Atoms, Molecules, and Ions

OpenStax Chemistry, 2nd edition

Chapter 2: Atoms, Molecules, and Ions

Learning Objectives

2.1 Early Ideas in Atomic Theory

·  State the postulates of Dalton’s atomic theory 

·  Use postulates of Dalton’s atomic theory to explain the laws of definite and multiple proportions

2.2 Evolution of Atomic Theory

·  Outline milestones in the development of modern atomic theory

·  Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford

·  Describe the three subatomic particles that compose atoms

·  Define isotopes and give examples for several elements

2.3 Atomic Structure and Symbolism

·  Write and interpret symbols that depict the atomic number, mass number, and charge of an atom or ion

·  Define the atomic mass unit and average atomic mass

·  Calculate average atomic mass and isotopic abundance

2.4 Chemical Formulas

·  Symbolize the composition of molecules using molecular formulas and empirical formulas

·  Represent the bonding arrangement of atoms within molecules using structural formulas

2.5 The Periodic Table

·  State the periodic law and explain the organization of elements in the periodic table

·  Predict the general properties of elements based on their location within the periodic table

·  Identify metals, nonmetals, and metalloids by their properties and/or location on the periodic table

2.6 Molecular and Ionic Compounds

·  Define ionic and molecular (covalent) compounds

·  Predict the type of compound formed from elements based on their location within the periodic table

·  Determine formulas for simple ionic compounds

2.7 Chemical Nomenclature

·      Derive names for common types of inorganic compounds using a systematic approach

Resources

2.1 Early Ideas in Atomic Theory

·  State the postulates of Dalton’s atomic theory 

·  Use postulates of Dalton’s atomic theory to explain the laws of definite and multiple proportions

Dalton’s atomic theory can be summarized in five postulates. 

1. Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.

2. An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element. 

3. Atoms of one element differ in properties from atoms of all other elements. 

4. A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the number of atoms of each of its elements are always present in the same ratio. 

5. Atoms are neither created nor destroyed during a chemical change, but instead rearrange to yield a different type(s) of matter. 

Law of conservation of matter

If atoms are neither created nor destroyed during a chemical change, then the total mass of matter present when matter changes from one type to another will remain constant (the law of conservation of matter).

Law of definite proportions or the law of constant composition: All samples of a pure compound contain the same elements in the same proportion by mass. 

The law of multiple proportions states that when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers.

2.2 Evolution of Atomic Theory

·  Outline milestones in the development of modern atomic theory

·  Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford

·  Describe the three subatomic particles that compose atoms

·  Define isotopes and give examples for several elements

Thompson (1897) - cathode ray tube - existence of the electron with negative charge with charge to mass ration

 of 1.759 × 1011 C/kg.

Millikan (1909) - oil drop experiment - determined the charge of the electron to be 1.602 × 10–19 C and mass of 9.107 × 10–31 kg. (Noble Prize in 1923.)

Rutherford (1911) - gold foil scattering experiment - established the atom as a very small (compared to the volume of the atom) positively charged nucleus surrounded by electrons.

Soddy (1913) -  Isotopes: Atoms of the same element that differ in mass. (Noble Prize in 1921.) 

Chadwick (1932) - Neutrons: Uncharged, subatomic particles with a mass approximately the same as that of protons. Neutrons are also found in the nucleus. (Noble Prize in 1935.)

Diameter of an atom ~ 10–10 m

Diameter of a nucleus is 100,000 times smaller ~ 10–15 m

* 1 amu = 1.6605 x 10-24 g

2.3 Atomic Structure and Symbolism

·  Write and interpret symbols that depict the atomic number, mass number, and charge of an atom or ion

·  Define the atomic mass unit and average atomic mass

·  Calculate average atomic mass and isotopic abundance

atomic number (Z) = number of protons

mass number (A) = number of protons + number of neutrons

charge = #protons - #electrons

atomic mass (actually average mass)

2.5 The Periodic Table

·  State the periodic law and explain the organization of elements in the periodic table

·  Predict the general properties of elements based on their location within the periodic table

·  Identify metals, nonmetals, and metalloids by their properties and/or location on the periodic table

Mendeleev (1869) started the development of the periodic table.

Periodic Law: The properties of the elements are periodic functions of their atomic numbers.

Named groups of the periodic table.

2.6 Molecular and Ionic Compounds

·  Define ionic and molecular (covalent) compounds

·  Predict the type of compound formed from elements based on their location within the periodic table

·  Determine formulas for simple ionic compounds

In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged.

Electrons participate in chemical reactions by being gained, lost, or shared.

The gain or lose of electrons, results in the formation of ions.

Ionic compounds are compounds that contains ions and are held together by ionic bonds - omni-directional electrostatic forces.  

Many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas.

    Group 1: lose one electron, form a cation with a 1+ charge

    Group 2: lose two electrons, form a cation with a 2+ charge

Many nonmetals gain enough electrons to give them the same number of electrons as an atom of the next noble gas.

    Group 17: gain one electron, form an anion with a 1– charge.

    Group 16: gain two electrons, form an anion with a 2– charge.

Polyatomic ions are electrically charged molecules (a group of bonded atoms with an overall charge).

Many ionic compounds contain polyatomic ions as the cation, the anion, or both.

    Treat polyatomic ions as discrete units. 

    Parentheses in a formula are used to indicate a group of atoms that behave as a unit.

    Example: Ca2+ and PO43– forms Ca3(PO4)2, calcium phosphate.

    Three Ca2+ ions gives six positive charges.

    Two PO43– ions gives six negative charges. 

Molecular compounds (covalent compounds) result when atoms share electrons.

Exist as discrete, neutral molecules.

Usually formed by a combination of nonmetals.

Often exist as gases, low-boiling liquids, and low-melting solids.

2.7 Chemical Nomenclature

·      Derive names for common types of inorganic compounds using a systematic approach

Seth's Compound Naming Flowchart