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Chemical Periodicity. A Brief History of the Periodic Table of Elements and Their Properties. Dimitri Mendeleev. Was a Russian Chemistry Teacher in Siberia Organized the known elements by atomic mass and chemical and physical properties Found that some “gaps” existed. Why He Gets Credit.
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Chemical Periodicity A Brief History of the Periodic Table of Elements and Their Properties
Dimitri Mendeleev • Was a Russian Chemistry Teacher in Siberia • Organized the known elements by atomic mass and chemical and physical properties • Found that some “gaps” existed
Why He Gets Credit • Instead of “overlooking” these gaps, Mendeleev decided that there were undiscovered elements. • He predicted the physical and chemical properties of the unknown elements. He also predicted their atomic masses. • When the elements were discovered, Mendeleev’s predictions proved to be accurate.
The Modern Periodic Table • In the early 1900’s, Henry Mosley arranged the elements by nuclear charge (atomic number). • The table consists of 7 horizontal rows and 18 vertical columns with 2 rows separate at the bottom. • The rows are called periods and the columns are groups or families.
Modern Periodic Table • There are currently two systems for numbering the columns on the table. • The newest methods numbers the columns, from left to right, 1-18. for example aluminum would be in column number 13. • The older numbering system uses two sets of numbers, A and B.
Modern Periodic Table • In the older system, the two columns on the left side and the six columns on the right side are called representative elements. These columns are numbered from left to right IA, IIA, IIIA,…(roman numerals are preferred). The last column can be called 0 or VIIIA. • This system actually gives more information but is harder to learn. Aluminum would be in group IIIA.
Periodic Law • When the elements are arranged by their atomic number, their physical and chemical properties are periodic. • This is not a “good” law, because it really is too vague. • Only the representative elements appear to come close to following the law. • Certain properties do fit the law well enough.
Atomic Radius • Atomic radius is the size of the atom. • Going down a particular group, energy levels are added, atoms get larger. • Going across a period, and energy level get filled, it becomes more condensed, and atoms get smaller.
Energy • Energy is the “money” of the universe. • Nothing will happen unless energy is exchanged. • If the energy value is positive, energy had to be paid to make it happen. (The situation did not favor the change) • If the energy value is negative, energy was returned (the situation favored the change)
Ionization Energy • The energy required to remove an electron from a neutral atom in the gas phase. • Shielding is a process whereby inner shell electrons “block” outer electrons from the effect of the nucleus. • The further from the nucleus an electron is, the easier it is to remove. • As an energy level becomes closer to being filled, electrons are more difficult to remove.
Electron Affinity • Electron Affinity is a measure of how much the atom wants to receive an additional electron. • If the value is negative, the atom wants the electron. • If the value is positive, the atom does not want the electron.
Ionic Radius • When an atom gains electrons it becomes larger. The electrons try to spread out to reduce repulsion. • When an atom loses electrons it becomes smaller. The nucleus pulls the remaining electrons in tighter.
Electronegativity • Electronegativity is a measure of how an atom shares electrons. • Atoms with high electronegativities tend to take electrons from other atoms • Atoms with low electronegativities tend to give electrons away. • The type of bond that exists between two atoms is determined by electronegativities.
Summation • The Periodic Table was developed in 1869. • The table has periods and groups or families. • Certain properties of the representative elements are periodic. • These properties are atomic radius, ionization energy, electron affinity, ionic radius and electronegativity.
