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Chapter 2 - Atoms & Elements
(Chapter 2 in 1st Edition)
Consider pouring some sugar in water. How would you
describe this action?
A Chemist says: Sugar is being dissolved in water
One chemical mixing with another
chemical
One compound getting near to another
compound
Chemists write in symbolic language:
Chemists envision:
Molecules bump into each other
Atoms in the molecules vibrating
Chemists draw water as:
or
-letter abbreviations for
Chemical symbols are
the names of the elements. Water is made up of two
elements: hydrogen (H) and oxygen (O).
Chemists need to know:
How many elements are there?
What are they?
How do they behave?
Are they similar or different?
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Periodic Table
Dmitri Mendeleev (late 1800's) outlined first modern
Periodic Table.
88 elements occur in nature
115 elements are currently known
Gases (11)
Helium, Neon, Argon, Krypton, Xenon, Radon
He
Ne Ar
Kr
Xe
Rn (noble gases)
, and Oxygen,
, are the main
Nitrogen,
components of air(78% and 21% respectively). The other
1% is from other gases.
A formula is the symbol for a stable compound.
Fluorine (symbol
Chlorine (symbol
Hydrogen (symbol
the presence of O2)
Liquids (2)
Bromine (symbol
Mercury (symbol
Remainder are Solids
, formula
, formula
, formula
, highly reactive)
, reactive)
, explosive in
, formula
, formula
, reactive)
, poisonous)
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Organization:
Representative and Transition elements
Groups and Periods
Group number and period numbers
Metals versus Non-metals versus Metalloids
Human body composition (by mass):
Oxygen (
, 65%)
, 18.3%)
Carbon (
, 10%)
Hydrogen (
Nitrogen (
, 3.3%)
, 1.7%)
Calcium (
, 1%)
Phosphorous (
many elements are essential in trace quantities for
maintaining proper health
e.g.
iodine (
, need ca. 50 mg/year)
,
, need ca. 1 g/year)
copper & fluorine (
,
need ca. 5 g/year)
iron & zinc (
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Atoms
What do we know about the composition of elements?
Greeks Philosophers believed that matter must contain small
particles called atomos.
John Dalton developed the first atomic theory in 1808.
Postulates of Atomic Theory:
1.
All matter is made up of tiny particles called atoms.
2.
All atoms of a given element are similar to one another
but different from atoms of other elements.
3.
Atoms of two or more elements combine to form
compounds. The composition of a particular compound
always has same kinds of atoms in the same ratios.
4.
A chemical reaction involves rearrangement, separation
or combination of atoms. Atoms are never created or
destroyed during a chemical reaction.
eg. ___H2 + O2 → ___H2O
What’s wrong with the above equation?
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Equations must always be
What is inside an atom?
.
subatomic particles: protons (p or p+, +)
electrons (e-, -)
neutrons (n or n0, 0)
Where are the particles inside the atom?
From the famous 1911 gold foil experiment, Ernest
Rutherford proposed that the mass of the atom was made
up of a small, dense region at the centre of the atom. This
region is called the nucleus. The rest of the atom is
essentially empty space which is occupied by the electrons.
IMPORTANT:
The # protons defines the element.
If the # protons changes, then it is not the same element.
eg. The carbon atom has 6 protons in the nucleus. If you
remove 1 proton from the carbon nucleus, you change the
nature of the element.
C - p → B
if you add 1 proton to the carbon nucleus you get nitrogen.
C + p → N
These are nuclear reactions. It takes a nuclear reaction to
change an element.
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Atomic Number and Mass Number
mass number
element
atomic number
12
6
C
Atomic number = # protons
Mass number = # protons + # neutrons
If an atom is neutrally charged, then
# electrons = # protons
NOTE: different periodic tables use different formats.
Mass number is not typically shown.
Questions:
Give the number of protons, neutrons and electrons for
each neutral atom?
1
1
16
8
H
O
4
2
56
26
He
Fe
24
12
Mg
197
79
Au
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Atoms of an element can be different if the number of
neutrons they have differ. This will result in different mass
numbers.
eg.
1
1
12
6
H
C
2
1
13
6
H
C
3
1
14
6
H
C
These are called isotopes of the element.
All isotopes coexist in the stable form of the element.
Under normal conditions, it is OK to treat the element as
an “averaged atom” with an “averaged mass”.
Atomic Mass
(as shown on the periodic table)
= Weighted average of all naturally occurring isotopes
Example:
chlorine has 2 isotopes
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35
17
Cl
75.8 %
34.97 amu
The average mass of Cl is:
37
17
Cl
24.2 %
36.97 amu
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The Electrons
“electrons = chemistry”
Electrons are the particles that interact with each other to
forms bonds.
When the interaction is broken, so is the bond between
atoms.
Electrons are constantly moving through the space
surrounding the nucleus and therefore must have
Do all electrons have the same energy?
.
What are the consequences of electrons having different
energies?
Electron Energy Levels
Electrons are arranged in specific energy levels called
shells.
The energy levels are numbered by “n”, n = 1, 2, 3, ...7.
As the energy levels increase so does the energy of the
electrons.
.
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Within each shell, electrons are grouped into subshells.
The different subshells are labelled s, p, d, and f.
The number of subshells is determined by the shell
number. Thus:
Shell
Number
n=4
n=3
n= 2
n=1
Number of
subshells
Types of
subshells
Within each subshell are orbitals. Orbitals are three
dimensional regions of space around the nucleus where the
electrons are most likely to be found.
2 IMPORTANT POINTS REGARDING ORBITALS:
1. The shapes of the orbitals represent electron density and
NOT the pathway the electron follows.
2. Orbitals may only hold up to two electrons.
The s subshell contains 1 s-orbital. It is spherical in shape.
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The p subshell contains 3 p-orbitals. They are dumbbell
shaped.
z
z
z
y
y
y
x
x
x
The d subshell contains 5 d-orbitals and the f subshell
contains 7 f-orbitals. You don’t need to know their shapes
right now.
Within a subshell, the order of increasing energy is:
5s
5p
5d
5f
4s
4p
4d
4f
3s
3p
3d
2s
2p
1s
5g
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Electron Configuration
The electron configuration of an atom describes the
organization of its electrons. Electrons are always
arranged starting with the lowest energy subshell, followed
by the next highest subshell and so on. In the Electron
Configuration notation, the number of electrons in a
subshell is written as a superscript.
e.g. hydrogen atom (
Number of e =
)?
, E. C. =
carbon atom (
Number of e =
)?
, E. C. =
chlorine atom (
Number of e =
)?
, E. C. =
Remember; always start with the lowest energy subshell
and only a maximum of two electrons in each orbital. The
subshells of lower numbers are the inner shells and the
subshells with the highest number represent the outermost
shell.
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Shorthand Notation.
What is the E.C. for Ar?
What is the E.C. for Br?
Therefore, we can write the Noble Gas notation for Br as:
Periodic Law
Elements in the vertical columns or groups of the periodic
table exhibit similar physical and chemical properties. As
we go from one group to the next there is also a regular
change in physical and chemical properties. This pattern
of change is known as the periodic law. There is also a
similarity in the electron arrangements for elements within
the same group.
What is the E.C. for:
H
Li
Na
K
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Therefore, among elements within a group, their similar
chemical and physical properties can be attributed to them
having the same number of electrons in the outermost shell.
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Important Concepts from Chapter 2
• Element name and symbols
• Periodic Table Organization (groups and periods)
• Atomic Number and Mass Number
• Calculating Average Atomic Mass
• Isotopes
• Electron Organization – Shells, Subshells, and Orbitals
(shape and energy)
• Electron Configuration