Unit 3 - Elements, Atoms and Ions
1. The Elements
2. Symbols for Elements
3. Dalton’s Atomic Theory
4. Formulas of Compounds
5. Structure of the Atom
6. Modern Concept of Atomic Structure
7. Isotopes
8. Introduction to the Periodic Table
9. Natural States of Elements
10. Ions
11. Compounds That Contain Ions
pages 82-113
1
Unit 3 - Elements, Atoms and Ions
Upon completion of this unit, you should be able to do the
following:
1.
2.
3.
4.
5.
6.
7.
8.
Determine the number of protons, neutrons and electrons in
an atom based on the element name or the atomic number
and atomic mass.
Write the symbols of common elements and write the
names of common elements given their symbols.
Differentiate between a chemical symbol and a chemical
formula.
Explain the significance of subscripts
.
Given a formula, state the number of atoms of each element
present.
Use the periodic table to predict the charge and formula of a
monatomic ion.
Use the periodic table to identify metals and non-metals.
Use the periodic table to identify alkali metals, alkaline earth
metals, halogens and noble gases.
2
The Elements
Element – a substance that cannot be broken
down into other substances by chemical methods.
• Sometimes chemists use the word element
is used to mean a single atom of that
element. This is the microscopic form.
• Other times the word element is used to
mean a sample large enough to weigh on a
balance. This is the macroscopic form.
• The term element is also used in generic
form. When we say the human body
contains sodium, that does not mean
elemental sodium is present, rather atoms
of some form of sodium are present.
3
The Elements
Currently there are 118 elements, 88 of which occur
naturally. The 9 most abundant elements listed below
account for 98% of the mass of the earth’s crust.
Element
Oxygen (O)
Silicon (Si)
Aluminum (Al)
Iron (Fe)
Calcium (Ca)
Sodium (Na)
Potassium (K)
Magnesium (Mg)
Hydrogen (H)
Mass Percent
49.2
25.7
7.50
4.71
3.39
2.63
2.40
1.93
0.87
4
Symbols for Elements
• Symbols are used as abbreviations for element
names
• One or two letters in symbol
• First letter is upper case, second letter is lower case
• Origin of some symbols come from Greek or Latin
name
• The symbol for lead is Pb from the Latin word for
heavy, plumbum.
• The name for bromine comes from a Greek word
meaning stench.
• Table 4.3 on page 87 lists 44 common elements
with their symbols – memorize them
5
The Elements Song
The Elements Song by
Tom Lehrer – Harvard scientist and musician
Background – song set to the tune of
“The Very Model of a Modern Major-General”
from The Pirates of the Penzance
by Gilbert and Sullivan
Daniel Radcliffe version
Karaoke - Your Turn
6
Dalton’s Atomic Theory
1. Most natural materials are mixtures of pure
substances.
2. Pure substances are either elements or
combinations of elements called compounds.
3. A given compound always contains the same
proportions (by mass) of the elements. This is
known as the law of constant composition.
For example, water always contains 8 grams of
oxygen for each gram of hydrogen.
7
Dalton’s Atomic Theory
1. Elements are made of tiny particles called atoms.
2. All atoms of a given element are identical.
3. The atoms of a given element are different from
those of any other element.
4. Atoms of one element can combine with atoms
of other elements to form compounds. A given
compound always has the same relative
numbers and types of atoms.
5. Atoms are indivisible in chemical processes.
Atoms are not created or destroyed in chemical
reactions. The reaction only changes the way
the atoms are grouped together.
8
Formulas of Compounds
Compound – a distinct substance that is composed
of the atoms of two or more elements and always
contains exactly the same relative masses of those
elements. A compound can be broken down into
those elements by chemical methods.
Example
Water (H2O) – can be broken down into hydrogen
and oxygen. Water always contains 2 hydrogen
atoms for each oxygen atom.
9
Formulas of Compounds
A chemical formula expresses the types of atoms
and numbers of each atom in a given compound.
Water has 2 atoms of hydrogen for each atom of
oxygen.
The chemical name for table salt is sodium chloride.
The chemical formula for sodium chloride is NaCl. It
contains 1 atom of sodium and 1 atom of chlorine.
10
Formulas of Compounds
Rules for writing formulas
1. Each atom present is represented by its element
symbol.
2. The number of each type of atom is indicated by a
subscript written to the right of the element symbol.
3. When only 1 atom of a given type is present, the
subscript 1 is not written.
Page 90, example 4.1
Self check exercise 4.1
11
Structure of the Atom
Scientists believed that elements consisted of
atoms and that compounds are a specific
collection of atoms bound together in some way.
But what is an atom like? Does it consist of
smaller parts, and if so, what are the subatomic
particles?
Video 1 - Atoms and the Periodic Table
Video 2 - J.J. Thomson and the electron
Video 3 - Ernest Rutherford and the nucleus
12
Structure of the Atom
13
Structure of the Atom
Atoms consist of:
Electron – a negatively charged particle
Proton – a positively charged particle, same size of
charge as an electron, but positive
Neutron – a neutral particle with a mass relatively
close to that of proton
Protons and neutrons are located in a small dense
center called the nucleus. Most of the mass of an
atom is due to the nucleus.
