Groups (columns) 3 through 12 are called "Transition Elements". The elements in the second group (column) of the Periodic Table are called Alkaline Earth Metals. The elements in the first group (column) of the Periodic Table are called Alkali Metals. Then, we fill the 5d orbitals, and finally the 6p orbitals. These elements are shown separately as the "Actinide series". After filling the 7s orbital with two electrons (Ra), and adding a single 7p electron (Ac), we then need to fill the 5f orbitals as they are higher in energy than the 6p1 orbital, but lower in energy than the 6p2 (and 5d) orbitals. The seventh period is analogous to the sixth period. Finally, we finish filling the 6p orbitals, ending on Rn. After filling the 4f orbitals, we then continue in period 6 by filling the 5f orbitals. One could argue that the Lanthanides, beginning with element number 58, should be listed in the sixth row following element number 57, La. These elements are shown in a separate row called the "Lathanide Series". Before filling the rest of the 6p orbitals, we must first fill the 4f orbitals. In the sixth period, we first fill the 6s orbitals (Cs and Ba), and then add one electron to the 6p orbital (La). Similarly, for the 5th period, we fill the 4d orbitals after the 5s orbitals, but before filling the 5p orbitals. Because the 3d orbitals are higher in energy than the 4s orbitals, but lower in energy than the 4p orbitals, we must first fill the 3d orbitals. After filling the 4s orbital with two electrons (Ca), you then fill the 3d orbitals with 10 electrons (Sc -> Zn). In the 4th period, there is an exception. In the 3rd period, you start with one electron in the 3s orbital (Na), and then add a second electron to the 3s orbital (Mg), and then fill the 3p orbital with six electrons as you progress from Al to Ar. Completing the 2nd period, Ne, a noble gas, has a completely filled valence electron shell with two 2s electrons and six 2p electrons. (Lithium) has one electron in the 2s shell, Be has two electrons in the 2s shell, B has three valence electrons (two 2s electrons, and one 2p electron). In the topmost period, the Hydrogen atom (H) has a single 1s electron, and Helium (He) has a filled valence shell with two electrons in the 1s shell. As you traverse a period from left to right, the valence shells are filled, following the. Each subsequent row (period) has a higher energy level for its ground state, since higher "n" quantum numbers indicate higher energy states. For example, all the elements in the first row have n=1, all the elements in the second period have n=2, etc. The elements in a given row (called a "period") all have the same quantum energy level (the "n" quantum number"). Elements within a group are called "homologues", because they exhibit similar chemical behavior. 
Similarly, in the last group (column) of the Periodic Table, all of the elements have their valence electron shells filled, so they are (for the most part) un-reactive with other elements for this reason, we call this last group the "noble gases".For example, in the first column (group) of the periodic table, the elements H, Li, and Na all have one valence electron in the s orbital, and thus only have one electron available for bonding with other atoms.In general, only the valence electrons take part in bonding with other atoms thus, elements in a group have similar bonding behavior since they have the same number of valence electrons.The elements in a given group (column) of the periodic table have similar chemical properties because they have the same number of valence electrons.As the atomic number increases, the number of protons, neutrons, and electrons in each subsequent element increases, and thus the mass of the element also increases.For example, Carbon-12 has 6 protons and 6 neutrons, Carbon-13 has 6 protons and 7 neutrons, and Carbon-14 has 6 protons and 8 neutrons.
An element is defined by the number of protons, but can have differing numbers of neutrons, resulting inĭifferent isotopes. The total atomic weight of the element is equal to the sum of the weights of the nucleons (protons and neutrons) and electrons. The atomic number indicates the number of protons in the nucleus.Hydrogen (H) has atomic number 1, Helium (He) has atomic number 2, etc. Each element in the table has an assigned atomic number.The following notes were taken by me while listening to Click on any of the squares above for more information on an element.
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