Chem 100
Atoms
► The smallest unit of matter “indivisible”.”
Helium
atom
Electron Shells
a)
Atomic number = number of Electrons
b)
Electrons vary in the amount of energy
they possess, and they occur at certain
energy levels or electron shells.
c)
Electron shells determine how an atom
behaves when it encounters other atoms
Electron Location

Each energy level (shell) consists of sublevels:

Shape of electron cloud
s = spherical
p = dumbbell
d = too complex
f = too complex

1st energy level has 1 sublevel – s

2nd energy level has 2 sublevels -- s and p

3rd energy level has 3 sublevels -- s, p, and d

4th energy level has 4 sublevels -- s, p, d and f
S Sublevel

Orbitals do not have sharp boundaries.
Orbitals


Describes the orientation in space within a
sublevel.

s = 1 orbital

p = 3 orbitals

d = 5 orbitals

f = 7 orbitals
ONLY 2 electrons go into any orbital!!!
Orbitals
The s and p types of sublevel
Orbitals

Spin: electrons in the same orbital must
have opposite spins.

One spins clockwise and the other spins
counter-clockwise. (+1/2, or -1/2)
Electron Configuration

Elements
have
different
electron
configurations; that is, different levels of
bonding.
 Electrons
to rules.
 The
are placed in shells according
1st shell can hold up to two
electrons, and each shell thereafter
can hold up to 8 electrons.
Electron Configuration
1. Aufbau Principle: electrons enter orbitals of
lowest energy first.
- Electrons do not fill in orbitals in consecutive
numerical order.
2. Pauli Exclusion Principle: - an atomic orbital
can hold a maximum of 2 electrons and
those 2 electrons must have opposite spins.
Electron Configuration
Orbital diagram: a box grouped by sublevel
containing arrow(s) to represent electrons
Electron Configuration
3. Hund’s Rule: When electrons occupy orbitals
of equal energy (same sublevel), one electron
enters each orbital with parallel spin before
pairing oppositely.
- E.g.: Helium atom
Electron Configuration
Electron Arrangements of the First 18 Atoms
on the Periodic Table
Classifying Electrons

Valence electrons – electrons in the
outermost (highest) principal energy level of
an atom.

Core electrons – inner electrons.

Elements with the same valence electron
arrangement show very similar chemical
behavior.
Electron Arrangement

Orbital filling and the periodic table
Atomic Properties

Metals and Nonmetals
- Metals tend to lose electrons to form positive ions.
- Nonmetals tend to gain electrons to form negative ions.
Atomic Size
• Size
tends
increase down
column. Why?
• Size
tends
decrease across
row. Why?
to
a
to
a
Electronegativity

Tendency for an atom to attract electrons to
itself when bonded to another element.
– Tends to decrease down a column
– Tends to increase across a row
Lewis Dot Structures

1
Symbols of atoms with dots to represent the
valence-shell electrons.
2
13
14
15
16
17
H
He:

Li
18
Be

Na Mg


C

 N

O
 B

F







:

:Ne:




Al
 Si
 P
 S
:Cl
:Ar:





Electron
Configuration
Worksheet