The Atomic Theory of Matter

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Chapter 2

Atoms, Molecules,

and Ions


The Atomic Theory of Matter

• Democritus (460–370 bce) and other early Greek

philosophers described the material world as made up of tiny, indivisible particles that they called atomos,

meaning “indivisible” or “uncuttable.”

• Plato and Aristotle did not embrace the atomic ideas of Leucippus and Democritus.

Matter had no smallest parts.

Different substances were composed of various proportions of fire, air, earth, and water.

• John Dalton (1766–1844) offered convincing evidence that supported the early atomic ideas of Leucippus and

Democritus 2


Dalton’s theory

Dalton’s Postulates


The law of conservation of mass

• The total mass of materials present after a chemical

reaction is the same as the total mass present before the reaction



The law of constant composition

• All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements.


Law of Multiple Proportions

• When two elements (call them A and B) form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers.

• An atom of A combines with either one, two, three, or more atoms of B (AB1, AB2, AB3, etc.).



The Discovery of Atomic Structure


Discovery of Subatomic Particles

• In Dalton’s view, the atom was the smallest particle

possible. Many discoveries led to the fact that the atom itself was made up of smaller particles.

• The atom is composed of subatomic particles:

 Electrons

 Protons

 Neutrons



The Electron (Cathode Rays)

• Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence.

• J. J. Thomson is credited with their discovery (1897).


The Electron

• Thomson measured the charge/mass ratio of the electron to be 1.76X108 coulombs/gram (C/g).


* The coulomb (C) is the SI unit for electrical charge.


Millikan Oil-Drop Experiment (Electrons)

• Robert Millikan determined the charge on the electron in 1909



• In 1896 the French scientist Henri Becquerel (1852–1908) discovered that a compound of uranium spontaneously emits high-energy radiation. This spontaneous emission of radiation is called radioactivity.

• Three types of radiation were discovered by Ernest Rutherford:

α particles (positively charged, +2)

β particles (negatively charged, -1, like electrons)

γ rays (uncharged)



The Nuclear Model of the Atom

• J. J. Thomson’s plum-pudding model of the atom.


Rutherford’s Gold Foil Experiment

• Ernest Rutherford shot α particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.



Rutherford’s Gold Foil Experiment

• The Rutherford experiment gave an unexpected result. A majority of the particles did pass directly through the foil, but some particles were deflected, and some

(approximately 1 in 20,000) even bounced back.


The Nuclear Atom

• Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.

• Most of the volume is empty space.

• Atoms are very small; 1 – 5 Å or 100 – 500 pm.

16 angstrom (A°), where 1 A° = 1 * 10-10 m


Subatomic Particles

Protons (+1) and electrons (–1) have a charge; neutrons are neutral.

Protons and neutrons have essentially the same mass (relative mass 1). The mass of an electron is so small we ignore it

(relative mass 0).

Protons and neutrons are found in the nucleus; electrons travel around the nucleus


Periodic Table

Each element is identified by a unique atomic number and with a unique chemical symbol.

The chemical symbol is either a one- or two-letter

abbreviation listed directly below its atomic number on the periodic table.



Atomic Mass

• Atoms have extremely small masses.

• The heaviest known atoms have a mass of approximately 4 × 10–22 g.

• A mass scale on the atomic level is used, where an atomic mass unit (amu) is the base unit.

 1 amu = 1.66054 × 10–24 g


Atomic Numbers, Mass Numbers,

• The atoms of each element have a characteristic number of protons. The number of protons in an atom of any

particular element is called that element’s atomic number.

• The mass number, is the number of protons plus neutrons in the atom




What does "X" represent in the following symbol? X

Determine the number of protons, neutrons and



• Isotopes are atoms of the same element with different masses.

• Isotopes have different numbers of neutrons, but the same number of protons




How many protons, neutrons and electrons does sulfur-31 and sulfur-32 have?


Determining the Number of Subatomic Particles in Atoms

How many protons, neutrons, and electrons are in an atom of (a) 197Au, (b) strontium-90?

24 Note: Gold has atomic number 79.

The atomic number of strontium is 38.


Atomic Weights

• Most elements occur in nature as mixtures of isotopes.

We can determine the average atomic mass of an element, usually called the element’s atomic weight

Atomic Weight = Ʃ [(isotope mass) × (fractional natural abundance)].

EX: carbon is composed of 98.93% 12C and 1.07% 13C.



Calculate the atomic mass of element "X", if it has 2 naturally occurring isotopes with the following masses and natural abundances:

X-45: 44.8776 amu 32.88%

X-47: 46.9443 amu 67.12%



The Mass Spectrometer

• The most accurate means for determining atomic weights is provided by the mass spectrometer


Periodic Table

Each element is identified by a unique atomic number and with a unique chemical symbol.

The chemical symbol is either a one- or two-letter

abbreviation listed directly below its atomic number on the periodic table.



Periodic Table





Classification of Elements

• Elements in the periodic table are classified as the following:

 Metals

 Nonmetals

 Metalloids (類金屬)



• Metals lie on the lower left side and middle of the periodic table and share some common properties:

 They are good conductors of heat and electricity.

 They can be pounded into flat sheets (malleability).

 They can be drawn into wires (ductility).

 They are often shiny.

