This unit deals with the nature of elements and their organization in the periodic table, and seeks to identify and explain trends among those elements. These trends are of properties such as ionization and reactivity, as well as atomic characteristics, such as electron configuration and atomic mass.
- Classify elements as being noble gases, metals, non-metals, or metalloids.
- Provide definitive names for groups 1, 2, 3-12, 17, & 18 in the periodic table.
- Explain the trends of atomic radius and electronegativity within groups and periods.
- Explain the trends of reactivity and electron affinity among metals and among non-metals.
- Provide a comparison of first, second, and third ionization energies for any given element.
- Explain the impact of ionization on atomic radius for any given element.
The Periodic Table
The modern periodic table sorts elements by atomic number from left to right in rows, known as periods. Elements are grouped in to columns by consistent properties, as first noted by Dmitri Mendeleev. Mendeleev noticed that elements followed predictable trends when ordered by mass, and grouped them based on this simple observation. This observation is now recognized as the Periodic Law--all elements, discovered or undiscovered, will follow various predictable trends. It is important to note that Mendeleev left gaps in his first table for elements he expected based on properties, but had not been discovered.
Periodic Table Trends
The grouped characteristics Mendeleev had used for grouping were actually the effects of the unique electron configurations in each element. The electron configuration of an element defines its reactivity and other observable chemical properties. Depending on the electron configuration of an element, it has certain tendencies as to its interactions with electrons (which are what every reaction involve).
The tendency of an element to interact with electrons in a certain way is a result of the forces caused by its nucleus and electrons in its surrounding energy levels. Electron shielding is repulsion between electrons, specifically between the electrons of inner energy levels and the outer energy level. Nuclear pull is the attraction of electrons towards the center of the atom caused by the nucleus' positive charge.
The electronegativity of an atom is its tendency to attract electrons. This is really a comparison of the relative strengths of the nuclear pull and electron shielding. If an element has a large nuclear pull as compared to its shielding, it is very electronegative. This occurs as we look from left to right across a period of the periodic table. Within a period the amount of electron shielding does not increase (the same number of inner energy levels exist), but the nuclear pull increases as atomic number increases. One major exception to this is the Noble Gases, which are the farthest to the right. In Noble Gases, the outmost principle energy level is full, so any incoming electron will be the first of the next energy level. This means it will experience a increase in shielding by an entire principle energy level. Looking down a group, electronegativity tends to decrease because each lower element has an entire principle energy level of shielding more than higher elements.
The atomic radius of an element is the distance between its center (nucleus)and outer most electron orbital when bonded. The atomic radius is affected by the same factors as the electronegativity--nuclear pull and electron shielding--and is highly indicative of an element's electronegativity (except among noble gases). Once again, the relative nuclear pull as compared to the electron shielding will give an element a large (where shielding is stronger) or small (where pull is stronger) radius.
An ion is an atom (or a molecule) that has lost or gained one or more electrons, making it positively or negatively charged. Positively charged ions are called cations (cat-eye-ons) and negatively charged ions are called anions (Anne-eye-ons). The ionization energy of an element is the amount of energy required to make an element a cation by removal of an electron. The electron affinity of an atom is the energy release when an electron is gained in the formation of an aion.
The rows in the periodic table are called periods, and are sorted from left to right based on atomic number.
The columns in the periodic table are called groups, and contain elements with similar outer electron configurations and similar properties. The only exceptions to this rule are hydrogen and helium, which have their outer electron configurations set up differently than those of other elements.