electron shell

Examples of electron shell in the following topics:

• Periodic Table Position and Electron Configuration

  • The electrons in the partially filled outermost shell (or shells) determine the chemical properties of the atom; it is called the valence shell.
  • The electron shell configurations of the first 18 elements in the periodic table.
  • The number of outer-shell electrons is represented by the right-most digit in the group numbers.
  • Each box includes representations of the electron shell structure for the element.
  • Position in the periodic table based on electron shell configuration.

• The Shielding Effect and Effective Nuclear Charge

  • The more electron shells there are, the greater the shielding effect experienced by the outermost electrons.
  • However, when more electrons are involved, each electron (in the n-shell) feels not only the electromagnetic attraction from the positive nucleus but also repulsion forces from other electrons in shells from 1 to n-1.
  • The shielding effect explains why valence shell electrons are more easily removed from the atom.
  • The valence shell is shell 2 and contains 8 valence electrons.
  • Once again, the electron configuration is the same as in the previous examples and the number of nonvalence electrons is 2 (by losing one electron, the valence shell becomes the n=2 shell).

• General Rules for Assigning Electrons to Atomic Orbitals

  • Here, electrons are arranged in energy levels, or shells, around the nucleus of an atom.
  • An atom's electron shell can accommodate 2n2 electrons, where n is the energy level.
  • For example, the first shell can accommodate 2 x (1)2 or 2 electrons.
  • The second shell can accommodate 2 x (2)2, or 8, electrons.
  • An element's electron configuration is the arrangement of the electrons in the shells.

• Periods 1 through 3

  • Elements of the same period have the same number of electron shells.
  • Electron are organized in energy levels or electron shells, which correspond to the periods on the periodic table.
  • In the periodic table, there are 2 electrons in period 1, while both periods 2 and 3 have 8 electrons in the filled level.
  • With the nucleus at the center with a positive charge, the electrons are "organized" in energy levels, or shells, at increasing distances from the nucleus.
  • The distance between the n = 2 and n = 3 shells in this illustration is the difference in energy between them.

• Electron Configurations of Atoms

  • Electron shell #1 has the lowest energy and its s-orbital is the first to be filled.
  • In the third period of the table, the atoms all have a neon-like core of 10 electrons, and shell #3 is occupied progressively with eight electrons, starting with the 3s-orbital.
  • The highest occupied electron shell is called the valence shell, and the electrons occupying this shell are called valence electrons.
  • The other members of group 8 have a characteristic valence shell electron octet (ns2 + npx2 + npy2 + npz2).
  • These atoms have only one electron in the valence shell, and on losing this electron arrive at the lower shell valence octet.

• Electron Configuration of Cations and Anions

  • Monatomic ions are formed by the addition or removal of electrons from an atom's valence shell.
  • For example, Group 1 element sodium (Na) has a single electron in its valence shell, with full shells of 2 and 8 electrons beneath.
  • Removal of this one electron leaves sodium stable: Its outermost shell now contains eight electrons, giving sodium the electron configuration of neon.
  • Sodium could gain electrons, but it would require seven more to achieve a full valence shell.
  • On the other hand, a chlorine atom (Cl) has seven electrons in its valence shell, which is one short of a stable, full shell with 8 electrons.

• Ions

  • If an atom has the same number of protons and electrons, it is electronically neutral.
  • Generally, the electrons are either added to or lost from the valence shell of an atom; the inner-shell electrons are more tightly bound to the positively charged nucleus and so do not participate in this type of chemical interaction.
  • The process is motivated by the achievement of more stable electronic configurations, such as the octet rule, which states that most stable atoms and ions have eight electrons in their outermost (valence) shell.
  • However, according to the octet rule, sodium would be more stable with 10 electrons (2 in its inner most shell, 8 in its outermost shell).
  • Chlorine naturally has 17 electrons but it would be more stable with 18 electrons (2 in its inner most shell, 8 in its second shell, and 8 in its valence shell).

• Hund's Rule

  • Hund's Rule defines the behavior of unpaired valence shell electrons, providing insight into an atom's reactivity and stability.
  • For example, take the electron configuration for carbon: 2 electrons will pair up in the 1s orbital, 2 electrons pair up in the 2s orbital, and the remaining 2 electrons will be placed into the 2p orbitals.
  • Therefore, two p orbitals will each get 1 electron and one will get 2 electrons.
  • When atoms come into contact with one another, it is the outermost electrons of these atoms, or valence shell, that will interact first.
  • An atom is least stable (and therefore most reactive) when its valence shell is not full.

• Electron Affinity

  • Mulliken used a list of electron affinities to develop an electronegativity scale for atoms by finding the average of the electron affinity and ionization potential.
  • A molecule or atom that has a more positive electron affinity value is often called an electron acceptor; one with a less positive electron affinity is called an electron donor.
  • Eea generally increases across a period (row) in the periodic table, due to the filling of the valence shell of the atom.
  • For instance, within the same period, a Group-17 atom releases more energy than a Group-1 atom upon gaining an electron because the added electron creates a filled valence shell and therefore is more stable.
  • Electron affinity follows the trend of electronegativity: fluorine (F) has a higher electron affinity than oxygen (O), and so on.

• The Building-Up (Aufbau) Principle

  • As electrons are added, they assume the most stable shells with respect to the nucleus and the electrons already present.
  • When there are two electrons in an orbital, the electrons are called an electron pair.
  • If the orbital only has one electron, this electron is called an unpaired electron.
  • Put one electron in each of the three p orbitals in the second energy level (the 2p orbitals) and then if there are still electrons remaining, go back and place a second electron in each of the 2p orbitals to complete the electron pairs.
  • In a hydrogen-like atom, which only has one electron, the s-orbital and the p-orbitals of the same shell in the Aufbau diagram have exactly the same energy.