III. Periodic Trends

Types of Periodic Trends

• Atomic size (atomic radius)• Ionic size (ionic radius)• Ionization energy• electronegativity

Trends in Atomic Size

• The atomic radius is the distance between the nucleus and the outer edge of the electron cloud.

Trends in Atomic Size

• In general, atomic size:– increases from top to bottom within a group and – increases from right to left across a period.

Atomic radius vs. atomic number

Ca

K

H He

Li

B

Be

C N O FNe

NaMg

Al Si P S Cl

Ar

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

Element

Ato

mic

Rad

ius

(pm

)

Trends in Atomic Size

Ions

• Ions form when electrons are transferred between atoms.• If an atom loses an electron: cation (positive charge)• METALS ALWAYS LOSE (electrons)

Ions

• If an atom gains an electron: anion (negative charge)• Non-metals always gain electrons

Trends in Ionic Size

• The size of ions:– Cations (LOSE ELECTRONS) are always smaller than the

original atoms. – Anions (GAIN ELECTRONS) are always larger than the original

atoms.

Trends in Ionic Size

• Trends in Ionic Size: Size of anions and cations increase from right to left

Siz

e g

en

era

lly in

crea

ses

Negative Ions (anions)Positive Ions (cations)

Trends in Ionization Energy

• Atoms hold onto its electrons with different “strengths”• The energy required to remove an electron from an atom is

called ionization energy.– Energy required to remove the first electron is called the first

ionization energy. – Energy required to remove an electron from an ion with a 1+ charge

is called the second ionization energy.

Trends in Ionization Energy

• First ionization energy:– increases from bottom to top within a group, and – increases from left to right across a period.

Ionization energy vs. atomic number

Ca

K

H

He

Li

B

Be C

N O

F

Ne

Na

MgAl

SiP S

Cl

Ar

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12 14 16 18 20

Element

Ioni

zatio

n en

ergy

(kJ

/mol

)

Trends in Electronegativity

*

* Fluorine has the highest electronegativity

Electronegativity is the ability of an atom to attract electrons when the atom is in a compound

Trends in Electronegativity

• Electronegativity is the ability of an atom to attract electrons when the atom is in a compound.

• Fluorine has the highest electronegativity.

• Noble gases have zero electronegativity (they already have a full valence shell)