Chemistry
Valence Electrons
Valence electrons are the outermost electrons in an atom that are involved in chemical bonding. They determine the reactivity and chemical properties of an element. The number of valence electrons can be used to predict an element's behavior in forming compounds and its placement in the periodic table.
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7 Key excerpts on "Valence Electrons"
Chemical bonding often involves an exchange or sharing of Valence Electrons, and the number of electrons in an atom’s out- ermost shell is called its valence. Chemists often express the importance of the number of outer electrons by saying that valence represents the combin- ing power of a given atom. The top three rows of the periodic table of the elements provide the key to understand- ing the varied strategies of chemical bonding (Figure 10-1). Different electron shells hold different numbers of electrons, which gives rise to the distinctive structure of the periodic table (see Figure 8-4). It turns out that by far the most stable arrangement of electrons—the electron configuration of lowest energy—has completely filled electron shells. A glance at the periodic table tells us that atoms with a total of 2, 10, 18, or 36 electrons (all the atoms that appear in the table’s extreme right-hand column) have filled shells and very stable configurations. Atoms with this many total electrons are inert gases (also called noble gases), which do not combine readily with other materials. Indeed, helium, neon, and argon, with atomic numbers 2, 10, and 18, respectively, have completely filled outer electron shells, and are thus the only common elements that do not ordinarily react with other elements. Every object in nature tries to reach a state of lowest energy, and atoms are no exception. Atoms that do not have the magic number of electrons (2, 10, 18, etc.) are more likely to react with other atoms to produce a state of lower energy. You are familiar with this kind of process in many other natural systems. If you put a ball on top of a hill, for example, it will tend to roll down to the bottom, creating a system of lower gravi- tational potential energy. Similarly, a compass needle tends to align itself spontaneously with Earth’s magnetic field, thereby lowering its magnetic potential energy.
eBook - PDF- Bernard Moody(Author)
- 2013(Publication Date)
- Arnold(Publisher)
52 The electronic theory of valency and the periodic classification that in certain cases, addition of valency towards oxygen and hydrogen for an element gives eight. This holds for Group IV onward and is an aspect of the 'rule-of-eight' proposed by Abegg in 1904. The basic assumptions of the electronic theory The magnitude of the energy change involved in a chemical reaction is only sufficient to disturb the outer electrons of the atoms interacting. For this reason, electrons are usually divided into core elec-trons, composing the inner electrons which are not affected, and valence or valency electrons, which are those regrouped. The chemical nature of the inert or noble gases was attributed, in the previous chapter, to posses-sion of completed shells of electrons although some of these could be expanded later to explain the existence of the various transition series. The basis of present theories about chemical combination may be traced back to propositions advanced independently in 1916 by W. Kossel and G. N. Lewis. The former was struck by the fact that in the Periodic Table strongly electronegative ele-ments are separated from strongly electropositive elements by the inert gases. It was suggested that: during a chemical reaction, the atoms of an element adjust their electronic configuration to that of the nearest inert gas. While a few elements close to helium have two electrons (the helium configuration) after the reac-tion, many others finally have eight electrons in the valency shell. They are said to obey 'the octet rule'. However, there are certain other configurations which result from chemical combination and these will be described in their proper context. The ionic bond and the formation of ions In 1916, Kossel suggested that an atom acquired the electronic configuration of an inert gas by the loss or gain of electrons.
