to form ions An ion is an atom or group of atoms that has a positive or negative charge Losing electrons results in a positive ion called a cation Gaining electrons results in a negative ion called an anion
Metals (left side of the table) form cations Cations are smaller than their atom counterparts because they are losing an electron (and sometimes an energy level) More positive charges have a greater pull on less negative charges
Nonmetals (right side of the table) form anions Anions are larger than their atom counterparts because they are gaining an electron Less positive charges cannot pull in the greater number of negative charges
Ionization energy is the energy required to remove an electron from an atom a low IE means it is easier to remove the electron Atoms can lose an electron, to form an ion They do this to achieve noble gas electron configuration (or 8 valence electrons) When an atom easily loses electrons, it is said to be active Metals tend to lose electrons
Ionization Energies of Some Common Elements Symbol First Second Third H 1312 He (noble gas) 2372 5247 Li 520 7297 11,810 Be 899 1757 14,840 C 1086 2352 4619 O 1314 3391 5301 F 1681 3375 6045 Ne (noble gas) 2080 3963 6276 Na 496 4565 6912 Mg 738 1450 7732 S 999 2260 3380 Ar (noble gas 1520 2665 3947 K 419 3096 4600 Ca 590 1146 4941
Ionization Energy Trend: Down a group ionization energy decreases As the valence electrons are farther from the nucleus, the atom gives them up with less energy Across a period ionization energy increase As the number of valence electrons increases in the same energy level, the atom is more resistant to giving up an electron (more energy) Greatest IE fluorine Least IE - francium
Electron affinity is the energy change required to gain an electron (released energy is a negative value) When an atom releases a lot of energy, it is said to be active Nonmetals tend to gain electrons (large energy change)
Electron Affinity Trend: Down a group electron affinity decreases (slightly) Distance from the positive nucleus decreases the pull on the electrons Across a period electron affinity increases As the number of valence electrons added to the same energy level increases, the atom easily accepts another electron (to reach 8) Greatest EA fluorine Least EA francium
Electronegativity is the measure of the ability of an atom in a chemical compound to attract electrons All values are based on fluorine Fluorine is most electronegative atom with a value of 4.0 The trend decreases in either direction from fluorine