Chapter 3
Atoms and the Periodic Table
Matter
• What accounts for matters secrets?
– Atomists – Democritus
– All things are made of Fire – Herclitus
– Four Elements – Aristotle
Conservation of Mass and Law of
Definite Proportions
• Robert Boyle (1627–1691):
Provided evidence for the
atomic nature of matter.
– Element - A substance that
cannot be chemically
broken down further
• Joseph Priestley (1733–
1804):
Conservation of Mass and Law of
Definite Proportions
• Antoine Lavoisier (1743-1794):
Conservation of Mass
•
Law of Mass Conservation: Mass is neither
created nor destroyed in chemical reactions.
Hg(NO3)2 + 2 KI → HgI2 + 2KNO3
3.25g + 3.32g → 4.55g + 2.02g
6.57g
=
6.57g
Law of Definite Proportions
• Law of Definite Proportions: In a unique
compound the elements will always be found in
the exact same ratio.
Dalton’s Atomic Theory
• John Dalton (1766–1844): Proposed explanations for
the laws of mass conservation and definite proportions.
– Postulate 1: Elements are made of atoms
– Postulate 2: Atoms of the same element have the same
properties (including mass). Atoms of different atoms have
different properties
– Postulate 3: Compound are comprise of elements joined
together in small whole ratios
– Postulate 4: Chemical reactions only rearrange the way the
atoms are combined
The Structure of Atoms: Electrons
• Elements are composed of atoms
• Compounds are composed of atoms of
elements in a given ratio
• What does an atom look like?
The Structure of Atoms
• Structure of the Atom:
The Structure of Atoms
• Comparison of Subatomic particles
Particle
Grams
amu
Coulombs
e
Electron
9.109382 x 10-28 0.0005485799 -1.602176 x 10-19
-1
Proton
1.672622 x 10-24 1.007276
1.602176 x 10-19
1
Neutron
1.674927 x 10-24 1.008665
0
0
Atomic Number
• # protons in an atom determines the
element
– atomic number (Z) - Atomic number is found
on the periodic table
• # electrons = # protons in a neutral atom
Mass Number
• mass of the atom ≈ # neutrons + # protons
– Mass number = # protons + # neutrons
– An element may have more than one mass #
• Isotopes: Atoms with identical atomic numbers,
but different mass numbers.
• Due to different # of neutrons NOT protons
Isotopes and Atomic Weight
• Atoms of an element that have a different mass
are called isotopes
Elements and Atomic Number
Symbol
# protons
K
# neutrons
Mass #
Atomic #
# electrons
20
30
34
25
118
11
50
Atomic Mass
• Atomic Mass: A weighted average of the
isotopic masses of an element’s naturally
occurring isotopes.
– Unit – atomic mass unit (amu)
Example
• Chlorine has two naturally occurring isotopes:
with an abundance of 75.77% and an isotopic
mass of 34.969 amu, and with an abundance of
24.23% and an isotopic mass of 36.966 amu.
What is the atomic mass of chlorine?
Isotopes and Atomic Weight
•
Bromine has two naturally occurring isotopes:
79Br and 81Br, calculate the atomic mass of
bromine.
Isotope
Abundance%
79Br
50.69
81Br
49.31
Problem
• Atoms X, Y, Z, and R have the following nuclear
compositions:
Which two are isotopes?
–
–
–
–
–
A.
B.
C.
D.
E.
X&Y
X&R
Y&R
Z&R
X&Z
Electronic Structure
• The Periodic Table Powerpoint
– Elements in a group react similarly
• Electronic Structure
– Atomic Size
– Why metals for cations and non-metals anions
Electronic Structure of Atoms
• Structure based on an accepted quantum
mechanical model
– Mathematical description of the location in
which an electron can be found
– Quantum mechanical model – electron cloud
can be divided into
• Shells
• Subshells
• Orbitals
Electronic Structure
Shell – Each Floor
Subshell – Row of rooms
Nucleus
Orbital - Room
2 beds per room –
opposite direction
Electronic Structure of Atoms
• Shells – layers in which the electrons are
grouped around the nucleus
– As a layer is added the atomic size increases
• Shells agree with the period # on periodic table
– Differ in energy – requires more energy to remove
an electron from a shell closer to the nucleus
– Electron capacity – increases with higher shells
cause e- can spread out more
Shell #
1
2
3
4
e- capacity
2
8 18 32
Electronic Structure of Atoms
• Subshells
– s, p, d, f
•
•
•
•
•
1st shell – only s
2nd shell – s, p
3rd shell – s, p, d
4th shell – s, p, d, f
Note: shell has the # of subshells = the shell #
• 3s electron – an electron is found in the s
subshell of the 3rd shell
Electronic Structure of Atoms
• Orbitals – Subshells can further be divided into orbitals
– regions of space within an atom where the specific electrons are
more likely to be found.
– Different orbitals have different shapes. Orbitals in s subshells
are spherical (a), while orbitals in p subshells are roughly
dumbbell shaped (b).
Electronic Structure of Atoms
• 2 electrons in each orbital with opposite spins
Electronic Structure of Atoms
•The overall electron distribution within an atom is
summarized in Table 3.2 below.
Electron Configurations
• Electron Configuration: The exact
arrangement of electrons in atom’s shells and
subshells. Rules to predict electron
configuration:
1. Electrons occupy the lowest energy orbitals
available first.
2. Each orbital can hold only two electrons, which
must be of opposite spin.
3. If two or more orbitals have the same energy,
each orbital gets one electron before any orbital
gets two.
Electron Configurations
•Order of orbital energy
levels:
►Electrons fill orbitals from
the lowest-energy orbitals
upward.
► Lower numbered shells fill
before higher numbered
shells at first.
►Some overlap in energy
levels occurs starting with
shell 3 and 4.
Electron Configurations
►Electron configurations are described by
► write the shell number
► subshell letter in order of increasing energy.
► number of electrons actually occupying each subshell is
indicated by a superscript.
►A graphic representation can be made by indicating
each orbital as a line and each electron as an arrow. The
head of the arrow indicates the electron spin.
►A shorthand using noble gas configurations is very
useful for large atoms.
Periodic Table and Electron
Configurations
Electron Configurations
Electron Configurations
Problem
• Select the correct electron configuration
for sulfur (Z = 16).
– A.
– B.
– C.
– D.
– E.
1s21p62s22p6
1s22s22p83s23p4
1s22s22p83s23p2
1s22s22p63s23p4
1s22s22p63s23d4
Problem
• Select the correct electron configuration
for Te (Z = 52).
– A.
– B.
– C.
– D.
– E.
[Kr]5s25p64d8
[Kr]5s25d105p4
[Kr]5s24d105p6
[Kr]5s24f14
[Kr]5s24d105p4
Problem
• The electronic structure
1s22s22p63s23p64s23d8 refers to the
ground state of
– A.
– B.
– C.
– D.
– E.
Kr
Ni
Fe
Pd
none of these choices is correct
Electron Configuration
• Allows determination of valence e- and core e• Indicates why elements of a group have similar
reactivity
• Determine which element is present by the sum
of the superscripts
– Ground state atom
– Excited state atom
Optional Homework
• Text – 3.31, 3.32, 3.33, 3.44, 3.46, 3.48,
3.50, 3.52, 3.56, 3.70, 3.72, 3.74, 3.76,
3.78, 3.82, 3.84, 3.86, 3.96, 3.100, 3.106,
3.108, 3.110, 3.111
• Chapter 3 Homework – from website