eBook - PDFChemistry
Structure and Dynamics
- James N. Spencer, George M. Bodner, Lyman H. Rickard(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
The magnitude of this achievement can be appreciated by noting that Lewis’s octet rule was gen- erated only five years after J. J. Thomson’s discovery of the electron and nine years before Ernest Rutherford proposed that the atom consisted of an infinitesi- mally small nucleus surrounded by a sea of electrons. The electrons in the outermost shell eventually became known as the Valence Electrons. This name reflects the fact that the number of bonds an ele- ment can form is called its valence. Because the number of electrons in the out- ermost shells in the Lewis theory controls the number of bonds the atom can form, these outermost electrons are the Valence Electrons. Figure 4.2, which utilizes the PES data from Chapter 3, shows the relative energies of the 1s, 2s, 2p, 3s, and 3p orbitals as the atomic number increases from H (Z 1) to Sc (Z 21). By the time we get to lithium (Z 3), we already begin to see a significant difference between the energy needed to remove an elec- tron from the valence 2s orbital and the 1s core orbital. This gap continues to widen, and, by neon (Z 10), the 1s electron is extremely difficult to remove. At argon (Z 18), we see that the 2s and 2p core electrons are much more tightly held than the 3s and 3p Valence Electrons. There is not much difference, however, between the energy required to remove an electron from the 2s and 2p orbitals or between the 3s and 3p orbitals. As the atomic number increases, the electrons in the core shells or subshells become increasingly more difficult to remove from the atom. It is only the electrons in the valence orbitals that are relatively easy to remove from an atom. The Valence Electrons are therefore those electrons on an atom that can be gained or lost in a chemical reaction.
eBook - ePubChemistry
Concepts and Problems, A Self-Teaching Guide
- Richard Post, Chad Snyder, Clifford C. Houk(Authors)
- 2020(Publication Date)
- Jossey-Bass(Publisher)
Nitrogen (N), a Group VA element, has ________ electrons in its outermost shell. Aluminum (Al), a Group IIIA element, has ____ electrons in its outermost shell.Answer: five; threeThe outer shell electrons are also known as Valence Electrons. The periodic table on the next page includes all of the symbols for the first 20 elements. The numbers of Valence Electrons are listed for lithium (Li), carbon (C), and argon (Ar). Fill in the number of Valence Electrons for each remaining element in this periodic table.Remember that helium (He) is an exception in Group VIIIA, since it only has a total of two electrons in its entire atom, both of which are Valence Electrons.Answer:The outer shell or Valence Electrons are especially important because those electrons are involved when atoms unite chemically to form compounds.When an atom of magnesium unites with another different atom to form a compound, what electrons of the magnesium atom are primarily involved?Answer: the two valence or outer shell electronsELECTRON DOT SYMBOLS (LEWIS SYMBOLS)
The outer shell or Valence Electrons may also be represented by a series of dots. Beryllium, with two Valence Electrons, can be represented as Be: (each dot represents one valence electron).represents a chlorine atom with how many Valence Electrons? ____Answer: sevenThe first 20 elements and their electron dot symbols are as follows.Note that some books represent boron as , aluminum as , carbon as , and silicon as . Such a representation is used to simplify the explanation of how many bonds a given element may have or may form with other elements, but it does not agree with the quantum mechanical concept of the atom (Chapter 1 ).We will discuss the number of bonds an element has in a compound in Chapter 14
eBook - PDFStructure of Matter
With Contributions in Memoriam Including a Complete Bibliography of His Works
- C. Guy Suits(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
On the other hand, the maximum negative valence is the number of electrons which the atom must take up in order to reach one of these stable configurations. For example, magnesium has a positive valence of two, since its atomic number is 12 while that of neon is 10. Sulfur has a positive valence of 6 since it has 6 electrons more than neon ; but it has a negative valence of two because it must take up two more electrons before it can assume a form like that of the argon atom. It is clear, however, that this theory of valence is not yet complete. 