Valence Shell of the Atom
- 1. Empowering Students’ Understanding of the Atomic Shell Structure – Part 2 Valence Shell, Valence Electrons, and the Valence Sheath B Dr. Renuka Rajasekaran Chemist and Chemistry Educator PhD (Chemistry); PhD (Chemistry Education) The greatest problem in understanding the atomic structure is in the peculiarities of the Valence Shell. Enjoy this primer on the Valence Shell, compile to help high school chemistry students and those in introductory chemistry courses at the college level. Progressive Chemistry Learning Series, Volume 2, 2006, pp. 1-12.
- 2. A quick review of structure of the atom Note The tendency to lose or gain electron comes from the combining capacity of atoms. (Losing or gaining can be a shared activity between atoms, which you will study later in bonding, under Covalent Bonds) The combining capacity of atoms is called the Valence or Valency. Picture courtesy: http://cmapspublic.ihmc.us/rid=1136790907031_992074287_3 141/Basic%20Atomic%20Structure.cmap
- 3. The Fundamentals of Atomic Structure 1. Atoms are the fundamental building units of matter. 2. Atom has two major parts: Nucleus and Shells. The nucleus is the small sized, dense body found at the center of the atom; shells are the circular orbits found around the nucleus. 3. There are three types of particles (generally called subatomic particles) inside an atom. They are: Electrons, Protons, and Neutrons. Electrons are negatively charged. Protons are Positively charged; Neutrons are Neutral. The mass of electron is negligible. The mass of a proton is the same as the mass of a neutron = 1 a.m.u. (atomic mass unit). 1 a.m.u = 1/6.023 x 10^23 g. Or 6.023 x 10^23 a.m.u = 1g; 6.023 x 10^23 is the Avogadro Number. 4. Avogadro Number represents the number of atoms in 1 mole of a monoatomic element. Atomic Number, Z, represents the number of protons in the nucleus of the atom of an element. Mass Number, A represents the total number of protons and neutrons present in the nucleus of an tom. Since number of electrons is equal to the number of protons, atomic number is also equal to the number of electrons; however, by definition, atomic number is equal to the number of protons only. 5. Atoms are electrically neutral even though they have charged particles inside of them. This means that the positive and the negative charges inside an atom are equal and they cancel out each other. Therefore, the number of Protons should be equal to the number of electrons. 6. Nucleus contains both the Protons and Neutrons. Therefore, the positive charge of the atom is entirely concentrated only in the Nucleus. Similarly the mass of the atom is concentrated only in the nucleus. 7. The negatively charged particles namely electrons revolve around the nucleus in shells. This is similar to the planetary motion Electrons also spin on their own axis like the planets. 8. Every shell can have only a definite number of electrons. (See Right). Class Work Task 1 Q Shell – maximum of 8 electrons only P Shell – maximum of 18 electrons O Shell – maximum of 32 electrons N Shell - maximum of 32 electrons M Shell –maximum of 18 electrons L Shell -maximum of 8 electrons K Shell –maximum of 2electrons Your task: Position Protons and Neutrons Class Work – Task 2: Draw Nucleus, Label Shells, Write the number of electrons per shell correctly, label valence shell, penultimate shell, antepenultimate shell, kernel, and valence sheath
- 4. The Fundamentals of Atomic Structure 9. The shells are arranged concentrically with nucleus at the center. 10. Each shell has a fixed diameter and energy. The shell is also called Energy Level. 11. The diameter as well as the energy of the shell increases as move from inside to the outside. The diameter of an atom is: 0.1 to 0.5 nanometers or 1 to 5 Angstrom units, Å; (nm = 1 × 10-9 m; Å = 1 × 10- 10 m so 10 Å = 1 nm ). 12. There are infinite number (countless) of shells in an atom. 13. We count shells from interior to the exterior. Shells are named K, L, M, N, O, P, Q, respectively from shell 1. 14. The last shell to have electrons is called the Valence Shell or the Outermost Shell 15. The shell immediately inner to the Valence Shell is called the Penultimate Shell 16. The shell immediately inner to the Penultimate shell is called the Antepenultimate shell. 17. The electrons present in the valence shell are called Valence Electrons. 18. All electrons other than the valence electrons are called inner electrons are core electrons. 19. Valence shell together with all the empty shells outside of the valence shell is called the Valence Sheath (Please see Next Slide) 20. Nucleus together with all the shells from K up to the Penultimate shell is called the Kernel. Both the diameter and the energy of the shells increase moving from inner to the outer. Energy Diameter
