Atoms and the Periodic Table
Chapter Three
Subatomic Particles
Atoms are composed of subatomic particles
Particle
Symbol
Mass (g)
Mass (amu)
Charge
Proton
p
1.672622 x 10-24
1.007276
+1
Neutron
n
1.674927 x 10-24
1.008665
0
Electron
e
9.109328 x 10-28
5.485799 x 10-4
-1
The Structure of an Atom
Attractive and Repulsive Forces in
Atomic Structure
Structure of the Atom
• Protons and neutrons reside in a small, dense
nucleus.
• Electrons are spread out in space outside the
nucleus. Most of the atom is empty space.
• Atoms are neutral overall, i.e., no net charge
• Atomic Number (Z) - The number of protons in
the nucleus of an atom.
– Determines the type of atom (which element it is)
– Also number of electrons in a neutral atom
• Mass Number (A) - The sum of the protons plus
neutrons in the nucleus of an atom.
• Therefore, # of neutrons = A - Z
Example Problem 3.2
• If Phosphorus has a mass number of A = 31, how many
protons, neutrons and electrons are there?
Atoms and Ions
• An atom is electrically neutral.
#e = #p
#e = Z
• Ion - An atom or other chemical species that
carries an overall positive or negative charge.
– Monatomic ion - a single atom with a positive or negative
charge
– Cation - a positive ion
(#e < #p)
– Anion - a negative ion
(#e > #p)
Isotopes
• Isotopes- Atoms of the same element that have different
mass numbers
Same number of protons
Different number of neutrons
How to Indicate Isotopes
• Nuclide - A nucleus with a particular mass
number
• Symbols of Nuclides
– Write the symbol (Sy) of the element.
– Put the atomic number (Z) at the lower left.
– Put the mass number (A) at the upper left.
A
Z
Sy
Sample Isotope Problem
12C
protons
neutrons
electrons
amu
13C
14C
Atomic Weight (Mass)
• Atomic Weight (Mass) - The average mass of all
of the isotopes of an element that occur in nature.
• Carbon-12 is assigned a mass of exactly 12 amu.
– Standard
At. Wt. = fract. abund1 x mass1 +
fract. abund2 x mass2 +
fract. abund3 x mass3 + ….
Atomic Weight of Magnesium
Isotope
Approximate Mass
(amu)
Abundance
24Mg
23.99
78.99%
25Mg
24.99
10.00%
26Mg
25.93
11.01%
The Periodic Table
• Periodic Table - A chart in which elements are
arranged in order of increasing atomic number
such that elements having similar chemical
properties lie beneath one another.
• The atomic number (Z), the symbol, and the
atomic mass of each element is given.
The Periodic Table
• Groups(Families) - The columns in the table
• Periods - The rows in the table
• Groups can be designated by numbers or names
• Representative Elements (Main Group Elements)
– Elements of groups 1,2 and 13-18 (old 1A, 2A and 3A - 8A)
– Alkali metals (1A), Alkaline earth metals (2A), Halogens (7A),
Noble gases (8A)
• Transition metals
– Elements of groups 3-12 (old 3B - 8B and 1B & 2B)
The Periodic Table
The Periodic Table
• Atomic Radius versus Atomic Number
Periodic Properties
• Atomic Size
– Decreases in going across a period
– Increases in going down a group
• Ionization Energy
– Energy required to remove an electron from an atom
– Increases in going across a period
– Decreases in going down a group
• Electron Affinity
– Energy released when an electron is added to an atom
– Increases in going across a period
– Generally decreases in going down a group (some exceptions)
Properties of Electrons
• All moving objects have properties of both
particles and waves
- If the particle is large, e.g., a soccer ball, the associated wave is not a
measurable property of the object
• Electrons are sufficiently small that they can be
described as both particles and waves
- wave-particle duality
• Current theories of the electronic structure of
atoms are based on the wave properties of the
electron
Quantum Mechanical Model of Atomic
Structure
• Electrons are not perfectly free to move about in an atom
• The energy of the electron is quantized
• Quantized- only certain specific values are permitted
-
Steps versus ramp analogy
• These values of energy are called energy levels
Electrons prefer to reside in the lowest possible energy level
• Electrons in an atom that are in the same energy level are
at about the same average distance from the nucleus
Electrons that are closer to the nucleus have lower
energies because they are attracted mores strongly by
the nucleus
Location of An Electron
• Shells- like layers of an onion, as you progress from the
center the larger it is, thus it can hold more electrons
• Subshells- each energy shell is further sub-divided into
energy sublevels where the number of sublevels is equal
to n which is the shell number
-
subshells (energy sublevels) are designated by letters
-
subshells increase in energy in the order of:
s<p<d<f
• Oribtals- regions in space within an atom where specific
electrons are most likely to be found
Orbitals
• The number of orbitals in a subshell increases
-
s subshells are composed of 1 orbital
p subshells are composed of 3 orbitals
d subshells are composed of 5 orbitals
f subshells are composed of 7 orbitals
• Each orbital can contain a maximum of 2
electrons of opposite spin
-
s subshells contain 2 electrons
p subshells contain 6 electrons
d subshells contain 10 electrons
f subshells contain 14 electrons
Shapes of s and p Orbitals
Summary of Electron Distribution
in Atoms
Energy Perspective of Electron
Distribution in Atoms
Electron Configurations
• Electron configurations- the exact arrangement
of electrons in the shells and subshells of atoms
• 3 Rules apply for placement of the electrons
-
Rule 1: Electrons occupy the lowest-energy orbitals
available and are filled in the order shown in Fig 3.5
Rule 2: Each orbital can hold only 2 electrons which must
be of opposite spin
Rule 3: Two or more orbitals of the same energy (i.e., p,d,f
subshells) are each half filled by 1 electron with the
same spin before any one oribtal is filled by the
second electron of opposite spin
Order of Filling Electron Orbitals
• Note that above the 3p level there is a crossover of energy
levels, thus 4s orbital must filled before 3d orbitals, 5s
orbital before 4d orbitals
Representation of Electron
Configurations
• Written
• Graphical
Electron Configuration and the
Periodic Table
• Relationship between rows (periods, shells) and columns
(groups, subshells)
• Main group elements (s or p)
• Transition group elements (d or f)
Valence Shell Electrons
• Valence electrons - electrons found in the
outermost shell (valence shell)
• Elements in the same group (column) of the
periodic table have similar electron
configurations in their valence shell
- Group 1 (1A) : ns1
- Group 2 (2a): ns2
- Group 13 (3A): ns2np1
- Group 17 (7A):ns2 np5
- Group 18 (8A): ns2np6
Where n is the shell number