Lesson : Periodic Trends: Ionic Radii, First Ionization Energy, Electronegativity

LEARNING GOALS

3.3.3 Using atomic theory, bonding, and the repeating pattern of electron configurations, explain the trends of atomic and ionic radii across periods and down columns.

3.3.4 Using atomic theory, bonding, and the repeating pattern of electron configurations, explain the trend of first ionization energy across periods and down columns.

3.3.5 Using atomic theory, bonding, and the repeating pattern of electron configurations, explain the trend of electronegativity across periods and down columns.

 

 

DISCUSSION

Periodic Trends: Ionic Radii

Just as atomic radii can be determined using Coulombic attraction concepts, ionic radii can be determined using the same concepts. As the name ionic radii implies, the trend for ionic radii is determining the size (radius) of an ion. The formation of a positively charged ion, a cation,  involves losing electrons. Less electrons means a decrease in the number of electron-electron replusions. Therefore, cations are smaller than their parent atoms.

chemistry atomic radii<br />ionic radii periodic<br />trends<br />laurajutex-semuatentangfi<br />sika.blogspot.com

 

The opposite is true for anions or atoms that gain electrons and form a negative charge. When electrons are added to an atom there are more electron-electron replusions causing electrons to spread out more in space. Thus, anions are larger than their parent atoms.

 

 

Periodic Trends: First Ionization Energy

The first ionization energy of an atom is the energy required to remove the first electron from a neutral atom. The greater the ionization energy, the more difficult it is to remove an electron. Using the same Coulombic attraction ideas, we can explain the first ionization energy trends on the periodic table. 

chemistry periodic trends<br />periodic table first<br />ionization energy 3D<br />www.chem.ufl.edu

 Across a period

             Trend: first ionization energies increase

             Reasoning: increase in protons, increase effective nuclear charge, more attraction

                                        = harder to remove an electron (takes more energy)

                    ** Makes sense: moving across the period towards nonmetals = tend to gain electrons (anions), not lose them

 Down a Group

              Trend: first ionization energies decrease

              Reasoning: more shells/energy levels, more distance from the nucleus + more shielding, decrease effective nuclear charge, less attraction

                                         = easier to remove an electron (takes less energy)

 

Periodic Trends: Electronegativity

 The tendency of an atom in a molecule to attract shared electrons to itself is called electronegativity. The greater an atom's electronegativity, the greater is its ability to attract electrons to itself. Once again, using the Coulombic attraction ideas the periodic trends for electronegativity can be explained. 

chemistry intramolecular<br />forces electronegativity<br />chart periodic table 3D<br />http://stoichiometricbasi<br />cs.blogspot.com/

 Across a Period

             Trend: electronegativity increases

             Reasoning: more protons, increase effective nuclear charge

                               stronger attraction = high electronegativity

Down a Group

              Trend: electronegativity decreases

              Reasoning: more shells/energy levels, farther distance from the nucleus + more shielding, decrease                                effective nuclear charge

                                 weaker attraction= low electronegativity 

** Hint: Fluorine is the most electronegavtive element. If you remember that fact, everything becomes easy, because electronegativity must always increase towards fluorine in the periodic table.

Missing the Nobel Gases??

Looking at the graph above, it ignores the noble gases. Historically this is because they were believed not to form bonds - and if they don't form bonds, they can't have an electronegativity value. Even now that we know that some of them do form bonds, data sources still don't quote electronegativity values for them.