- 5. Gilbert Newton Lewis, the Discoverer of the Covalent Bond, Valence Electron Pairs, Lewis Dot Structures, and Valence Bond Theory Glibert Newton Lewis fondly called Lewis proposed the concept of Valence Sheath. According to him, the valence electrons can be ripped off for sharing or transfer between atoms in bonding. The valence sheath serves as the basic field or platform for the atomic communications to take place during bonding. Valence sheath gets tightened and becomes unresponsive once the valence shell is complete, that is once bonding has taken place. Noble gases by virtue of their fully filled valence shell have a complete and closed Valence Sheath. You can learn more about Valence Sheath, Valence, Covalence, and electrovalence at: • https://books.google.com/books?id=DClAAQAA MAAJ&pg=PA64&lpg=PA64&dq=valence+sheath& source=bl&ots=K9lA4FV5Jk&sig=ZSmM04l3ILF7Jc NmI9jiW5sUCMc&hl=en&sa=X&ved=0ahUKEwjLq u7AwufRAhWGYyYKHWk1BB0Q6AEITTAM#v=one page&q=valence%20sheath&f=false Gilbert Newton Lewis Medal for meritorious achievement in the theoretical aspects of chemistry. One of the easiest ways to understand Lewis idea of valence electrons is to look at the model of the metallic bond in which the positively charged metal ions are visualized to be in a sea of electrons. The sea of electrons is due to the mobile valence electrons. An illustration of the metallic bond is shown below. The orange circles with the central black dot represent the positively charged ions. The yellow circles outside of the orange circles are the valence electrons.
- 6. Four Pairs of Electrons and Four Bonds: The True Drive of Chemical Reactions We are aware that the valence shell is the last shell to have electrons and it can hold a maximum of eight electrons. So the fully filled state of A VALENCE SHELL MEANS EIGHT ELECTRONS OR FOUR PAIRS OF ELECTRONS. Chemical reactions involve electrons. Atoms gain or lose electrons in order to attain the noble gas configuration – that is the octet configuration in the valence shell. Therefore, through chemical reactions atoms attain the fully filled state of the valence shell, which represents eight electrons or four bonds (remember: a bond requires two electrons). See below: Carbon tetra chloride (left) and carbon dioxide (right) Four single bonds or two double bonds make four bonds. In carbon tetrachloride both carbon and chlorine attain the octet configuration in the valence shell. Similarly, in carbon dioxide bot carbon and oxygen attain the octet configuration of the valence shell. Valence Shell is like the Front Office of the Atom Thus it is clear that in chemical reactions, only the valence shell and the valence electrons are directly involved – loss or gain of electrons take place at the valence shell only. It is the valence electrons, which are lost or gained, to attain the nearest noble gas configuration. The inner electrons are never directly involved in chemical reactions. Similarly, the nucleus never participates in chemical reactions. This gives us a picture of valence shell functioning like the front office of the atom. The valence shell takes care of the atomic correspondences and communications and relationship between different atoms and actual bonds are formed only at the valence shell. Not All Can Form the Octet in Chemical Reactions: Three Inevitable Exceptions However, attaining octet configuration is difficult for some atoms and they make less than four bonds. On the other hand, some atoms are able to make more than four bonds. These two conditions of (i) making less than or (ii) making more than four bonds constitute EXCEPTIONS TO THE OCTET RULE. Case 1: Less than four bonds is due Reduced Octet or Electron Deficiency (Elements up to Boron are electron deficient) Case 2: More than four bonds is due to Expanded Octet or Electron Richness (Elements such as Phosphorus and Sulfur are electron rich). Case 3: SEEN IN FREE RADICALS, WHICH CONTAIN ODD NUMBER OF ELECTRONS RESULTING FROM HOMOLYTIC BOND CLEAVAGE. Actually, this is a case of Electron Deficiency but in Electron Deficient atoms there are even number of electrons and bonds whereas free radicals contain odd electrons and odd number of bonds, with one solitary electron.
