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Periodic Trends An Introduction to Using the Periodic Table Introduction Mendeleev put the periodic table together by grouping elements together by properties. All alkali metals (Li, Na, K, and so forth) are soft, silvery metals with low density and high reactivity with water. All halogens are non-metals with high reactivity to metals and organic compounds. We now know that there are other properties that are periodic as well. Introduction As we examine the trends in the properties of elements, we will be talking about the changes when ... we move down a group we move across a period Periodic Trends We will be examining the trends in ... atomic radius ionization energy electron affinity ionic radius electronegativity Atomic Radius Monoatomic binary molecules: atomic radius, r, is ½ the distance, d, between the nuclei of the two atoms. e.g., hydrogen, H2: d d = 0.74 Å r = (0.74 Å)/2 = 0.37 Å Atomic Radius Multiatomic molecules: r is d minus known radius e.g., methane, CH4: distance between C and H dCH dC-H = 1.14 Å C H rC = dC-H - rH atomic radius of H • rC = 1.14 Å- 0.37 Å = 0.77 Å Atomic Radius Trend: decreases as we move from left to right increases as we move from top to bottom Atomic Radius Trend: decreases as we move from left to right increases as we move from top to bottom Why is this so? As we move across the period, we add more protons (increasing the + charge of the nucleus) while we add more electrons to the same shell. The added + charge draws the electrons closer Atomic Radius Trend: decreases as we move from left to right increases as we move from top to bottom Why is this so? As we move down a group or family, we are adding electrons to new shells at a much greater distance from the nucleus. This causes the atom to increase in size. Ionization Energy Ionization energy (IE) is the energy needed to remove an electron from an atom. As we move across a period, the distance of the electron from the nucleus decreases. The amount of energy needed to remove an electron increases. (Why?) Energy Energy ++ Ionization Energy Ionization energy (IE) is the energy needed to remove an electron from an atom. As we move down a group, the distance of the electron from the nucleus increases. The amount of energy needed to remove an electron Energy decreases. (Why?) + Ionization Energy Trend: increases as we move from left to right decreases as we move from top to bottom Electron Affinity Electron affinity is the change of energy in an atom when an extra electron is added. As we move across a period, the distance of the electron from the nucleus decreases. The amount of energy needed to add an electron decreases. (Why?) We say that affinity of the element for the electron has increased. −− Energy Energy Electron Affinity Ionization energy (IE) is the energy needed to remove an electron from an atom. As we move down a group, the distance of the electron from the nucleus increases. The amount of energy needed to add an electron increases. (Why?) Energy We say that affinity of the element for the electron has decreased. − Electron Affinity Trend: increases as we move from left to right decreases as we move from top to bottom Remember that the affinity of the element increases if the amount of energy to add an electron decreases. Ionic Radius Ionic radius is just what it says: it is the radius of the most common ion. This is a little complicated since elements to the left (metals) are usually cations (+ charged) and the elements to the right (non-metals) are usually anions (− charged). − + In general, the ionic radius of the cations decrease as we move from left to right for the same reason that the atomic radius decreases as we move from left to right. (Why is that?) Ionic Radius Ionic radius is just what it says: it is the radius of the most common ion. This is a little complicated since elements to the left (metals) are usually cations (+ charged) and the elements to the right (non-metals) are usually anions (− charged). − + Similarly, the ionic radius of the anions decrease as we move from left to right. (Why is that?) Ionic Radius Ionic radius is just what it says: it is the radius of the most common ion. This is a little complicated since elements to the left (metals) are usually cations (+ charged) and the elements to the right (non-metals) are usually anions (− charged). − + However, when we change from cations to anions, there is a jump in ionic radius. (Why?) Ionic Radius Ionic radius is just what it says: it is the radius of the most common ion. This is a little complicated since elements to the left (metals) are usually cations (+ charged) and the elements to the right (non-metals) are usually anions (− charged). − + In general, the ionic radius of all ions increase as we move from top to bottom for the same reason that the atomic radius increases as we move from top to bottom. (Why is that?) Ionic Radius Trend: generally, decreases as we move from left to right increases as we move from top to bottom Electronegativity Electronegativity is the tendency of an element to draw electron density toward itself when bonded to a different element. This makes electronegativity different from other properties. it is a property of an element bonded with other elements. The other properties we have looked at are only related to the element itself. Electronegativity In general, electronegativity increases as the size of the element decreases. Electronegativity does not apply to the noble gas elements. (Why?) This means that F is the most electronegative element (at 4.0) and Fr is the least electronegative element (at 0.7). The scale for electronegativity is arbitrary and the values are only for comparison of relative magnitudes. Electronegativity Trend: increases as we move from left to right decreases as we move from top to bottom Summary As we move from left to right: atomic radius decreases ionization energy increases electron affinity increases ionic radius decreases electronegativity increases As we move from top to bottom: atomic radius increases ionization energy decreases electron affinity decreases ionic radius increases electronegativity decreases