Chemistry 100
Chapter 4
Chemical Foundations: Atoms & Elements
Elements & Compounds
Element: is a substance consists of identical atoms.
Cannot be divided by chemical & physical methods.
Carbon, Hydrogen, Oxygen
116 elements – 88 in nature
Millions of compounds are composed of these 116 elements.
Element abundance on earth
H: 0.9%
Ti: 0.6%
Others: 0.9%
O-O
O2
Almost half of the mass is oxygen.
20% of atmosphere.
Dalton’s Atomic theory
1. All matter is made up of very tiny, indivisible particles (atoms).
2. All atoms of a given element have the same chemical properties.
3. Compounds are made up of two or more different kinds of atoms.
A compound has the same relative numbers and types of atoms.
Law of constant composition: Compound always has the same composition.
H2O
H
O
CO
C
O
=
=
2
1
1
1
or
2:1
or
1:1
Compound always has the same properties.
Dalton’s Atomic theory
4. Atoms are not created or destroyed in chemical reactions.
A chemical reaction changes the way the atoms are grouped together.
Law of conservation of mass:
C
+
O

CO
Molecular models
O
H 2O
H
Molecular
formula
H
Structural
formula
H
CH4
H
C H
H
Ball-and-stick
model
Space-filling
model
Atom
+
-
-
- -
-
-
-
(Source of  particles)
Atom
Nucleus: positive charge
Atoms are neutral.
Atomic mass unit (amu) = 1.6605×10-24 g
mass of proton = 1 amu
mass of neutron = 1 amu
mass of electron = 5.48×10-4 amu
Atom
Mass number:
Protons + Neutrons
Atomic number:
Protons
Mass number (A)
12
Atomic number (Z)
6
C
Isotopes
Isotopes: atoms with the same number of protons and electrons but
different numbers of neutrons.
different mass number
12
6
C
13
6
C
14
6
C
6P+6N
6P+7N
6P+8N
Carbon-12
Carbon-13
Carbon-14
Almost the same properties
Atomic Weight
Atomic weight: of an element is average of the masses (in amu)
of its isotopes found on the Earth.
Cl
35
Cl
17
37
Cl
17
34.97 amu
36.97 amu
(75.77/100 × 34.97 amu) + (24.23/100 × 36.97 amu) = 35.45 amu
Atomic number
Atomic weight
17
Cl
35.45
main-group elements: 1A to 8A
transition elements: 1B to 8B (3 – 12)
inner transition elements: between B3 & B4
(58 to 71 and 90 to 103)
Column: the same properties (main group)
Row or Period (7 rows)
Group 1A: Alkali metals
Li-Na-K-Rb-Cs-Fr
too reactive, unstable, solid metal
2Na + 2H2O
2Na + Cl2
2NaOH + H2
2NaCl
Group 2A: Alkaline metals or earth metals
Be-Mg-Ca-Sr-Ba-Ra
reactive, solid metal
Group 7A: Halogens
F-Cl-Br-I-At
reactive, colored, gas, nonmetal
Group 8A: Noble gases
He-Ne-Ar-Kr-Xe-Rn
non reactive, stable, gas, nonmetal
Classification of the elements
metals
nonmetals
metalloids
Metals:
solid (except mercury), shiny, conductors of electricity and heat, ductile, malleable
Nonmetals: solid, liquid or gas, do not conduct electricity (except graphite)
Metalloids (Semimetals): between metals and nonmetals
Most elements are reactive and they cannot be found in nature in free form.
They occur in compounds.
Noble metals: unrecative metals
Ag, Au, Pt
Monatomic
Ar
He
Noble gases
Diatomic
N2
Polyatomic
O2
S8
Allotropes
Different forms of an element
Carbon:
Diamond
Graphite
Buckminsterfullerene
& Nanotube
More metallic
Metallic properties
More metallic
Ions
Total number of protons = Total number of electrons
Atom is neutral (zero net charge).
Ion: atom with any charges (positive or negative).
Na → Na+ + e-
Cation
Cl + e- → Cl-
Anion
Number of protons and neutrons in the nucleus remains unchanged.
Ions & Ionization
Na + energy → Na+ + eion
1110-
11+
Na
11+
+ e-
Na+
The size of the cation is smaller than the neutral atom.
Ions & Ionization
Cl + e- → Clion
17-
18-
17+
Cl
+ e-
17+
Cl-
Problem 1
Determine the charge of each ion:
a) a magnesium ion with 10 electrons
b) A sulfur ion with 18 electrons
Ion charge = #P+ – #ea) Magnesium’s atomic number is 12.
Ion charge = 12 – 10 = 2+
(Mg2+)
b) Sulfur’s atomic number is 16.
Ion charge = 16 – 18 = 2-
(S2-)
Problem 2
Determine the number of protons and electrons in the Ca2+ ion:
Ion charge = #P+ – #e-
Calcium’s atomic number is 20.
2+ = 20 – #e#e- = 20 – 2 = 18
Ionization Energy
Ionization energy: the energy required to remove the most
Ionization energy
loosely held electron from an atom in the gaseous state.
Ionization energy
Metals: lose 1, 2 or 3 e-
Cation (Y+)
Ions
Nonmetals: gain 1, 2 or 3 e-
1A 2A
Anion (X-)
3A 4A 5A 6A 7A 8A
Transition elements
Ionic Compounds
• Ionic compounds are salts containing cations and anions.
• The positive charge of the cation(s) must balance the negative
charge of the anion(s) to form a neutral compound.