- 7. Valence Shell: A Quick Recap Valence shell (the last shell to have electrons) together with the outer empty shells makes what is called a Valence Sheath The nucleus together with all the inner shells makes the kernel of the atom. You can visualize the atom as orange fruit or a corn with the sheath. The skin is like the valence sheath and the corn or the fruit is the kernel or core of the atom. 1. There are indefinite number (countless) of shells in an atom. 2. We count shells from interior to the exterior. Shells are labeled K, L, M, N, O, P, Q. There are only 7 shells that have been found with electrons. We have not yet found elements that have electron in Shell 8 or above. 3. The last shell to have electrons is called the Valence Shell or the Outermost Shell 4. The shell immediately inner to the Valence Shell is called the Penultimate Shell 5. The shell immediately inner to the Penultimate shell is called the Antepenultimate shell. 6. The electrons present in the valence shell are called Valence Electrons. 7. All other electrons are called inner electrons are Core electrons or Inner electrons. 8. Effective Nuclear Charge is a measure of the influence of the nucleus on the valence shell, which is determined by the inner or core electrons. Effective Nuclear Charge is = Number of protons minus Number of Inner Electrons. Effective Nuclear Charge has the symbol: Zeff, its value helps determine “Shielding Effect.” Shielding effect can be described as how much an outer electron is hidden from the nucleus because of the electrons in the intermediate shell. It is also referred to as the Screening Effect or Atomic Shielding. 9. Although the effective nuclear charge was originally aimed only for the valence electrons, one can use the same formula to determine the effective nuclear charge on any electron. A representation of shielding effect from http://chem.libretexts.org/Core/Physi cal_and_Theoretical_Chemistry/Quan tum_Mechanics/09._The_Hydrogen_ Atom/Atomic_Theory/Electrons_in_At oms/Multi-electron_Atoms
- 8. VALENCE OR VALENCY: COMBINING CAPACITY OF ATOMS Valence shell is where chemical reactions take place. Inner shells and inner electrons are not involved in chemical reactions Elements gain or lose electrons so as to attain the octet configuration in the valence shell. How many electrons are lost or gained represents the combining capacity of atoms, called Valence or Valency. So valence or valency is a whole number and can never be in decimal, because full electrons are lost or gained or shared. Valency can be easily determined from the position of an element on the periodic table relevant to the nearest Noble Gases. • Valency/Valence and the Group Charge on the Periodic Table Transition elements +2/+3 Inner Transition elements +3/+4
- 9. VALENCE SHELL: OCTET RULE and EXCEPTIONS TO OCTET RULE • Shells contain electrons. The maximum capacity of a shell for electrons is given by 2n2 where n represents which shell – first, second, third, fourth, etc. • Thus, the maximum number of electrons in shells are as follows” • Shell 1 or K shell: 2 x 1x 1 = 2 • Shell 2 or L shell: 2 x 2x 2 = 8 • Shell 3 or M shell: 2 x 3x 3 = 18, --- etc. • However, the valence shell of an atom cannot hold more than 8 electrons. • This is called the famous Octet Rule, which helps us understand that elements enter into chemical reactions, in order to attain the completely filled valence shell. (Please read Exceptions to Octet Rule on next slide). • The Octet Rule is a very important rule and guides the force behind most of the chemical reactions. • Octet configuration of the valence shell represents Stability. In order to understand the stability aspect, let us have a look at the noble gases. All the noble gases (excepting helium) have the octet configuration. They are therefore quite stable, inert, and unreactive. • All other elements undergo chemical reactions to attain the noble gas configuration of the valence octet. • For example sodium attains neon configuration by losing one electron and fluorine gains one electron to attain neon configuration. However, there are exceptions to the Octet Rule (Slide 10).
