The student is expected to use isotopic composition to calculate average atomic mass of an element AND express the arrangement of electrons in atoms through electron configurations and Lewis valence electron dot structures.
Atoms of the same element with the same number of protons but a different number of neutrons are called isotopes. The average atomic mass of each element can be calculated using the weighted average of the masses of all of the naturally occurring atoms and isotopes of that element.
The electron configuration of an element can be written to show the location and number of electrons in an atom. In this model, energy levels are divided into four sublevels, which correspond to particular zones on the Periodic Table. The sublevels are filled in order with the lowest energy available orbitals filled first.
A Lewis valence electron dot structure is used to visually represent the outer electrons of an atom. This model uses the chemical symbol of an element surrounded by dots, where each dot represents an electron that are found within the outermost electron shell.
ISOTOPES
All atoms of the same element are similar, but not necessarily identical. There can be different masses of atoms for the same element. For example, carbon has six protons in each atom. In fact, every carbon atom contains six protons in the nucleus. Those six protons are what makes carbon carbon. However, different atoms of carbon could contain different numbers of neutrons. Carbon cold have six, seven, or eight neutrons, which are named carbon-12, carbon-13, or carbon-14 respectively. Atoms of the same element with the same number of protons but a different number of neutrons are called isotopes.
The average atomic mass of each element can be calculated using the weighted average of the masses of all of the naturally occurring isotopes of that element. In this formula, atomic masses of isotopes are represented by each w, and natural abundances (percentages) of isotopes are represented by each y. It can be expressed in the following equation:
W = w1y1 + w2y2 + … +wnyn
ELECTRON CONFIGURATION
The electron configuration of an element can be written to show the location and number of electrons in an atom. A certain orbit links to a corresponding energy level. In this model, energy levels are divided into four sublevels, which correspond to particular zones on the Periodic Table.
The sublevels are filled in order with the lowest energy available orbital filled first. The ground stateis the state with the lowest energy. By contrast, configurations other than those in ground state are in an excited state. An atomic structure with a certain configuration of excited state always has the tendency to return to the ground state with minimum energy level.
LEWIS VALENCE ELECTRON DOT STRUCTURES
Lewis valence electron dot structures are used to illustrate the valence electrons, and only the valence electrons that may be found surrounding a neutral atom as described in Gilbert N. Lewis’s 1916 paper titled The Atom and the Molecule.
The valence electrons of an atom represent the highest energy electrons that occupy the s and porbitals of that atom. In this model, a specific number of dots (between 1 and 8) surround the chemical symbol of the element, with each dot representing one valence electrons that may be found in a neutral atom of that element.
If the element has eight valence electrons, it is said to have a full octet, as the s and p orbitals are “full.”
ELECTRON MODELS
Another model, called an electron shell diagram, may be used to represent visually all of the electrons that may be found in the electron shells surrounding a neutral element. Remember that Lewis valence electron dot structures and electron shell diagrams are two different models commonly used to represent visually the electrons found in a neutral element.
By using chemical symbols, the Lewis valence electron dot structures can represent every atom in a molecule structure. Note, chemical bonds between atoms are represented with lines and excess electrons are drawn as dots.