 They tend to lose electrons when they undergo chemical changes.




• Nonmetals lie on the upper right side of the periodic table.

• Nonmetals as a whole tend to

 be poor conductors of heat and electricity.

 be not ductile and not malleable.

 gain electrons when they undergo chemical changes


Metalloids (類金屬)

Metalloids are sometimes called semimetals.

They are elements that lie along the zigzag diagonal line that divides metals and nonmetals.

They exhibit mixed properties.

Several metalloids are also classified as semiconductors because of their intermediate (and highly temperature- dependent) electrical conductivity.



Molecules and Molecular Compounds

• Most of the oxygen in air consists of molecules that contain two oxygen atoms. (chemical formula O2 )

• The subscript tells us that two oxygen atoms are present in each molecule. A molecule made up of two atoms is called a diatomic molecule.

• Compounds composed of molecules contain more than


Types of Formulas

• Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound.

• Molecular formulas give the exact number of atoms of each element in a compound.

• If we know the molecular formula of a compound, we can determine its empirical formula. The converse is not true!


For C4H8, the greatest common factor is 4. The empirical formula is therefore CH2.

For C3H6, the greatest common factor is 3. The empirical formula is therefore CH2.

For CCl4, the only common factor is 1, so the empirical formula and the molecular formula are identical.



Structural formula

• A structural formula shows which atoms are attached to which, as in the following example


Molecular Models

A ball-and-stick molecular model represents atoms as balls and chemical bonds as sticks; how the two connect reflects a molecule’s shape.

In a space-filling molecular model, atoms fill the space between each other to more closely represent our best

estimates for how a molecule might appear if scaled to visible size.



Ions and Ionic Compounds

• The nucleus of an atom is unchanged by chemical processes, but some atoms can readily gain or lose

electrons. If electrons are removed from or added to an atom, a charged particle called an ion is formed. An ion with a positive charge is a cation; a negatively charged ion is an anion.



Give the chemical symbol, including superscript indicating mass number, for (a) the ion with 22 protons, 26 neutrons, and 19 electrons.






Polyatomic Ion

• Many common ionic compounds contain ions that are themselves composed of a group of covalently bonded atoms with an overall charge. This group of charged species is called polyatomic ions.

 NH4+ (ammonium ion)

 SO42- (sulfate ion),



Predicting Ionic Charge


Ionic Compounds

• Ionic compounds are generally formed between metals and nonmetals. (Ex: NaCl)

• Electrons are transferred from the metal to the nonmetal.

The oppositely charged ions attract each other. Only empirical formulas are written.



Identifying Ionic and Molecular Compounds

Which of these compounds would you expect to be ionic: N2O, Na2O, CaCl2, SF4?



The formula for a salt is XBr. The X-ion in this salt has 46 electrons. The metal X is ________



Writing Formulas

• Because compounds are electrically neutral, one can determine the formula of a compound this way:

 The charge on the cation becomes the subscript on the anion.

 The charge on the anion becomes the subscript on the cation.


Names and Formulas of Ionic Compounds

• Ionic compounds are usually composed of metals and nonmetals. (EX: MgSO4 )

• Ionic compounds can be categorized into two types, depending on the metal in the compound.

The first type contains a metal whose charge is invariant from one compound to another.

The second type of ionic compound contains a metal with a charge that can differ in different compounds



Names and Formulas of Ionic Compounds

Cations formed from nonmetal atoms have names that end in -ium:



Names and Formulas of Ionic Compounds


Polyatomic anions containing oxygen have names ending in either -ate or –ite and are called oxyanions.

the one with more oxygen atoms has the ending –ate

the one with fewer has the ending -ite.




• If there are more than two ions in the series then the prefixes hypo-, meaning less than, and per-, meaning more than, are used.


Naming Ionic Compounds



Names and Formulas of Acids

• Binary acids have H+ cation and nonmetal anion.

Write a hydro- prefix.

Follow with the nonmetal name.

Change ending on nonmetal name to –ic.

Write the word acid at the end of the name

EX: HCl hydrochloric acid

• Oxyacids have H+ cation and polyatomic anion contain oxygen.


Nomenclature of

Binary Molecular Compounds



Nomenclature of

Binary Molecular Compounds

• The ending on the second element is changed to -ide.

CO2: carbon dioxide

CCl4: carbon tetrachloride

• If the prefix ends with a or o and the name of the

element begins with a vowel, the two successive vowels are often elided into one.


Nomenclature of Organic Compounds

Organic chemistry is the study of carbon.

Organic chemistry has its own system of nomenclature.

The simplest hydrocarbons (compounds containing only carbon and hydrogen) are alkanes.

The first part of the names just listed correspond to the number of carbons (meth- = 1, eth- = 2, prop- = 3, etc.)



Nomenclature of Organic Compounds

• When a hydrogen in an alkane is replaced with

something else (a functional group, like -OH in the

compounds above), the name is derived from the name of the alkane.

• The ending denotes the type of compound.

EX: An alcohol ends in -ol.



Compounds with the same molecular formula but different arrangements of atoms are called isomers.

EX: 1-propanol and 2-propanol are structural isomers




An ion has 8 protons, 9 neutrons, and 10 electrons. The symbol for the ion is ________

How many protons (p) and neutrons (n) are in an atom of






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