Electron Configurations and

Periodicity

Chapter 8

Isoelectronic Species

Species that have the same electronic configuration.

Having the same number of electrons is not sufficient.

• H-, He, Li+, Be2+ are isoelectronic

• Mn-, Fe, Co+ are NOT isoelectronic

• Br, Cl, I , F are NOT isoelectronic

Isoelectronic Species

Periodic Properties

The periodic law states that when the elements are arranged by atomic number, their physical and chemical properties vary periodically.

We will look at three periodic properties:Atomic radius

Ionization energy

Electron affinity

Periodic Properties

Atomic radius

Within each period (horizontal row), the atomic radius tends to decrease with increasing atomic number (nuclear charge).Within each group (vertical column), the atomic radius tends to increase with the period number.

Periodic Properties

Two factors determine the size of an atom.

One factor is the principal quantum number, n. The larger is “n”, the larger the size of the orbital.

The other factor is the effective nuclear charge, which is the positive charge an electron experiences from the nucleus minus any “shielding effects” from intervening electrons.

Figure 8.17: Representation of atomic radii (covalent radii) of the main-group elements.

Atomic Radii

Periodic Properties

Ionization energy

The first ionization energy of an atom is the minimal energy needed to remove the highest energy (outermost) electron from the neutral atom.

For a lithium atom, the first ionization energy is illustrated by:

e)s1(Li)s2s1(Li 212

Ionization energy = 520 kJ/mol

Periodic Properties

Ionization energyThere is a general trend that ionization energies increase with atomic number within a given period.

This follows the trend in size, as it is more difficult to remove an electron that is closer to the nucleus.

For the same reason, we find that ionization energies, again following the trend in size, decrease as we descend a column of elements.

Figure 8.18: Ionization energy versus atomic number.

Ionization Energies

Periodic Properties

Ionization energyThe electrons of an atom can be removed successively.

The energies required at each step are known as the first ionization energy, the second ionization energy, and so forth.

Table 8.3 lists the successive ionization energies of the first ten elements.

Ionization Energies

Periodic Properties

Electron AffinityThe electron affinity is the energy change for the process of adding an electron to a neutral atom in the gaseous state to form a negative ion.

For a chlorine atom, the first electron affinity is illustrated by:

)p3s3]Ne([Cle)p3s3]Ne([Cl 6252 Electron Affinity = -349 kJ/mol

Periodic Properties

Electron AffinityThe more negative the electron affinity, the more stable the negative ion that is formed.

Broadly speaking, the general trend goes from lower left to upper right as electron affinities become more negative.

Table 8.4 gives the electron affinities of the main-group elements.

Electron Affinities

Atomic vs. Ionic Radii

In the case of a positive ion vs its parent atom the positive atom has a smaller radius.

There are more positive charges pulling on the electrons

In the case of a negative ion vs its parent atoms the parent atom has a smaller radius.

The extra electrons create Coulombic repulsions which increase the size of the outer shell.

Atomic vs. Ionic Radii

Metallic Behavior

The metallic character also exhibits a periodic trend.

The further down a group an element is located the more metallic it is.

• C, Si, Pb

The further to the right the element is located the lower its metallic character.

• Ca, Br, Kr

Operational Skills

Applying the Pauli exclusion principle.

Determining the configuration of an atom using the Aufbau principle.

Determining the configuration of an atom using the period and group numbers.

Applying Hund’s rule.

Applying periodic trends.

Molecules, Ions and Their Compounds

Chemistry 101

Chapter 3

Virginia State University

Dr. Victor VilchizSummer 2008

Types of Bonding• There are three different types of bonding.

– Ionic bonding, which involves a metal and a nonmetal (or polyatomic ions)

• Ionic bonding requires the exchange of electrons

– Covalent bonding, which involves two nonmetals or metalloids.

• Covalent bonding requires the sharing of electrons

– Metallic bonding, which involves two metals.• Metallic bonding requires pooling of the valence

electrons.

Chemical Formulas; Molecular and Ionic Substances

• The chemical formula of a substance is a notation using atomic symbols with subscripts to convey the relative proportions of atoms of the different elements in a substance.

Consider the formula of aluminum oxide, Al2O3. This formula implies that the compound is composed of aluminum atoms and oxygen atoms in the ratio 2:3.

Chemical Formulas; Molecular and Ionic Substances

• Molecular substancesA molecule is a definite group of atoms that are chemically bonded together – that is, tightly connected by attractive forces.

A molecular substance is a substance that is composed of molecules, all of which are alike.

Chemical formula provides information regarding to the composition of the compound.

Chemical Formulas• There are at least four types of

chemical formulasEmpirical formula shows the smallest ratio of elements possible in a compound.

A molecular formula gives the exact number of atoms of elements in a molecule.

Structural formulas show how the atoms are bonded to one another in a molecule.

Condensed formula shows every atom present in a formula showing the bonding atoms but not the exact structural positioning.

(molecules)

Although many substances are molecular, others are composed of ions.