1 It is not applicable to those cases where we have usually taken valences of 4 for sul-fur, or 3 and 5 for chlorine, etc. But more especially it does not explain the structure of organic compounds and such substances as H 2 , Cl 2 , 0 2 , N 2 H 4 , PC1 3 , etc. J. J. Thomson, Stark, Bohr, and others had suggested that pairs of electrons held in common by two adjacent atoms may function in some cases as chemical bonds between the atoms, but this idea had not been combined with the con-ception of the stable groups of electrons or octets. G. N. Lewis, in an important paper in 1916, advanced the idea that the stable configurations of electrons in atom could share pairs of electrons with each other and he identified these pairs of electrons with the chemical bond of organic chemistry. This work of Lewis has been the basis and the inspiration of my work on valence and atomic structure. As result of the sharing of electrons between octets, the number of octets that can be formed from a given number of electrons in increased. For example, two fluorine atoms, each having seven electrons in its outside shell, would not be able to form octets at all except by sharing electrons. By sharing a single pair of electrons, however, two octets can be formed since two octets holding a pair in common require only 14 electrons. This is clear if we consider two cubes with electrons at each of the eight corners.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
STEP 2 Determine how the atoms are connected—atoms with the highest valency should be placed at the center and monovalent atoms should be placed at the periphery. 1.3 Electrons, Bonds, and Lewis Structures 5 FIGURE 1.3 A periodic table showing group numbers. Transition Metal Elements 1A 8A 2A 3A 4A 5A 6A 7A H Li Be Na Mg K Ca Rb Sr Cs Ba B C Al Si Ga Ge n Sn Tl Pb N O P S As Se Sb Te Bi Po F Ne He Cl Ar Br Kr Xe At Rn The Lewis dot structure of an individual atom indicates the number of Valence Electrons, which are placed as dots around the periodic symbol of the atom (C for carbon, O for oxygen, etc.). The placement of these dots is illustrated in the following SkillBuilder. At that point, the force of repulsion between the nuclei begins to overwhelm the forces of attrac- tion, causing the energy of the system to increase if the atoms are brought any closer together. The lowest point on the curve represents the lowest energy (most stable) state. This state determines both the bond length (0.74 Å) and the bond strength (436 kJ/mol). Drawing the Lewis Structure of an Atom Armed with the idea that a bond represents a pair of shared electrons, Lewis then devised a method for drawing structures. In his drawings, called Lewis structures, the electrons take center stage. We will begin by drawing individual atoms, and then we will draw Lewis structures for small molecules. First, we must review a few simple features of atomic structure: • The nucleus of an atom is comprised of protons and neutrons. Each proton has a charge of +1, and each neutron is electrically neutral. • For a neutral atom, the number of protons is balanced by an equal number of electrons, which have a charge of −1 and exist in shells. The first shell, which is closest to the nucleus, can contain two electrons, and the second shell can contain up to eight electrons. • The electrons in the outermost shell of an atom are called the Valence Electrons.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
We show how simple Lewis theory gives a good approximation of the distribution of electrons within molecules, and how we can use this to predict molecular geometry using valence-shell-electron-pair repulsion theory. We then consider in detail the two predominant theories used to describe covalent bonding: valence bond theory and molecular orbital theory. 5.1 Fundamentals of bonding LEARNING OBJECTIVE 5.1 Describe qualitatively the bonding in molecules. We learned in the chapter on atomic energy levels that the size, energy levels and electron configuration of an atom determine its chemical properties. In molecular compounds, electrons on atoms interact and are shared between atoms. In ionic compounds, electrons are transferred completely between atoms to form positively and negatively charged ions. Since electrons have both particle- and wave-like properties, bonding interactions can be described from either viewpoint. Keep in mind, though, that either model might be better at explaining certain aspects of reality. The electrostatic energy between two charged species is proportional to the magnitudes of the charges and inversely proportional to the distance between them. Charges of opposite sign attract one another, but like charges repel. The relationship is given by Coulomb’s law. E electrostatic = k q 1 q 2 r In this equation: E electrostatic = electrostatic potential energy (J) q 1 = charge on species 1 (C) q 2 = charge on species 2 (C) r = separation distance between the pair of charges (m) k = 9.00 × 10 9 N m 2 C −2 (Coulomb’s constant in vacuum) FIGURE 5.2 When electrons are in the region between two hydrogen nuclei, attractive electrostatic forces exceed repulsive electrostatic forces, leading to the stable arrangement of a chemical bond. attraction repulsion + + ‒ electron nucleus ‒ The equation describes the potential energy of one pair of charges. Molecules, however, contain two or more nuclei and two or more electrons.
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