October 05, Periodic_Trends_Presentation student notes.notebook. Periodic Trends. Periodic Trends: Effective Nuclear Charge

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1 Periodic Trends: tomic Radius Ionization nergy lectronegativity Metallic haracter Ionic Radius Periodic Trends Five main trends in the periodic table will be discussed: The sizes of atoms Ionization energy lectronegativity Metallic character The sizes of ions These five periodic trends are affected by these main factors: ffective nuclear charge oulomb's Law Shielding from inner electrons ffective Nuclear harge In a many electron atom, electrons are both attracted to the nucleus and repelled by other electrons. The nuclear charge that an electron experiences depends on both factors. For example, here's sodium. ffective Nuclear harge The effective nuclear charge, Z eff, is found this way: Z eff = Z S Where Z is the atomic number and S is a screening constant, usually about equal to the number of inner electrons. In this example, the outer electron of sodium is attracted towards the nucleus by an effective charge (Z eff) of 1 proton. Moving across the periodic table increases Z eff, and therefore increasing the force of attraction between the electrons and the nucleus. ffective Nuclear harge For any given atom, the electrons closest to the nucleus experience the greatest effective nuclear charge (they have the least shielding). The electrons farthest from the nucleus experience the least effective nuclear charge (they are shielded the most). s you move across the periodic table (from left to right), the effective nuclear charge felt by the outermost electron increases. tomic Radius In general, as you move from left to right across the periodic table the atoms have a smaller radius s you move down from row to row, the radius increases Helium has the smallest radius Francium has the largest radius 1

2 Summary of tomic Radius Trends tomic radius decreases going left to right across a period. cross a period, effective nuclear charge increases. So the force of attraction between the nucleus and outer electron(s) gets stronger. Outer electrons are pulled in more strongly, so radius gets smaller. Size of orbitals stays approximately the same (across a given period) Summary of tomic Radius Trends tomic radius increases going down a group. The size of orbitals increases significantly. Outer electrons are located farther from the nucleus in each successive period. 1 The effective nuclear charge acting on an electron is larger than the actual nuclear charge. True False The "r" in oulomb's law increases so the force of attraction between nucleus and outer electrons weakenss lectron electron repulsion increases with greater atomic number. 2 selenium atom has 34 electrons. lectrons in the subshell experience the lowest effective nuclear charge. 3 In which orbital does an electron in an arsenic (s) atom experience the greatest shielding? 2p 1s 4p 3p 3p 3d 3s 4p 1s 5p 2

3 4 In which orbital does an electron in a calcium atom experience the greatest effective nuclear charge? 5 tomic radius generally increases as we move. 1s 2s 2p 3s down a group and from right to left across a period up a group and from left to right across a period down a group and from left to right across a period 3p up a group and from right to left across a period down a group; the period position has no effect 6 Which one of the following atoms has the smallest radius? Ions When a neutral atom gains or loses electrons, it becomes an ion. ny atom losing electrons becomes a positive ion: O F S l Na Na + + e (neutral (sodium (electron) sodium) ion) ny atom gaining electrons becomes a negative ion: F + e F (neutral (electron) (fluorine fluorine) ion) Ions ations are positive and are formed by elements on the left side of the periodic chart. nions are negative and are formed by elements on the right side of the periodic chart. toms tend towards having complete outer shells of electrons (remember stability). full outer shell has: Ions 2 electrons in the s subshell and 6 electrons in the p subshell Thus, atoms tend towards having a total of: 8 Valance lectrons This is the Noble Gas electron configuration. 3

4 Valence lectrons The outer most electrons in an atom are called valence electrons. Number of valence electrons in neutral atoms: Which ion below has a noble gas electron configuration? Li 2+ e N 2 Ionization nergy The ionization energy is the amount of energy required to remove an electron from the ground state of a gaseous atom or ion. The first ionization energy is that energy required to remove first electron. a a + + e The second ionization energy is that energy required to remove second electron, etc. a + a 2+ + e Trends in First Ionization nergies s one goes down a column, less energy is required to remove the first electron. For atoms in the same group, Z eff is essentially the same, but the valence electrons are farther from the nucleus, "r" increases, so it is easier to remove the outermost electron Trends in First Ionization nergies Trends in First Ionization nergies Generally, as one goes across a row, it gets harder to remove an electron. However, there are two apparent discontinuities in this trend. s you go from left to right, Z eff increases, making it harder to remove an electron 4

