Transcript Electrons
Periodic Trends General Periodic Trends 1. 2. 3. 4. Atomic Radius Ionic Radius Ionization energy Electronegativity Essential Questions How can I use the periodic table to help me predict: The size of an atom? The size of an ion? The amount of energy it would take to remove an electron from an atom? The ability of an atom to attract an electron? Periodic Table Trends Higher effective nuclear charge Electrons held more tightly More energy levels = larger orbitals. Electrons held less tightly. Atomic Radius Atomic Radius: defined as half the distance between nuclei of two like atoms that are chemically bonded together. Atomic Radius Radius decreases or goes DOWN as you go across a period. Radius increases or goes UP as you go down a group. Why does the radius decrease as you across a period? Size decreases across a period because of the increased positive charge from the protons. Each added electron feels a greater and greater + charge because the protons are pulling in the same direction, where the electrons are scattered. Large Small Why does the radius increase as you down a group? As you go down a group, you add more energy levels making the valence electrons further from the nucleus, therefore creating weaker attraction. Shielding effect: each additional energy level “shields” the electrons from being pulled in toward the nucleus. Check for understanding: Which is Bigger (has a bigger atomic radius)? Na or K ? Na or Mg ? Al or I ? K Na Al Ionic Radius Atoms become ions by losing or gaining electrons to become more stable Ionic Radius: same as atomic radius but with ions that are bonded together! Ionic Radius Li,152 pm 3e and 3p Does+ the size go up+ or down Li , 60 pm when an 2e and losing 3p electron to form a cation? Cations: Atoms that lose or give away electrons and become positively charged ions. Ionic Radius + Li,152 pm 3e and 3p CATIONS Li + , 78 pm 2e and 3 p Forming a cation. are SMALLER than the atoms from which they come. The electron/proton attraction has gone UP and so size DECREASES. Ionic Radius Does the size go up or down when gaining an electron to form an anion? Anions: Atoms that gain or steal electrons and become negatively charged ions. Ionic Radius F, 71 pm 9e and 9p F- , 133 pm 10 e and 9 p Forming an anion. ANIONS are LARGER than the atoms from which they come. The electron/proton attraction has gone DOWN and so size INCREASES. Trends in ion sizes are the same as atom sizes. Trends in Ion Sizes Figure 8.13 Check for Understanding: Which is Bigger? Cl Cl or ? K+ or K ? Ca or Ca+2 ? I or Br ? Cl- K Ca I- Work with a partner to complete Periodic Trends – worksheet 6-1 & 6-2 Ionization Energy Ionization Energy: the energy required to remove one electron from a gaseous atom. (ie, the energy required to make an +1 ion) Ionization Energy Mg (g) + 738 kJ ---> Mg+ (g) + e- This is called the FIRST ionization energy because we removed only the OUTERMOST electron Mg+ (g) + 1451 kJ ---> Mg2+ (g) + eThis is the SECOND IE. Ionization Energy Ionization energy increases across a period because of the increase in positive protons to hold onto the electrons. Metals lose electrons more easily than nonmetals. Nonmetals lose electrons with difficulty (they like to GAIN electrons). Ionization Energy Ionization energy decreases down a group As the atomic radius size increases the shielding effect increases making it easier to remove an electron. Check for understanding: st Which has a higher 1 ionization energy? Mg or Ca ? Al or S ? Cs or Ba ? Mg S Ba Electronegativity is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling 1901-1994 Electronegativity Electronegativity increases across a period of elements More protons with the same number of energy levels means atoms can better attract electrons. Electronegativity Electronegativity decreases DOWN a group of elements. Atomic radius is smaller with fewer energy levels so the nucleus can attract electrons better (ie, less shielding) Electronegativity Electronegativity http://www.youtube.com/watch?feature=player _embedded&v=93G_FqpGFGY#! Check for understanding: Which is more electronegative? F or Cl ? Na or K ? Sn or I ? F Na I Remember it with a song http://www.youtube.com/watch?v=MGTKVaZ cq8Y Complete the worksheets included in the note taking guide