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.Negatively chargedare constantly moving, but theelectrons orbit the nucleus in distinctmovement of individual elec-energy levels called electron shells.Introns is restricted by the elec-a carbon atom, two electrons occupytron shell that they occupy.the first electron shell, and fourelectrons occupy the valence shell.Each shell can hold a specificensehleallVsthsreilFl Dmitry Mendeleyev 53and limited number of electrons.The innermost shell of an atomcan hold two electrons, the second and third shells can hold up toeight electrons, and the fourth and fifth shell can hold up to 18 elec-trons.The shells are filled successively.If an atom has 12 electrons,the first shell would contain two (the maximum number it can hold),the second shell would contain eight (the maximum number it canhold), and the third shell would only contain two.Atoms with filledouter shells are more stable; thus, when forming a chemical bondwith another atom, the outer shell may gain or lose electrons in anattempt to fill its outermost shell.The valence of an atom is the num-ber of electrons it must gain or lose in order to fill its outer shell.The valence equals the number of single bonds that an atom mayform.Chemical properties of elements are dictated by the numberof electrons contained in their outermost shells, or valence shells.Years spent examining the structure-function relationship of chemi-cal substances prepared Mendeleyev for a pioneering development.Finally, a Unifying PrincipleWhile teaching at the University of St.Petersburg, Mendeleyevwrote a very successful two-volume chemistry textbook, ThePrinciples of Chemistry (1868 70).While conducting research inpreparation for writing the text, he became interested in the rela-tionships among the chemical elements.The physical states of thepure elements differed: solid, liquid, or gas.Some of the elementswere hard and others were soft.They were different colors andexhibited a variety of chemical properties.Yet Mendeleyev was con-vinced there was an underlying logical connection and was deter-mined to uncover it.He searched for a clear and logical manner topresent the elements to his readers.Mendeleyev proceeded by cre-ating a deck of cards, with each card representing one of the 63known elements.The cards also included other information such asvalence and other chemical and physical properties.Mendeleyev arranged and rearranged his cards according to theproperties listed on them.Then one day he awoke from an after-noon nap with an epiphany.He realized that atomic weight was themost logical characteristic by which to categorize the elements.Atomic weight is the mass of an atom and is largely dependent on 54 ChemistryThe Periodic Table of the ElementsThe periodic table is a convenient means for introducing the chemicalelements.(A modern periodic table of the elements may be found atthe back of this book.) The recurrence of chemical properties of the ele-ments when arranged by atomic number is referred to as periodic law.The elements are represented by alphabetical symbols inside squares thatare arranged into seven rows.The atomic number of each element is list-ed above the chemical symbol.The concept of atomic number, or num-ber of protons in the nucleus of an atom, was not developed until 1914by the English physicist Henry Moseley.Mendeleyev used atomicweight, which today is more correctly referred to as atomic mass.Thoughatomic mass is related to atomic number in that the number of protonsconstitutes a significant proportion of the total mass, atomic number laterproved to be a better factor for assigning placement in the periodic table.Atomic number increases in distinct increments with the addition of sin-gle protons, and the table was modified accordingly so that now the ele-ments are identified by atomic number
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