Chapter 4

Chemical Foundations: Atoms & Elements

Chemistry 100

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.

Elements & Compounds

H: 0.9%Ti: 0.6%Others: 0.9%

Element abundance on earth

Almost half of the mass is oxygen.

O2

20% of atmosphere.

O - O

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.

Dalton’s Atomic theory

Law of constant composition: Compound always has the same composition.

Compound always has the same properties.

HO

2= 1

or 2 : 1

CO

1=

1or 1 : 1

H2O

CO

4. Atoms are not created or destroyed in chemical reactions.

A chemical reaction changes the way the atoms are grouped together.

Dalton’s Atomic theory

Law of conservation of mass:

C + O CO

O

H HH2O

Molecularformula

Structuralformula

Ball-and-stickmodel

Space-fillingmodel

Molecular models

C

H

H

H

HCH4

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

C12

6

Mass number (A)

Atomic number (Z)

Isotopes

Isotopes: atoms with the same number of protons and electrons butdifferent numbers of neutrons.

different mass number

C12

6 C13

6 C14

6

6 P + 6 N 6 P + 7 N 6 P + 8 N

Almost the same properties

Carbon-12 Carbon-13 Carbon-14

Atomic Weight

Atomic weight: of an element is average of the masses (in amu) of its isotopes found on the Earth.

Cl Cl35

17Cl37

1734.97 amu 36.97 amu

(75.77/100 × 34.97 amu) + (24.23/100 × 36.97 amu) = 35.45 amu

Cl17

35.45

Atomic number

Atomic weight

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-Frtoo reactive, unstable, solid metal

2Na + 2H2O 2NaOH + H2

2Na + Cl2 2NaCl

Group 2A: Alkaline metals or earth metals

Be-Mg-Ca-Sr-Ba-Rareactive, solid metal

Group 7A: Halogens

F-Cl-Br-I-Atreactive, colored, gas, nonmetal

Group 8A: Noble gases

He-Ne-Ar-Kr-Xe-Rnnon reactive, stable, gas, nonmetal

nonmetals

metals

metalloids

Classification of the elements

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

Noble metals: unrecative metals

Most elements are reactive and they cannot be found in nature in free form.They occur in compounds.

Ag, Au, Pt

Monatomic

Ar He

Diatomic

N2 O2

Polyatomic

S8Noble gases

Allotropes

Different forms of an element

Carbon:

Diamond Graphite Buckminsterfullerene & Nanotube

More metallic

Mo

re m

etal

lic

Metallic properties

Ions

Total number of protons = Total number of electrons

Atom is neutral (zero net charge).

Na → Na+ + e-

Cl + e- → Cl-

Ion: atom with any charges (positive or negative).

Cation

Anion

Number of protons and neutrons in the nucleus remains unchanged.

Na + energy → Na+ + e-

ion

Ions & Ionization

Na+

+ e-

Na

11+

11-10-

11+

The size of the cation is smaller than the neutral atom.

Cl + e- → Cl-

ion

Ions & Ionization

Cl-

+ e-

Cl

17- 18-

17+17+

Problem 1

Determine the charge of each ion:

a)a magnesium ion with 10 electronsb)A sulfur ion with 18 electrons

Ion charge = #P+ – #e-

a) 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: the energy required to remove the most loosely held electron from an atom in the gaseous state.

Ionization energy

Ion

izat

ion

en

erg

y

Ionization Energy

Metals: lose 1, 2 or 3 e- Cation (Y+)

Nonmetals: gain 1, 2 or 3 e- Anion (X-)

Ions

Transition elements

1A 2A 3A 4A 5A 6A 7A 8A

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.