Chapter 3: Electron Structure and the Periodic Law

Transcription

1 Chapter 3: Electron 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. group/family period PERIODIC TABLE GROUP OR FAMILY 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. PERIODIC TABLE PERIOD A horizontal row of elements arranged according to increasing atomic numbers. Periods are numbered from top to bottom of the periodic table. APPEARANCE OF A MODERN PERIODIC TABLE In a modern table, elements and are not placed in their correct periods, but are located below the main table 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 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

2 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 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. Indicate the number and type of orbitals in each of the following: A. 4s sublevel B. 3d sublevel C. n = 3 A. 4s sublevel one 4s orbital B. 3d sublevel five 3d orbitals C. n = 3 one 3s orbital, three 3p orbitals, and five 3d orbitals

3 The number of A. electrons that can occupy a p orbital is 1) 1 2) 2 3) 3 B. p orbitals in the 2p sublevel is 1) 1 2) 2 3) 3 C. d orbitals in the n = 4 energy level is 1) 1 2) 3 3) 5 D. electrons that can occupy the 4f sublevel are 1) 2 2) 6 3) 14 The number of A. electrons that can occupy a p orbital is 2) 2 B. p orbitals in the 2p sublevel is 3) 3 C. d orbitals in the n = 4 energy level is 3) 5 D. electrons that can occupy the 4f sublevel are 3) 14 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 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. Members of Group IIA(2) Valence Electrons The valence electrons Determine the chemical properties of the elements. Are the electrons in the highest energy level. Are related to the Group number of the element. Example: Phosphorus has 5 valence electrons 5 valence electrons P Group 5A(15) 1s 2 2s 2 2p 6 3s 2 3p 3 22 magnesium calcium strontium Groups and Valence Electrons All the elements in a group have the same number of valence electrons. Example: Elements in Group 2A(2) have two (2) valence electrons. Be 1s 2 2s 2 Mg 1s 2 2s 2 2p 6 3s 2 Ca [Ar] 4s 2 Sr [Kr] 5s ELECTRON OCCUPANCY OF SHELLS 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. What other element on this chart has similar properties to Mg, Ca, and Sr? Periodic Table and Valence Electrons Beryllium

4 State the number of valence electrons for each: A. O 1) 4 2) 6 3) 8 B. Al 1) 13 2) 3 3) 1 C. Cl 1) 2 2) 5 3) A. O 2) 6 B. Al 2) 3 C. Cl 3) A. Calcium 1) 1 2) 2 3) 3 B. Group 6A (16) 1) 2 2) 4 3) 6 C. Tin 1) 2 2) 4 3) A. Calcium 2) 2 B. Group 6A (16) 3) 6 A. 1s 2 2s 2 2p 6 3s 2 3p 1 B. 1s 2 2s 2 2p 6 3s 2 C. 1s 2 2s 2 2p 5 A. 1s 2 2s 2 2p 6 3s 2 3p 1 3 B. 1s 2 2s 2 2p 6 3s 2 2 C. 1s 2 2s 2 2p 5 7 C. Tin 2) Electron Configuration An electron configuration Lists the sublevels filling with electrons in order of increasing energy. Uses superscripts to show the number of electrons in each sublevel. For neon is as follows: number of electrons THE ORDER OF SUBSHELL FILLING 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. sublevel 1s 2 2s 2 2p

5 THE 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 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. Write the orbital diagrams for A. carbon B. oxygen H He Li Be B C N Ne C. magnesium Write the orbital diagrams for A. carbon 1s 2s 2p B. oxygen 1s 2s 2p C. magnesium 1s 2s 2p 3s 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. Subshells are represented by small colored rectangles. Orbitals within the subshells are represented by circles. 41 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 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 A. The correct electron configuration for nitrogen is 1) 1s 2 2p 5 2) 1s 2 2s 2 2p 6 3) 1s 2 2s 2 2p 3 B. The correct electron configuration for oxygen is 1) 1s 2 2p 6 2) 1s 2 2s 2 2p 4 3) 1s 2 2s 2 2p 6 C. The correct electron configuration for calcium is 1) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 2 2) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3) 1s 2 2s 2 2p 6 3s 2 3p

6 A. The correct electron configuration for nitrogen is 3) 1s 2 2s 2 2p 3 B. The correct electron configuration for oxygen is 2) 1s 2 2s 2 2p 4 C. The correct electron configuration for calcium is 2) 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 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. EXAMPLES OF THE USE OF NOBLE GAS CONFIGURATIONS Magnesium: [Ne]3s 2. The symbol [Ne] represents the electronic configuration of neon, 1s 2 2s 2 2p 6. Iron: [Ar]4s 2 3d 6. The symbol [Ar] represents the electronic configuration of argon, 1s 2 2s 2 2p 6 3s 2 3p 6. Galium: [Ar]4s 2 3d 10 4p 1. The symbol [Ar] represents the electronic configuration of argon, 1s 2 2s 2 2p 6 3s 2 3p Using the periodic table, write the electron configuration and abbreviated configuration for each of the following elements: A. Cd B. Sr A. Cd 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 [Kr] 4s 2 3d 10 B. Sr 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 [Kr] 5s 2 Give the symbol of the element that has A. [Ar]4s 2 3d 6 B. Four 3p electrons C. Two electrons in the 4d sublevel C. I C. I 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 5 [Kr] 5s 2 4d 10 5p 5 D. The element that has the electron configuration 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d Give the symbol of the element that has A. [Ar]4s 2 3d 6 Fe B. Four 3p electrons S PERIODIC TABLE CLASSIFICATIONS OF THE ELEMENTS 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. CLASSIFICATION ACCORDING TO DISTINGUISHING ELECTRONS The distinguishing electron is the last electron listed in the electronic configuration of the element. C. Two electrons in the 4d sublevel Zr D. Electron configuration Ti 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d

7 REPRESENTATIVE, TRANSITION AND INNER- TRANSITION ELEMENTS Elements are, again, classified according to the type of distinguishing electron they contain. METALS, METALLOIDS AND NONMETALS 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 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. 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 SCALE DRAWINGS OF REPRESENTATIVE ELEMENT ATOMS TRENDS IN FIRST IONIZATION ENERGY 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. TRENDS IN CHEMICAL REACTIVITY 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

8 Electronegativity Trends Electronegativity, is a chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself. Electron Affinity Trends electron affinity describes the ability of an atom to accept an electron. Unlike electronegativity, electron affinity is a quantitative measure that measures the energy change that occurs when an electron is added to a neutral gas atom Melting Point Trends Melting points are the amount of energy required to break a bond(s) to change the solid phase of a substance to a liquid. Generally, the stronger the bond between the atoms of an element, the higher the energy requirement in breaking that bond. Since temperature is directly proportional to energy, a high bond dissociation energy correlates to a high temperature. Melting points are varied and don't generally form a distinguishable trend across the periodic table. However, certain conclusions can be drawn from the following graph Melting Point Trends Metals generally possess a high melting point. Most non-metals possess low melting points. The non-metal carbon possesses the highest boiling point of all the elements. The semi-metal boron also possesses a high melting point. 67 8