chm 101 – chapter two

CHM 101 - ReevesDepartment of Chemistry and Biochemistry

CHM 101 – Chapter Two

1. Dalton’s Atomic Theory2. Discovery of atomic structure3. Modern view of atomic theory4. Atomic Weights5. The Periodic Table6. Molecules and Molecular Compounds7. lons and ionic compounds8. Naming inorganic compounds


CHM 101 – Atomic TheoryBetween 1803 and 1807, John Dalton proposed his Atomic Theory to explain experimental observations about gases and chemical reactions occurring in the gas phase.

• Each element is composed of extremely small particles called atoms.

• All atoms of a given element are identical; atoms of different elements are different and have different properties.


CHM 101 – Atomic TheoryBetween 1803 and 1807, John Dalton proposed his Atomic Theory to explain experimental observations about gases and chemical reactions occurring in the gas phase.

•Atoms are not changed into different atoms; atoms are neither created nor destroyed.

•Compounds are formed when atoms of at least two different elements combine; a given compound always has the same kind and ratio of atoms.


CHM 101 – Experimental EvidenceIn the late 19th and early 20th century, experimental evidence began to accumulate that contradicted some of Dalton's basic ideas


CHM 101 – Modern View of the Atom

The experiments of the early 20th century dispelled the myth of the indestructible atom. Instead, we now believe that the atom is itself composed of simpler particles. It is convenient for chemists to focus on protons, neutrons and electrons.


CHM 101 – Atomic Number & Mass

Atoms are identified by the atomic number (Z), which is the number of protons ( ) in the nucleus. The atomic mass is,approximately, the sum of the atom's protons and the neutrons ( ). For example, all Mg atoms have 12 protons.


CHM 101 – Atomic Number & MassBecause atoms are electrically neutral, the number of electrons = the number of proton. Ions are atoms that have more (anions) or less (cations) electrons than protons.

Symbol Protons Neutons Electrons Name

K39

537453

356Fe


• The atomic mass unit is defined by assigning the mass of one atom of is exactly 12 amu. This leads to the relationship

C12

CHM 101 – Average Atomic Mass

gxamu 241066.11

• However, the mass of carbon in the Periodic Table is 12.01 amu

6

C1 2 .0 1


Although carbon-12 is its most common isotope (98.93%), carbon also contains 1.03% of the isotope C-13, with an atomic mass of 13.00335 amu. Thus, the average atomic weight of Carbon is:

CHM 101 – Average Atomic Mass6

C1 2 .0 1


Copper has two important isotopes, 63Cu and 65Cu. Based on the atomic weight reported in the Periodic Table, what are the approximate abundances of the two isotopes?

CHM 101 – Average Atomic Mass29

Cu6 3 .5 6 4

90% 63Cu10% 65Cu

75% 63Cu25% 65Cu

50% 63Cu50% 65Cu

25% 63Cu75% 65Cu


CHM 101 – The Periodic TableWhen the elements are arranged in the order of increasing atomic number, their properties are observed to vary in a repeating or periodic pattern.

The Periodic Table arranges atoms in order of increasing atomic number such that elements with similar chemical and physical properties appear in the same column (group) of the table.


CHM 101 – The Periodic Table


CHM 101 Molecules and Molecular Compounds

• Most atoms are metals at room temperature. Metallic character increases as you go to the left and down the Table. Nonmetals are concentrated to the right and top of the Table. Metalloids are between metals and nonmetals.

• Nonmetals combine together to form molecules by sharing some of their valence electrons. Seven nonmetal elements are most commonly found in nature as diatomic molecules.

• When two or more different nonmetals combine, a molecular compound is formed.



Compounds are represented by a variety of formulae that present different types of information:

• Space-filling model: represents the molecule's actual spatial arrangement of atoms

• Structural formula: shows how atoms are connected, using symbols to represent atoms.



• Molecular formula: provides number and symbol for each atom in the compound

• Empirical formula: provides smallest whole-number ratio of atoms in the compound.

• Binary molecular compounds are named using a prefix before each atom to indicate how many there are in the molecule. (cf pg 66 & Table 2.6)



Compounds are represented by a variety of formulae that present different types of information:

N NO

OO

ON 2O 4 N O 2

SO

OO SO 3 SO 3

Space-Filling Model

Nam eStructural Formula

Molecular Formula

Em pirical Formula

Sulfur trioxide

Dinitrogen tetraoxide


CHM 101 Ions and Ionic Compounds

Positive ions (cations) form when atoms loose electrons.



• A-group metals form only one kind of cation, consistent with their position in the Periodic Table. Cations have the same name as the atoms from which they are derived.



Cations form when atoms loose electrons. • A-group metals form only one kind of cation, consistent with

their position in the Periodic Table. Cations have the same name as the atoms from which they are derived.

• B-group (transition) metals can form two or more different ions. To name them, a Roman Numeral following the symbol is used to indicate the charge on the ion.

Cr3+:

Fe2+:

Ag+:



Negative ions (anions) form when atoms gain electrons. Nonmetals consistently form anions.



A-group elements form only one kind of monatomic ion, consistent with their position in the Periodic Table. They are named by replacing the last syllable with "ide"



Anions may also be polyatomic, consisting of two or more atoms covalently bonded together, with one or more extra electrons.

• Some common polyatomic anions have names ending in “ide”

• Oxyanions are polyatomic ions that contain one or more oxygen atoms. Their names end in “ite” or “ate”.

• See rules on p. 62 regarding naming conventions. Memorize common ions in Tables 2.4 & 2.5



• All stable materials are electrically neutral.• Ionic compounds are formed through the attraction of anions and

cations and are named by combining the cation and anion names

sodium chloride



To be electrically neutral, the ions are combined so that the total positive charge = total negative charge

Cation Anion Name Formula

chm 101 – chapter two

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