5 Trends in First Ionization nergies Trends in First Ionization nergies The first is between Groups 2 and 3. In this case the electron is removed from a p 1 orbital rather than an s orbital. The electron removed is farther from nucleus, there is a small amount of repulsion by the s electrons, and the atom gains stability by having a full s subshell. The second is between Groups 15 and 16. The electron removed comes from doubly occupied p orbital. Repulsion from the other electron in the orbital aids in its removal. The atom gains stability by having a half full p orbital Summary of Trends in Ionization nergy Summary of Trends in Ionization nergy Ionization nergy decreases going down a group Ionization nergy increases left to right across a period The size of orbitals increases significantly y adding one proton and one electron you are increasing q 1 and q 2 in oulomb's Law Therefore, the force of attraction between the nucleus and outermost electrons is strengthened Thus, it takes more energy to remove the outermost electron The distance between the nucleus and outer electrons increases So the force of attraction between the nucleus and outer electron is less lso, there is more shielding by inner electrons as you go down a group nd more electron electron repulsion with increasing number of electrons Ionization nergy It requires more energy to remove each successive electron. ie: second ionization energy is greater than first, third ionization energy is greater than second, etc. 8 Which noble gas has the lowest first ionization energy? Give the atomic number. When all valence electrons have been removed, the ionization energy takes a huge jump. 5

6 9 Of the following atoms, which has the largest first ionization energy? 10 Of the following elements, which has the largest first ionization energy? r Na O l Se P I l r 11 Of the elements below, has the largest first ionization energy. Li K Rb Na H lectronegativity Recall that atoms gain stability when they have a full orbitals. Fluorine has 7 valence electrons. Neon has 8 valence electrons. n atom of fluorine would be much more stable if it gained an electron, and became the fluoride ion (with the same electron configuration as neon). Fluorine atoms "like" to acquire electrons lectronegativity lectronegativities lectronegativity is a measure of the ability of atoms in a molecule to attract electrons to themselves. On the periodic chart, electronegativity increases as you go from left to right across a row. from the bottom to the top of a column. 6

7 lectronegativities 12 The ability of an atom in a molecule to attract electrons is best quantified by the. In general we will not be concerned with the electronegativites of transition metals. paramagnetism diamagnetism electronegativity electron change to mass ratio first ionization potential 13 lectronegativity from left to right within a period and from top to bottom within a group. decreases, increases increases, increases increases, decreases stays the same, increases increases, stays the same 14 Of the atoms below, is the most electronegative. r O l N F 15 Of the atoms below, is the most electronegative. 16 Of the atoms below, is the least electronegative. Si l Rb a S Rb F Si l a 7

8 17 Which of the elements below has the largest electronegativity? Metallic haracter The metallic character of an element is a measure of how loosely it holds onto its outer electrons. Si Mg P S Na For a metal to conduct electricity or heat, it needs to have electrons that are free to move through it, not tightly bound to a particular atom. So the metallic character of an element is inversely related to its electronegativity. On the periodic chart, metallic character increases as you go from right to left across a row. from the top to the bottom of a column. Metallic haracter Metals, Nonmetals, and Metalloids ll atoms can be classified as metals, nonmetals or metalloids (also called semi metals) There are 7 semi metals: oron, Silicon, Germanium, rsenic, ntimony, Tellurium, and statine (85) 18 Of the elements below, is the most metallic. 19 Of the elements below, is the most metallic. sodium barium magnesium calcium cesium Na Mg l K r 8

9 Ionic size depends upon: ations are always smaller than their parent atom. The nuclear charge. The number of electrons. The orbitals in which electrons reside ut... ations are always smaller than their parent atom. nions are always larger than their parent atoms Neutral Lithium Li Lithium ation Li + + Free electron e nions are always larger than their parent atoms Ionic size depends upon: The nuclear charge. The number of electrons. Neutral Fluorine F + Free electron e Fluorine nion F The orbitals in which electrons reside. ations (pink) are smaller than their parent atoms (gray). The outermost electron is removed and repulsions between electrons are reduced. The more electrons removed, the smaller the cation becomes nions (blue) are larger than their parent atoms (gray). lectrons are added and repulsions between electrons are increased. The more electrons added, the larger the anion becomes 9

10 Ions increase in size as you go down a group. This is due to increasing value of n (adding energy levels). In an isoelectronic series, ions have the same number of electrons. Ionic size decreases with an increasing nuclear charge. The following ions are isoelectronic with Neon. Z is the nuclear charge (atomic number) neon Z = 8 Z = 9 Z = 10 Z = 11 Z = 12 Z = is isoelectronic with argon and is isoelectronic with neon. l, F l, l + F+, F Ne, Kr+ Ne, r + 21 Which of the following is an isoelectronic series? 5, Sr 4, s 3, Te 2 F, l, r, I S, l, r, K Si 2, P 2, S 2, l 2 O 2, F, Ne, Na + 22 Which of the following has the largest radius? r Kr r Sr 2+ Rb + 10