Electrons are located outside of the nucleus in a
negatively charged “cloud”. Most of the volume of
an atom is due to the electrons.
14
Modern View of Atomic Structure
If all atoms are composed of the same components,
why do different atoms have different chemical
properties?
The answer lies in the number and arrangement of
the electrons. The space in which the electrons
move accounts for most of the atomic volume. The
electrons are the parts of the atom that intermingle
when atoms combine to form molecules. The
number of electrons a given atom possesses greatly
affects the way it can interact with other atoms.
15
Modern View of Atomic Structure
Electron cloud
1 Angstrom = 1 x 10-10 meters
16
Isotopes
• The identity of an element is determined by the
number of protons it has. The number of protons is
called the atomic number.
• Isotopes of an element have the same number of
protons, and therefore the same atomic number, but
different numbers of neutrons.
• The total number of protons and neutrons in an atom
is called the mass number.
• The mass number is used in naming to identify
isotopes, such as Carbon-14, which has a mass
number of 14. Carbon-12 has a mass number of 12.
17
Isotopes
A, mass number =
Number of protons
and neutrons
Z, atomic number =
Number of protons
Ag
108
47
X, element
symbol
Silver
18
Isotopes
Iodine - 127; 53 protons, 74 neutrons
Iodine-123 (half-life 13 hours) is the isotope of choice
for nuclear medicine imaging of the thyroid gland,
which naturally accumulates all iodine isotopes
HW - Page 117-118
Problems – 38 and 42
19
20
Metals
Good conductors of heat and electricity
Shiny (have luster)
Malleable - can be hammered into sheets
Ductile - can be drawn into wires
21
Nonmetals
Good insulators (absorb electricity)
Dull appearance
Most are gases or easily vaporized solids
and liquids
Solids are brittle
22
Metalloids
Exhibits some properties of both metals and nonmetals.
23
Groups
All elements in a group have similar chemical properties.
Group 1 Alkali Metals
Group 2 Alkaline Earth Metals
Groups 3-12 Transition Metals
Group 17 Halogens
Group 18 Noble Gases
24
Natural State of Elements
Gas
Liquid
Solid
25
Diatomic Molecules
Mnemonic to help remember - H2, I2, N2, Cl2, Br2, O2, F2 HINClBrOF
Or 7 from 7 + H
26
Ions
An atom has a certain number of protons in its
nucleus and an equal number of electrons in the
space around the nucleus, giving the atom a net
zero charge.
Adding or removing an electron from a neutral
atom will create a charged ion.
Ions are always formed by adding or removing
electrons, not by changing the number of protons.
27
Ions
A positive ion, called a cation, is produced when an atom
loses one or more electrons.
Na → Na+ + eA cation is named using the name of the parent atom.
Na+ is called the sodium ion.
28
Ions
A negative ion, called an anion, is produced when an atom
gains one or more electrons.
Cl + eClAn anion is named by taking the root name of the atom and
changing the ending. Cl- is called the chloride ion.
29
Ions
Similarly, other Group 17 nonmetals gain one
electron to form anions. The fluorine anion is called
fluoride, the bromine anion is called bromide and the
iodine anion is called iodide.
Oxygen and sulfur add two electrons to form anions.
The oxygen anion is called oxide and the sulfur anion
is called sulfide.
30
Ions
Atoms do not form ions on their own. Most
commonly, ions are formed when metallic elements
combine with nonmetallic elements.
When metals and nonmetals react, the metal atoms
tend to lose one or more electrons, which are gained
by the atoms of the nonmetals.
Reactions between metals and nonmetals tend to
form compounds that contain metal cations and
nonmetal anions.
31
Ions
Group 1 metals form ions with a +1 charge
Group 2 metals form ions with a +2 charge
Group 13 metals form ions with a +3 charge
Many Group 3-12 metals form ions with multiple charges
Group 17 atoms form ions with a -1 charge
Group 16 atoms form ions with a -2 charge
Metals always form cations.
The tendency to lose electrons is a fundamental
characteristic of metals.
32
Compounds that contain Ions
Sodium Chloride example
NaCl is a solid and does not conduct electricity
Pure (distilled) water does not conduct electricity
NaCl dissolved in water does conduct electricity
Why?
33
Compounds that contain Ions
Electric current can travel when electrons are free to
move.
In ionic substances, ions carry the current
Substances that contain ions can conduct an electric
current only if the ions can move.
Distilled water contains no ions.
When NaCl is dissolved in water it ionizes and the
electrons are free to move.
34
Compounds that contain Ions
• Whenever a compound is formed between a metal
and non-metal, it can be expected to contain ions.
• These substances are called ionic compounds. A
chemical compound must have a net charge of zero.
• There must be positive ions (cations) and negative
ions (anions) present.
• The number of cations and anions must result in a
net charge of zero.
• When writing the formula for a compound, the
cation is always written first.
• Use subscripts to balance charges on compounds.
35
Compounds that contain Ions
Examples:
Practice
Writing Formulas
Na+ + Cl- → NaCl
Mg 2+ + Cl- → MgCl2
Al 3+ + Cl- → AlCl3
Li + + N 3- → Li3N
Ca 2+ + Cl- →
Na + + S2- →
Ca 2+ + P 3- →
Homework
Page 119 – problems 76, 78, 84
36