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Periodic Table A Tutorial The periodic table arranges elements, both naturally occurring and man made, in an orderly arrangement by increasing atomic number. Remember, the atomic number is, by definition, the number of protons. The number of protons determine the properties of the element as well as the reactivity of the element. Because the number of protons does not change it makes sense that the elements would be arranged by atomic number. In an unbonded atom, the number of protons will equal the number of electrons. The periodic table deals with unbonded atoms of all the elements. Because of this, the periodic table can tell us a lot about an element. The periodic table is made of columns and rows. The columns are called groups, they are read top to bottom, and are labelled with roman numerals I through VIII. The rows are called periods, they are read left to right, and are labelled with Arabic numbers 1-7. These rows and columns, these periods and groups assign each element with two specific numbers, much like the coordinates on a point on graph paper. I 1 2 3 4 5 6 7 H Li Na K Rb Cs Fr II Be Mg Ca Sr Ba Ra III B Al Ga In Ti IV C Si Ge Sn Pb V N P As Sb Bi VI O S Se Te Po VII F Cl Br I At VIII He Ne Ar Kr Xe Rn By definition: Groups tell us the number of valence electrons Periods tell us the number of energy shells found on an atom Look at Be. It is found in Group II and Period 2. This information tells us the following: Because it is in Group II, Be has 2 valence electrons. Because it is in Period 2, Be has 2 energy shells, K - L If we put those two pieces of information together, we can write the electron configuration as: K - L 2 - 2 Now that we know the electron configuration, we can add those two numbers together and get the total number of electrons. 2+2 = 4 electrons. Remember: the number of protons is equal to the number of electrons in an unbonded atom. The periodic table deals with unbonded atoms. With that said, if Be has 4 electrons then Be has 4 protons. If Be has 4 protons that will give us the atomic number for Be. The atomic number, the number of protons, of Be is 4. The Group tells us the number of valence electrons. If we know the number of valence electrons then we can write the electron dot. Remember the electron dot shows only the valence electrons found on an atom. Be has 2 valence electrons. Be Metals have less than 4 valence electrons. Be has 2 valence electrons, making Be a metal. Let’s list what we have found out about Be: It is a metal, It has 2 valence electrons ( Group II). It is a metal (less than 4 valence electrons) It has 2 energy shells (Period 2). It has an electron configuration of 2-2 The total number of electrons is 4 The number of protons is 4 The atomic number is 4. The electron dot is Be Let’s take a look at another element. Cl Chlorine, Cl, is found in Group VII Period 3 What do we know? Cl has 7 valence electrons (Group VII) Cl has 3 energy shells (Period 3) K-L-M The electron configuration is 2 – 8 – 7 The total number of electrons is 17 The total number of protons is 17 The atomic number is 17 It is a nonmetal, more than 4 valence electrons Its electron dot is Cl