Chapter 3: Electron Structure and the Periodic Law

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1 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. Chapter 3: Electron Structure and the Periodic Law 1 PERIODIC TABLE 2 PERIODIC TABLE GROUP OR FAMILY 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. A vertical column of elements that have similar chemical properties. Traditionally designated by a Roman numeral and a letter (either A or B) at the top of the column. Designated only by a number from 1 to 18 in a modern but as yet not universally-used designation. group/family period 3 PERIODIC TABLE PERIOD 4 APPEARANCE OF A MODERN PERIODIC TABLE A horizontal row of elements arranged according to increasing atomic numbers. Periods are numbered from top to bottom of the periodic table. In a modern table, elements and are not placed in their correct periods, but are located below the main table

2 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, period 4 Silver, Ag, element # 47: group IB, period 5 Sulfur, S, element # 16: group VIA, period 3 THE BOHR THEORY OF ELECTRON BEHAVIOR IN HYDROGEN ATOMS 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 Click here to play Coached Problem 7 8 THE QUANTUM MECHANICAL MODEL OF ELECTRON BEHAVIOR IN ATOMS 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 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 third shell (n = 3) would be designated as a 3p subshell. 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

3 ATOMIC ORBITALS The description of the location and energy of an electron moving around a nucleus is completed in the quantum mechanical model by specifying an 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 move. 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 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 THE ENERGY OF ELECTRONS IN ATOMS 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. RELATIONSHIPS BETWEEN SHELLS, SUBSHELLS, ORBITALS AND ELECTRONS

4 ELECTRON OCCUPANCY OF SHELLS What do magnesium and ELECTRONS AND 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. calcium have in common? 2 electrons in valence shell What predictions can be made about the number of electrons in strontium s valence shell? Members of Group IIA(2) Sr has similar chemical properties to Mg and Ca, so it likely has 2 electrons in its valence. What other element on this chart has similar properties to Mg, Ca, and Sr? magnesium calcium strontium Beryllium 19 ELECTRONIC CONFIGURATIONS 20 THE ORDER OF SUBSHELL FILLING 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. 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. 21 HUND'S RULE 22 THE PAULI EXCLUSION PRINCIPLE 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. 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

5 FILLING ORDER FOR THE FIRST 10 ELECTRONS 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 in atoms results. FILLING ORDER FOR ALL SUBSHELLS IN ATOMS The filling order for any number of electrons is obtained by following the arrows in the diagram. Shells are represented by large rectangles. H He Li Be B C N Ne Subshells are represented by small colored rectangles. Orbitals within the subshells are represented by circles AID TO REMEMBER SUBSHELL FILLING ORDER 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. SUBSHELL FILLING ORDER AND THE 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 EXAMPLES OF ELECTRON CONFIGURATIONS FOR ATOMS OF VARIOUS ELEMENTS The following electronic configurations result from the correct use of any of the diagrams given earlier. Magnesium, Mg, 12 electrons: 1s 2 2s 2 2p 6 3s 2 Silicon, Si, 14 electrons: 1s 2 2s 2 2p 6 3s 2 3p 2 Iron, Fe, 26 electrons: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 Galium, Ga, 31 electrons: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 1 NOBLE GAS CONFIGURATIONS With the exception of helium, all noble gases (group VIIIA) have electronic configurations that end with completely filled s and p subshells of the highest occupied shell. These configurations are called noble gas configurations. Noble gas configurations can be used to write electronic configurations in an abbreviated form in which the noble gas symbol enclosed in brackets is used to represent all electrons found in the noble gas configuration

6 EXAMPLES OF THE USE OF NOBLE GAS CONFIGURATIONS PERIODIC TABLE CLASSIFICATIONS OF THE ELEMENTS Magnesium: [Ne]3s2. The symbol [Ne] represents the electronic configuration of neon, 1s22s22p6. Iron: [Ar]4s23d6. The symbol [Ar] represents the electronic configuration of argon, 1s22s22p63s23p6. Galium: [Ar]4s23d104p1. The symbol [Ar] represents the electronic configuration of argon, 1s22s22p63s23p6. The periodic table can be used to classify elements in numerous ways: by Distinguishing Electron. by status as Representative, Transition, or Inner-Transition Element. by status as Metal, Nonmetal, or Metalloid. 31 CLASSIFICATION ACCORDING TO DISTINGUISHING ELECTRONS 32 REPRESENTATIVE, TRANSITION AND INNERTRANSITION ELEMENTS The distinguishing electron is the last electron listed in the electronic configuration of the element. Elements are, again, classified according to the type of distinguishing electron they contain. 33 METALS, METALLOIDS AND NONMETALS 34 PROPERTY TRENDS WITHIN THE PERIODIC TABLE Properties of elements change in a systematic way within the periodic table. The Elements of Group VA(15) arsenic antimony nitrogen phosphorous bismuth METALLIC AND NONMETALLIC PROPERTIES Most metals have the following properties: high thermal conductivity, high electrical conductivity, ductility, malleability and metallic luster. Most nonmetals have properties opposite those of metals and generally occur as brittle, powdery solids or as gases

7 Metalloids are elements that form a diagonal separation zone between metals and nonmetals in the periodic table. Metalloids have properties between those of metals and nonmetals, and often exhibit some characteristic properties of each type. TRENDS IN METALLIC PROPERTIES Elements in the same period of the periodic table become less metallic and more nonmetallic from left to right across the period. Elements in the same group of the periodic table become more metallic and less nonmetallic from top to bottom down the group SCALE DRAWINGS OF REPRESENTATIVE ELEMENT ATOMS TRENDS IN THE SIZE OF ATOMS For representative elements in the same period, atomic size decreases from left to right in the period. For representative elements in the same group, atomic size increases from top to bottom down the group. 39 TRENDS IN FIRST IONIZATION ENERGY 40 TRENDS IN CHEMICAL REACTIVITY The first ionization energy is the energy required to remove one electron from a neutral gaseous atom of an element. For representative elements in the same period, the general trend is an increase from left to right across the period. For representative elements in the same group, the general trend is a decrease from top to bottom down the group. Based on the photo, what is the trend for chemical reactivity with ethyl alcohol in group 1A(1)? lithium sodium potassium As the atomic number increases in group 1A(1), the chemical reaction becomes more vigorous. The rate of gas formation and the size of the bubbles indicate that reactivity increases from top to bottom in this family