Chapter 2 The Periodic Table Flashcards
What if effective nuclear charge (Zeff)?
As the positivity of the nucleus increases, the electrons surrounding the nucleus (including those in the valence shell) experience a stronger electrostatic pull toward the center of the atom. This causes the electron cloud to move closer and bind more tightly to the nucleus. The electrostatic attraction between the valence shell electrons and the nucleus is known as the effective nuclear charge, a measure of the net positive charge experienced by the outermost electrons.
Describe the periodic trend for effective nuclear charge?
Zeff increases from left to right across a period
Zeff remains relatively constant through a group.
How is the principal quantum number impacted as one moves own a group? What does this mean? How does this play a role in effective nuclear charge?
The principal quantum number increases by one each time as you move down a group
This means that the valence electrons are increasingly separated from the nuclear by a greater number of inner shells (filled principal energy levels).
The increased separation results in a reduction of electrostatic attraction between valence electrons and nucleus. Outermost electrons are held less tightly as principal quantum number increases. As one goes down a group, the increased shielding created by inner shell electrons cancels the increased positivity of the nucleus, thus Zeff is relatively constant within a group.
What is the atomic radius?
The atomic radius is ½ of the distance between centers of two atoms of an element that are briefly in contact with each other.
Describe the trend of atomic radii across a period? Why is this?
The atomic radius decreases from left to right across a period.
As you move across a period from left to right, protons and electrons are being added to the outermost shell (the number of inner shell electrons remains constant). Because electrons are only added to the outermost shell, the increasing positive charge of the nucleus pulls the outer electrons more closely inward and holds them more tightly, thus Zeff increases left to right across a period, and thus the atomic radius decreases from left to right across a period.
Describe the trend of atomic radii down a group? Why is this?
As you move down a group, the increasing principal quantum number implies that the valence electrons will be further from the nucleus (because the number of inner shells increasing), separating the valence shell from the nucleus. Thus, the atomic radius increases down a group.
Describe the atomic radius trend within a periodic table?
The atomic radius decreases from left to right across a period.
The atomic radius increases down a group.
What is ionization energy?
Ionization energy (aka ionization potential) is the energy required to remove an electron from a gaseous species. Removing an electron is an endothermic process because it requires the input of heat.
Describe the ionization trend within a period and group as seen in the periodic table? How does Zeff impact ionization energy?
The greater the Zeff, the more tightly bound the valence electrons are to the nucleus, making it more difficult to remove an electron. Thus:
ionization energy increases from left to right across a period
and
ionization energy increases from bottom to top in a group
Why does the removal of a second or third (etc) electron require increasing amounts of energy?
This is because the removal of more than one electron means that the electrons are being removed from an increasingly cationic (positive) species
What are first and second ionization energy?
first ionization energy: the energy required to remove the first electron
second ionization energy: the energy required to remove the second electron from the univalent cation (X+) to form the divalent cation (X2+)
What is electron affinity?
Electron affinity refers to the energy dissipated by a gaseous species when it gains an electron. Gaining an electron is exothermic (heat is expelled).
Electron affinity is essentially the exact opposite concept of ionization energy.
Describe the trend for electron affinity?
The stronger the electrostatic pull (Zeff) between the nucleus and the valence shell electrons, the greater the energy release will be when the atom gains the electron. Thus, the electron affinity increases across a period from left to right.
Because the valence shell is farther away from the nucleus as the principal quantum number increases, the electron affinity increases from bottom to top
What is electronegativity? How are electronegativity and ionization energy related?
Electronegativity is a measure of the attractive force than an atom will exert on an electron in a chemical bond.
The greater the electronegativity, the more it attracts electrons within a bond.
Electronegativity values are related to ionization energy values: the lower the ionization energy, the lower the electronegativity (and vice versa).
What are the exceptions in electronegativity trends?
The first three noble gasses are exceptions. They have high ionization energies, but negligible electronegativity because they rarely form bonds.
