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Chapter 3: Electronic Structure and the Periodic Law

PERIODIC LAW This is a statement about the behavior of the elements when they are arranged in a specific order. In its present form, the statement is: Elements with similar chemical properties occur at regular (periodic) intervals when the elements are arranged in order of increasing atomic numbers.

PERIODIC TABLE A periodic table is a tabular arrangement of the elements based on the periodic law. In a modern periodic table, elements with similar chemical properties are found in vertical columns called groups or families. 18 groups/families 7 periods

PERIODIC TABLE GROUP OR FAMILY A group or family is a vertical column of elements that have similar chemical properties. Traditional designation uses a Roman numeral and a letter (either A or B) at the top of the column. Modern (but not universally-used) designation uses only a number from 1 to 18.

PERIODIC TABLE PERIOD A period is a horizontal row of elements arranged according to increasing atomic numbers. Periods are numbered from top to bottom of the periodic table.

MODERN PERIODIC TABLE Elements 58-71 and 90-103 are not placed in their correct periods, but are located below the main table.

GROUP & PERIOD IDENTIFICATION ELEMENTS AND THE PERIODIC TABLE Each element belongs to a group and period of the periodic table. EXAMPLES OF GROUP AND PERIOD LOCATION FOR ELEMENTS Calcium, Ca, element 20: group IIA (2), period 4 Silver, Ag, element 47: group IB (11), period 5 Sulfur, S, element 16: group VIA (16), period 3

BOHR THEORY Bohr proposed that the electron in a hydrogen atom moved in any one of a series of circular orbits around the nucleus. The electron could change orbits only by absorbing or releasing energy. This model was replaced by a revised model of atomic structure in 1926.

QUANTUM MECHANICAL MODEL According to the quantum mechanical model of electron behavior, the precise paths of electrons moving around the nucleus cannot be determined accurately. Instead of circular orbits, the location and energy of electrons moving around the nucleus is specified using the three terms shell, subshell, and atomic orbital.

SHELL The location of electrons in a shell is indicated by assigning a number n to the shell and all electrons located in the shell. The value of n can be 1, 2, 3, 4, etc. The higher the n value, the higher is the energy of the shell and the contained electrons.

SUBSHELL Each shell is made up of one or more subshells that are designated by a letter from the group s, p, d, or f. The number of the shell to which a subshell belongs is combined with the letter of the subshell to clearly identify subshells. For example, a p subshell located in the fourth shell (n = 4) would be designated as a 4p subshell.

SUBSHELL (continued) The number of subshells located in a shell is the same as the number of the shell. Thus, shell number 3 (n = 3) contains three subshells, designated 3s, 3p, and 3d. Electrons located in a subshell are often identified by using the same designation as the subshell they occupy. Thus, electrons in a 3d subshell are called 3d electrons.

ATOMIC ORBITALS The last descriptor of the location and energy of an electron moving around a nucleus is the atomic orbital in which the electron is located. Each subshell consists of one or more atomic orbitals, which are specific volumes of space around the nucleus in which electrons of the same energy move.

ATOMIC ORBITALS (continued) Atomic orbitals are designated by the same number and letter used to designate the subshell to which they belong. Thus, an s orbital located in a 2s subshell would be called a 2s orbital. All s subshells consist of a single s orbital. All p subshells consist of three p orbitals. All d subshells consist of five d orbitals. All f subshells consist of seven f orbitals.

ATOMIC ORBITALS (continued) According to the quantum mechanical model, all types of atomic orbitals can contain a maximum of two electrons. Thus, a single d orbital can contain a maximum of 2 electrons, and a d subshell that contains five d orbitals can contain a maximum of 10 electrons.

ATOMIC ORBITAL SHAPES Atomic orbitals of different types have different shapes.

ENERGY OF ELECTRONS Electron energy increases with increasing n value. Thus, an electron in the third shell (n = 3) has more energy than an electron in the first shell (n = 1). For equal n values but different orbitals, the energy of electrons in orbitals increases in the order s, p, d and f. Thus, a 4p electron has more energy than a 4s electron.

RELATIONSHIP SUMMARY

CHEMICAL PROPERTIES The valence shell of an atom is the shell that contains electrons with the highest n value. Atoms with the same number of electrons in the valence shell have similar chemical properties. Members of Group IIA(2) magnesium calcium strontium

ELECTRON SHELL OCCUPANCY What do magnesium and calcium have in common? 2 electrons in valence shell What predictions can be made about the number of electrons in strontium’s valence shell? Sr has similar chemical properties to Mg and Ca, so it likely has 2 electrons in its valence shell. What other element on this chart has similar properties to Mg, Ca, and Sr? Beryllium

ELECTRONIC CONFIGURATIONS Electronic configurations give details of the arrangements of electrons in atoms. The notation used to represent electronic configurations is 1s22s22p6…, where the occupied subshells are indicated by their identifying number and letter such as 2s and the number of electrons in the subshell is indicated by the superscript on the letter. Thus, in the example above, the 2s2 notation indicates that the 2s subshell contains two electrons.

SUBSHELL FILLING ORDER Electrons will fill subshells in the order of increasing energy of the subshells. Thus, a 1s subshell will fill before a 2s subshell. The order of subshell filling must obey Hund's rule and the Pauli exclusion principle.

HUND'S RULE According to Hund's rule, electrons will not join other electrons in an orbital of a subshell if an empty orbital of the same energy is available in the subshell. Thus, the second electron entering a p subshell will go into an empty p orbital of the subshell rather than into the orbital that already contains an electron.

PAULI EXCLUSION PRINCIPLE Electrons behave as if they spin on an axis. According to the Pauli exclusion principle, only electrons spinning in opposite directions (indicated by ↑ and ↓) can occupy the same orbital within a subshell.

FILLING ORDER When it is remembered that each orbital of a subshell can hold a maximum of two electrons, and that Hund's rule and the Pauli exclusion principle are followed, the following filling order for the first 10 electrons results: H He Li Be B C N Ne

FILLING ORDER (continued) The filling order for any number of electrons is obtained by following the arrows in the diagram. Shells are represented by large rectangles. Subshells are represented by small colored rectangles. Orbitals within the subshells are represented by circles.

FILLING ORDER MEMORY AID The diagram provides a compact way to remember the subshell filling order. The correct order is given by following the arrows from top to bottom of the diagram, going from the arrow tail to the head, and then from the next tail to the head, etc. The maximum number of electrons each subshell can hold must also be remembered: s subshells can hold 2, p subshells can hold 6, d subshells can hold 10, and f subshells can hold 14.

FILLING ORDER & PERIODIC TABLE Notice the order of subshell filling matches the order of the subshell blocks on the periodic table, if the fill occurs in the order of increasing atomic numbers.