Lesson 21 Periodic Trends

Objectives:

-         The student will predict changes in atomic radius, shielding effect, ionization energy, electron affinity, and electronegativity, based on element positions on the periodic table.

 PA Science and Technology Standards: 3.1.10.C; 3.4.10.A; 3.4.12.A; 3.4.10.B

I.   The periodic table contains vertical and horizontal trends

a.      Trends in the table can be found in the horizontal periods and the vertical groups or families.

b.      Knowing these trends allows you to predict the behavior of elements when you don’t know much about them.

II. Atomic Radius increases within a family

a.      Because the nucleus is so small, the element’s size is determined by the electron cloud surrounding it

b.      Often these measurements are made by measuring the distance between two nuclei in a diatomic molecule, and then dividing by two.

c.      Atomic radius is defined as one-half the distance from center to center of two like atoms.

d.      Spherical representations of the atoms are useful, although not completely accurate.

e.      Trend 1: Atomic radius increases as you progress down through the elements in each group

                         i.      Because of the addition of an energy level every time you go to the next period, it makes sense that the atomic radius increases.

f.        Elements with large numbers of electrons show the shielding effect – the reduction of the attractive force between a nucleus and its outer electrons due to the blocking effect of the inner electrons

g.      Trend 2: Shielding effect increases as you progress down through the elements in a group.

h.      As you move across a period, you add electrons to the same level, but don’t create any new levels.

i.        Trend 3: Shielding effect does not change as you move across a period.

III. Atomic size decreases from left to right across a period

a.      Trend 4: Atomic radii generally decrease as you move across a period from left to rightb.      As you add electrons within the same period, no new principle energy levels are added.c.      The nucleus has a greater positive charge, and therefore can pull on the electrons more, and they end up closer to the nucleus.d.      As you move further across the period, the electrons end up being pulled closer together by the nucleuse.      Because electrons repel each other, there comes a point where the repulsion of the electrons among themselves and the attraction of the nucleus for

them levels out, and the atomic radius does not continue to shrink

f.       Tables illustrating atomic radii: 

Atomic radii in the case of ion formation

+ ions will decrease in size

- ions will increase in size

IV.   Ionization energy follows a periodic trend.

a.       Atoms are normally electrically neutral.b.      An ion forms when an atom loses or gains electronsc.      Ion – an atom or groups of atoms that has gained or lost one or more electrons to acquire a net electric charge.d.      The amount of energy needed to pull one electron away from

an atom is known as the ionization energye.      Ionization energy - the amount of energy needed to remove an electron from a specific atom or ion in its ground state in the gas phase.f.       Trend 5: Ionization energy generally decreases as you move down through a group.g.      As you move down in a group, the electrons are further away from the nucleus, and therefore, easier to remove.h.      Trend 6: Ionization energy generally increases as you move across a period.i.       As you move across a period, the electrons in the outer level are held in place with a stronger attractive force from the nucleus

Table of Ionization Energies:

V. Electron affinity decreases within a family and increases within a period.

a.      Electron affinity is the energy change that accompanies the addition of an electron to an atom in the gas phase.

b.      Trend 7: Electron affinity values generally become more negative (read as “increases”) as you move from left to right across a

period.

c.      This is because as you move left to right, the atom has more attractive charge from the nucleus for electrons, the shielding

effect doesn’t change, and the electrons are close to the nucleus.

d.      Trend 8: Electron affinity tends to become less negative (read as “decreases”) as you move from top to bottom within a group.

e.      As the atom gets larger, the attractive force gets larger also, but the distance more than offsets the larger attraction. It becomes harder for the atom to attract electrons.

I. Electronegativity decreases within a family and increases within a period

a.      Electronegativity is the tendency for an atom to attract electrons to itself when it is combined with another atom.

b.      It is an arbitrary scale, with Fluorine (the most electronegative) being assigned 4, and 0 being the lowest, or no attraction.

c.      The noble gases are not assigned a value, as they do not form a significant number of compounds.

d.      It is related to electron affinity and ionization energy

e.      Trend 9: Electronegativity values generally decrease going down a group

f.       As you go down a group, the atom becomes larger, and therefore cannot exert as strong a pull on a new electron.

g.      Trend 10: Electronegativity values generally increase going across a period

h.      As you go across, the shielding effect doesn’t change, but the positive attraction of the nucleus does.

Summary of Periodic Trends