Help Window: The Periodic Table of the Elements
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The Periodic Table is a listing of the elements according to increasing atomic number and similarities in chemical behavior. It was developed in the 19th century by the Russian chemist Mendeleev, before knowledge of modern atomic theory.
The Table is a chart having rows and columns.
- Each row is called a period.
- Each period/row is numbered 1, 2, 3...7. The period number is the same as the principal quantum number n for the valence level electrons (= the highest energy electrons).
- Each column is called a Group or Family. Elements listed in the same vertical column (group, family) have similar chemical behavior.
- Each group is numbered. In the international system of group numbers, the columns are simply numbered 1, 2, 3...18. In an older numbering system used in North America, groups are labeled with Roman Numerals and letters A or B. An older European label also is like this, but with different meanings for A and B. In CHM 102, the newer international group numbers (1-18) are used.
- Each box or entry in the Table contains the symbol of one element, its atomic number, and atomic weight.
- The first element listed has atomic number 1 (hydrogen). Each succeeding entry in the Table has the atomic number increase by 1 unit. Please remember that atomic number = number of protons in nucleus. The number of protons is the only property which invariably distinguishes one element from another. Since the atoms of the elements are neutral (charge = 0), the atomic number also = the number of electrons in an atom of the element. The atomic weight is the apparent average mass of the element as it is found in nature as a mixture of its various isotopes (if it has any).
- Elements in black are metals: conductors of electricity, give up electrons to form cations, and if a solid, can be worked into sheets and wires.
- Elements in red are non-metals: non-conductors of electricity, gain electrons to form anions, often not solids, and if they are solid, tend to be brittle and not easily worked into wires and sheets.
- Elements in green are metalloids: semi-conductors, intermediate in behavior to metals and non-metals.
It is not necessary to memorize the filling-order of electrons in atoms since these can be read from the Periodic Table. In fact, the reason that elements in the same Group have similar chemical behavior is because their electronic configuration is similar -- only the valence level electrons are at different energy levels.
To use the Table this way it is necessary to remember:
- Each orbital can describe at most two electrons.
- Each element has one more electron than the preceding element.
- In any shell (energy level = quantum number n) there can be only one s-type orbital, three p-type orbitals (for n = or > 2), five d-type orbitals (for n = or > 3), and seven f-type orbitals (for n = or > than 4).
- The "filling order" in which electrons usually occupy energy levels in atoms is s orbital, then p orbitals, then d orbitals, then f orbitals. When there are several orbitals of the same type in a shell (p, d, or f) each orbital of the same type must have at one electron in it before the others of that type can start to be filled with two electrons.
- Elements in Group 1 and 2 have their valence level (= highest energy) electron in a s-type orbital. Elements in Groups 3-12 have their valence electrons in d-type orbitals, and those in groups 13-18 have valence electrons in p-type orbitals.
Li has atomic number 3. The n = 1 shell is filled with electrons in the same arrangement as He, so these can be represented as [He]. Since Li is in Group 1, period 2, it has its one valence electron at the n = 2 quantum level in an s orbital. We write [He] 2s1.
Al has atomic number 13. Its first 10 electrons are arranged like Ne. There are thus 3 valence electrons in Al at the n = 3 energy level. Al is in the third column, so the first two electrons are in an s orbital, and the last in a p orbital. We write: [Ne] 3s2, 3p1.
Br has atomic number 35. The first 18 electrons are arranged like Ar. The remaining 17 start filling orbitals at the energy level 4s (because Br is in period 4). We write [Ar] 4s2, 3d10, 4p5. (Remember that there are three p-type orbitals, each of which may describe only 2 electrons at most. The designation 4p5 means 4px2, 4py2, 4pz1.)
Copyright 1997, Larry McGahey, The College of St Scholastica, Duluth, Minnesota.
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