An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons.

Sodium chloride is a substance made up of ions. (NaCl)

Chemical Formulas; Ionic Substances

• Ionic substances

When an atom picks up extra electrons, it becomes a negatively charged ion, called an anion.

An atom that loses electrons becomes a positively charged ion, called a cation.

An ionic compound is a compound composed of cations and anions..

An ionic compound is held together by the An ionic compound is held together by the coulombic attractionscoulombic attractions of the opposite sign of the opposite sign charges.charges.

Chemical Formulas; Ionic Substances

• Ionic substances

The formula of an ionic compound is written by giving the smallest possible whole-number ratio of different ions in the substance that will yield a neutral species.

Ionic substances do not have molecular formulas since a cation is surrounded by many anions to be able to distinguish where a molecule ends and the next starts.

Ionic compounds instead have formula units of the substance is the group of atoms or ions explicitly symbolized by its formula.

Chemical Formulas; Ionic Substances

• Ionic substances

Most ionic compounds contain metal and nonmetal atoms; for example, NaCl.

You name an ionic compound by giving the name of the cation followed by the name of the anion.

A monatomic ion is an ion formed from a single atom.

Figure 3.7 shows the common monatomic ions some elements.

Chemical Substances; Formulas and Names

• Ionic compounds

Charges on Common Charges on Common IonsIons

Charges on Common Charges on Common IonsIons

+3

-4 -1-2-3+1

+2

By losing or gaining e-, atom has same By losing or gaining e-, atom has same number of e-’s as nearest Group 8A atom.number of e-’s as nearest Group 8A atom.

Most of the main group metals form cations with the charge equal to their group number.

Example: Aluminum is in group IIIA; therefore an aluminum ion will be Al3+

The charge on a monatomic anion for a nonmetal equals the group number minus 8.

Example: Nitrogen is in group VA; therefore a nitrogen ion will be N(5-8),N3-

Chemical Substances; Formulas and Names

• Rules for predicting charges on main group monatomic ions

Transition Metal Ions

A lot of the transition metal ions are confused as to how many electrons they wish to donate; therefore, they form more than one ion, each with a different charge. (see Table).

You should know the ones for Cu, Fe, Pb, and Hg.

Predicting Charges on Monatomic IonsPredicting Charges on Monatomic Ions

There are two types of cations: elemental (Monatomic) and Ammonium Ion

Monatomic cations are named after the element and the word Ion is appended. For example, Al3+ is called the aluminum ion.

If there is more than one cation of an element, a Roman numeral in parentheses denoting the charge on the ion is used. This often occurs with transition elements. Example: Fe2+ and Fe3+; Iron(II) ion and Iron(III) ion

Naming Cations• Rules for naming cations

Naming Anions

Monatomic Anions: Use the root of the element’s name and change ending to – ide and append the word ion.

example, Br Br – Bromine becomes Bromide ion.

Polyatomic Anions: If it contains Oxygen (EOn

m-)use the following rules:

• Rules for naming anions

Naming Anions• (EOn

m-); Keep the root of the name of element “E” then:– if there is only one ion involving E and

O ending of “E” is changed to –ate.• Example: CO3

2-; E= Carbon Carbonate

– If there are two ions between E and O, the ion with most O’s has name ending in –ate the one with least O’s has name ending in –ite.

• Example NO3- and NO2

-; E=Nitrogen• NO3

- = Nitrate and NO2- = Nitrite

Naming Anions– if there are FOUR ions involving E and O

ending of “E” is changed to -ate for the two with most O’s and to –ite for the two with least O’s.

• Only Happens when E= Br, I, or Cl

• To distinguish between the two –ate and the two –ite: to the one with 4 O’s add the prefix PER and to the one with 1 O add the prefix HYPO.

• Ex: ClO-, ClO2-, ClO3

-, ClO4-

Hypochlorite, Chlorite, Chlorate and Perchlorate

Naming Anions

• If the polyatomic ion does not contain Oxygen then consult table 3.1.

Note: many O containing anions have names ending in –ate (or -ite).

Note: many O containing anions have names ending in –ate (or -ite).

Naming Ionic Compounds

• NaF -

• LiCl -

• MgO -

Naming Ionic Compounds

• NaF- Sodium Ion and Fluoride ion

• LiCl- Lithium Ion and Chloride Ion

• MgO- Magnesium Ion and Oxide Ion

Naming Ionic Compounds

• NaF - Sodium Fluoride

• LiCl - Lithium Chloride

• MgO - Magnesium Oxide

More PracticeCd(OH)2

Cadmium

Hydroxide

Ca(ClO)2

Calcium

Hypochlorite

AgCN

Silver Cyanide

Na2SO4

Sodium Sulfate

Na2SO3

Sodium Sulfite

KClO4

Potassium

Perchlorate

Ions You Should Know

http://sest.vsu.edu/~vvilchiz/ionsacids.htm

Properties of Molecular & Ionic Compounds