Periodic Trends: Atomic Radii

Elemental Properties and Patterns

The Periodic Law

Dimitri Mendeleev (1869/1871) was the first scientist to publish an organized periodic table of the known elements.

He was taking a chemistry course in Russia and tried to find a way to organize the periodic table.

The Periodic Law

Mendeleev even went out on a limb and predicted the properties of 2 at the time undiscovered elements.

He was very accurate in his predictions, which led the world to accept his ideas about periodicity and a logical periodic table.

The Periodic Law

Mendeleev understood the ‘Periodic Law’ which states:

When arranged by increasing atomic number, the chemical elements display a regular and repeating pattern of chemical and physical properties.

The Periodic Law

Atoms with similar chemical properties and behavior appear in groups or families (vertical columns named by Roman numerals with A or B) on the periodic table.

They are similar because they all have the same number of valence (outer shell) electrons, which governs their chemical behavior.

Periods– horizontal rows on periodic table

Periodic Trends

There are several important atomic characteristics that show predictable trends that you should know.

Atomic properties— Deal with only single atoms

The full, positive nuclear charge is “SHIELDED” from

outer shell electrons by all the other electrons between them

Electrons closer to the nucleus shield other electrons further away. Full attractive force is not felt by outer electrons

Electrons have different energies and therefore, located at different regions surrounding the nucleus Based on electrostatic field + positively charged nucleus

Electron Shielding

The amount of nuclear charge influencing or

attracting an electron

= actual nuclear charge—screening influence of other electrons

Effective Nuclear Charge (Zeff)

An atom’s size affects chemical and physical

properties of an atom

Atom’s size is related to the radius of the atom Radius of atom = ½ distance between nuclei of 2

adjacent atoms of the same element Radius is the distance from the center of the nucleus to the “edge”

of the electron cloud. half the distance between the nuclei of 2 bonded atoms.

Atomic Radii

Atomic Radius General Trend

Decreases

Increases

Atomic Radius

The effect is that the more positive nucleus has a greater pull on the electron cloud. The nucleus is more positive and the electron cloud is

more negative. The increased attraction pulls the cloud in,

making atoms smaller as we move from left to right across a period—MOVING ACROSS TABLE

More and more electrons are added and placed at higher and higher energy levels so atomic size increases—MOVING DOWN TABLE

Half the distance between nuclei of adjacent

atoms in a metal

Radius of metallic atoms

> 75% of elements are metals

1. Metallic Radii

2. Covalent Radius

Half the distance between the nuclei of 2 adjacent, bonded atoms in a chemical compound.

Atoms MUST be identical Radius of nonmetallic atoms (Ex. Cl2. Br2)

Smaller radii than metallic—covalent bonds hold the atoms close

Ex. Br2

2.86

Å1.43 Å

1.43 Å

Ions

Atoms that have gained or lost electrons (anion, cation)

Electrons gained/lost from valence shell (outer electron shell)

Defined by the distance between the nuclei of 2 ions Studies with crystal structures Radii related to original atomic radii

Ionic Radii

1) Anions

LARGER size than original neutral atom Generally nonmetals

Additional electrons cause the atom’s size to increase

Electrons repel as increase in number, causes size to increase

Ionic Radii (cont.)

2) Cations

SMALLER size than original neutral atom Generally metals

Removal of electrons causes the atom’s size to decrease more attractive force from nucleus, nucleus can

pull remaining electrons towards it.

Ionic Radii (cont.)

Element

Covalent Radius (nm)

Ion Ionic Radius (nm)

F 0.064 F- 0.136

Cl 0.099 Cl- 0.181

Li 0.123 Li+ 0.068

Na 0.157 Na+ 0.095

Compare atomic sizes of Na and Cl

Neutral Ion Na > Cl Na+ < Cl-

Example 1

Applies to either atoms or ions

Atoms or ions with the SAME number of electrons So electron configuration is the same as well

Does not state the protons stay the same As the proton number increases but the electrons

stay the same, the positive nucleus holds electrons more tightly

Isoelectric

Atom/Ion Radius (nm) Electron Configuration

O2- 0.140 1s22s22p6

F- 0.136 1s22s22p6

Ne 0.112 1s22s22p6

Na+ 0.095 1s22s22p6

Mg+2 0.065 1s22s22p6

Which of the following ions has the LARGEST

radius?

A) Na+B) Mg+2

C) S2-

D) Cl-

E) Se2-

Example 2:

Which of the following ions has the SMALLEST

radius? Why?

A) K+

B) Li+

C) Be+2

D) O2-

E) F-

Example 3:

Which of the following isoelectric ions is the

LARGEST? Why?

A) Mn+7

B) P3-

C) S2-

D) Sc3+

E) Ti4+

Example 4:

Atomic Theory III Worksheet #1-17

Homework