- 10. Four Pairs of Electrons and Four Bonds: The True Drive of Chemical Reactions We are aware that the valence shell is the last shell to have electrons and it can hold a maximum of eight electrons. So the fully filled state of A VALENCE SHELL MEANS EIGHT ELECTRONS OR FOUR PAIRS OF ELECTRONS. Chemical reactions involve electrons. Atoms gain or lose electrons in order to attain the noble gas configuration – that is the octet configuration in the valence shell. Therefore, through chemical reactions atoms attain the fully filled state of the valence shell, which represents eight electrons or four bonds (remember: a bond requires two electrons). Atoms execute losing and gaining of electrons strategically depending on individual electronic configuration (as expressed in its position on the periodic table). Often times, losing and gaining can be intelligently done by sharing. See below: Carbon tetra chloride (left) and carbon dioxide (right) Four single bonds or two double bonds make four bonds. In carbon tetrachloride both carbon and chlorine attain the octet configuration in the valence shell. Similarly, in carbon dioxide both carbon and oxygen attain the octet configuration of the valence shell. Valence Shell is like the Front Office of the Atom Thus it is clear that in chemical reactions, only the valence shell and the valence electrons are directly involved – loss or gain of electrons take place at the valence shell only. It is the valence electrons, which are lost or gained, to attain the nearest noble gas configuration. The inner electrons are never directly involved in chemical reactions. Similarly, the nucleus never participates in chemical reactions. This gives us a picture of valence shell functioning like the front office of the atom. The valence shell takes care of the atomic correspondences and communications and relationship between different atoms and actual bonds are formed only at the valence shell. Not All Can Form the Octet in Chemical Reactions: Three Inevitable Exceptions However, attaining octet configuration is difficult for some atoms and they make less than four bonds. On the other hand, some atoms are able to make more than four bonds. These two conditions of (i) making less than or (ii) making more than four bonds constitute EXCEPTIONS TO THE OCTET RULE. Case 1: Less than four bonds is due Reduced Octet or Electron Deficiency (Elements up to Boron are electron deficient) Case 2: More than four bonds is due to Expanded Octet or Electron Richness (Elements such as Phosphorus and Sulfur are electron rich). Case 3: SEEN IN FREE RADICALS, WHICH CONTAIN ODD NUMBER OF ELECTRONS RESULTING FROM HOMOLYTIC BOND CLEAVAGE. Actually, this is a case of Electron Deficiency but in Electron Deficient atoms there are even number of electrons and bonds whereas free radicals contain odd electrons and odd number of bonds, with one solitary electron.
- 11. ATOMIC STRUCTURE FOUNDATIONS: Ring Diagram of the Shell Structure of a Fully Filled Atom B D C D D Q Shell – one S subshell and one P subshell – maximum of 8 electrons only P Shell one S subshell, one P subshell, and one D subshell – Maximum of 18 electrons O Shell - one S subshell, one P subshell, one D subshell, and one F subshell – Maximum of 32 electrons N Shell – one S subshell, one P subshell, one D subshell, and one F subshell – Maximum of 32 electrons M Shell – one S subshell, one P subshell, and one D subshell – Maximum of 18 electrons L Shell – one S subshell and one P Subshell – Maximum of 8 electrons K Shell – one S subshell – Maximum of TWO electrons See the Octet Configuration of the Valence Shell S subshells filled with electrons P subshells filled with electrons F subshells filled with electrons D subshells filled with electrons Note that the Nucleus contains Neutrons and Protons. Neutrons and protons have equal masses (1 a.m.u). Proton is positively charged; Neutron is neutral. Note that the Kernel or Core (shaded in blue color) is everything from the nucleus up to the Penultimate Shell. It is like the inside fruit of an orange. K L M N O P Q Valence Shell Antepenultimate Shell Penultimate Shell Nucleus K L M N O P Q A shell has subshells. Subshells are the regions of the shell where electrons are present. Note: Glibert Newton Lewis fondly called Lewis proposed the concept of Valence Sheath. According to him, the valence electrons can be ripped off for sharing or transfer between atoms in bonding. The valence sheath serves as the basic field or platform for the atomic communications to take place during bonding. Valence sheath gets tightened and becomes unresponsive once the valence shell is complete, that is once bonding has taken place. Noble gases by virtue of their fully filled valence shell have a complete and closed Valence Sheath. It cannot be detached like a hard and dried orange peel. Valence Sheath Valence Shell and all Empty shells outside of the valence shell merge together to form the Valence Sheath. The region shown in double headed arrows is Valence Sheath. The Valence Sheath can be visualized somewhat like a loose outer cover of the atom, like the skin of the orange fruit. The kernel of the atom is like the fruit inside of the peel. However, for fully filled valence shell, the orange peel will be tight and hard and difficult to remove. This is the case with Noble Gases and complete compounds.