This period contains eight elements from sodium (Z = 11) to argon (Z = 18). The period ends with neon (Z = 10) in which the second shell is complete (2s 2 2p 6). It starts with lithium (Z = 3) in which one electron enters the 2s-orbital. In all, these four orbitals have a capacity of eight electrons and, therefore, second period has eight elements in it. The second period contains 8 elements because for n = 2, there are four orbitals (one 2s and three 2p) in second energy shell (L). This means that there can be only two elements. 1s) which can accommodate only two electrons. First energy shell (K) has only one orbital (i.e. The first period corresponding to n = 1 is unique because it contains only two elements. Reading across a period from left to right, the number of electrons in the outer shell or subshell of the element increases. Each successive period in the periodic table is associated with the filling of the next higher principal energy level (n = 1, n = 2, n = 3, etc.) There are seven periods and each period starts with a different principal quantum number. In terms of electronic structure of the atom, a period constitutes a series of elements whose atoms have the same number of electron shells i.e. These are discussed below:Ī period may be defined as horizontal row in the periodic table. The modern periodic table consists of horizontal rows called periods and vertical columns called groups. Therefore the distinguishing electron must occupy either the 5 s or 5 p subshell.The modern periodic table was constructed by the Russian chemist Dmitri Mendeleyev in 1869. For example, iodine is a representative element in the fifth period. The value of n, the principal quantum number for the distinguishing electron, can be quickly determined by counting down from the top of the periodic table. As a general rule, in the case of the representative elements, the distinguishing electron will be in an ns or np subshell. In the third period the 3 s subshell is filling for Na and Mg, and therefore Al, Si, P, S, Cl, and Ar. Across the second period Li and Be have distinguishing electrons in the 2 s subshell, and electrons are being added to the 2 p subshell in the atoms from B to Ne. In the first period the distinguishing electrons for H and He are in the 1 s subshell. The first three horizontal rows or periods in the modern periodic table consist entirely of representative elements. ![]() Formulas for chlorides of the first dozen elements that show the periodic variation of valence Element This agrees with the valence rules derived from the periodic table, and results in formulas for chlorides of the first dozen elements that show the periodic variation of valence. For representative elements the number of valence electrons is the same as the periodic group number, and the number needed to match the next noble-gas configuration is 8 minus the group number. That is, the valences of the representative elements may be predicted on the basis of the number of valence electrons they have, or from the number of electrons that would have to be added in order to attain the same electron configuration as an atom of a noble gas. Many of the chemical properties of the representative elements can be explained on the basis of Lewis diagrams. ![]() Most of the elements whose chemistry and valence we have discussed so far fall into this category. The representative elements are those in which the distinguishing electron enter an s or p subshell. ![]() The type of subshell ( s, p, d, f)into which the distinguishing electron is placed is very closely related to the chemical behavior of an element and gives rise to the classification shown by the color-coding on the periodic table seen here. This last electron is called the distinguishing electron because it distinguishes an atom from the one immediately preceding it in the periodic table. Since it is the outermost (valence) electrons which are primarily involved in chemical interactions between atoms, the last electron added to an atom in the building-up process is of far more interest to a chemist than the first. The commonly used long form of the periodic table is designed to emphasize electron configurations.
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