1.4 periodic trends - Introductory University Chemistry I...

Introductory University
Chemistry I
Unit 1
Periodic Table
and
Atomic Properties

Introduction
• The periodic table is arranged according to the atomic
number of the elements – Mendeleev & Moseley
• The basis of the periodic table is the electron
configuration of elements
• The physical and chemical properties of the elements
show regular periodic trends

Core & valence electrons
-
-
+
+
Core
Valence
-
-
+
+
electrons
electrons
Do not take part in
bonding
Affect the periodic
properties
Take part in bonding
Determine the periodic
properties

Electrostatic attractions between
electrons and the nucleus
Understanding the nature of electrostatic attractions between
electrons and nucleus helps to predict periodic trends
Q
1
x Q
2
Energy ∝
d

Effect of nuclear charge (Z)
• Increasing nuclear charge is expected to
increase the attraction (energy)

Effect of Shielding (S)
Electron in the same shell as the electron of interest shields
the nucleus attraction somewhat but not very effective
For example:
He (g) He
+
(g) + e (Easier)
He
+
(g) He
2+
(g) + e (Harder)

Effect of Shielding (S)
Stronger attraction
(Harder to remove)
Electrons in shells with lower quantum numbers than electron of
interest shield nucleus better than electrons in the same shell.

Effect of Distance
• The greater the distance an electron from the
nucleus, the smaller is the attraction (energy)
Closer to the nucleus
Harder to remove
Further from nucleus
Easier to remove

Moving across a period
Moving
down a
group
circle6 What is the periodic trend moving down a group?
circle6 What is the periodic trend moving across a period?

Atomic radius
• The radius of an atom is
difficult to define due to
uncertainty of the atomic
boundary.
• The atomic radii are measured
by determining the inter-nuclear
distance.

• Covalent radius – ½ distance between two
nuclei formed by two identical atoms by a single
covalent bond.
• Metallic radius - ½ distance between two nuclei
formed by two identical atoms by a metallic bond.

The inter-nuclei distance of
Cl – Cl bond is 199 pm
Covalent radius of Cl is 199 / 2 ≈ 100 pm
The covalent radius of C - Cl is 177 pm
Covalent radius of carbon
= 177 pm – 100 pm = 77 pm

Trends in atomic size

Trends in atomic size
Atomic size increases
down a group
Atomic size decreases
across a period
Atomic size of transition
elements does not show
drastic change

Trends in atomic size
Atomic radius increases going down
groups
• Increase principal quantum number,
n, also increases orbital size

4s electron