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CHAPTER 2

ATOMS, MOLECULES &

IONS

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Learning objectives

To list the postulates of atomic theory To describe the structure of the atom To write the nuclide symbol for a given nuclide To identify groups and periods on periodic table To determine chemical formulas, molecular and ionic

substances To write chemical equations

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Learning outcomes students will be able to

Solve the qualitative problems related to write chemical equations, list the postulates of atomic theory, describe the structure of the atom, write the nuclide symbol for a given nuclide, identify groups & periods on periodic table, determine chemical formulas, molecular and ionic substances

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2.1 Atomic theory of matterDalton’s Atomic Theory All matter is composed of indivisible atoms.

An atom is an extremely small particle of matter that retains its identity during chemical reactions.

An element is a type of matter composed of only one kind of atom.

A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions.

A chemical reaction involves the rearrangement of the atoms present in the reacting substances to give new chemical combinations present in the substances formed by the reaction.

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Atomic symbol: represent an atom corresponding to a particular element.

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Law of Multiple ProportionsWhen two elements form more than one compound, the masses of one element in these compounds for a fixed mass of the other element are in small, whole-number ratios.

Example: compare the mass of oxygen in the two oxygen compounds of carbon:

carbon monoxide 12 g C 16 g O

carbon dioxide 12 g C 32 g O

The ratio of oxygen in CO2 to CO

2g16

g32

COinoxygen

COinoxygen 2

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2.2 The structure of the atom

The atom is composed of two parts:

Nucleus

Central core

Positively charged

Contains most of the atom’s mass

Electrons

Very light

Negatively charged

Exist in the region around the nucleus

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Discovery of the ElectronJ. J. Thomson discovered that the particles that make up the cathode ray are negative and are part of all matter.

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Nuclear Model of the Atom

Ernest Rutherford used positively charged alpha particles to bombard very thin gold foil.

Most alpha particles passed through the foil But a few were scattered at large angles, sometimes

almost straight backward.

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2.3 Nuclear structure: isotopes Proton A nuclear particle having a positive charge

atomic number The number of protons in an atom

Neutron A nuclear particle having a mass almost equal to that of the proton but no electrical charge

Mass number total number of protons and neutrons in the nucleus

Isotopes have the same atomic number but different numbers of neutrons

Nuclide An atom characterized by a certain atomic number and mass number

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Nuclear structure-exercise

[1] What is the nuclide symbol for the nucleus that contains 14 protons and 14 neutrons?

[2] An atom contains 11 protons and 11 neutrons. What is the nuclide symbol for the nucleus?

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[3] The following table gives the number of protons and neutrons in the nuclei of various atoms. Which atom is the isotope of atom A? Which atom has the same mass number as atom A?

Protons Neutrons

Atom A 32 39

Atom B 32 38

Atom C 38 50

Atom D 33 38

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[4] What is the name of the element represented by each of the following atomic symbols?

a) Ar

b) Zn

c) Ag

d) Mg

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2.4 Atomic masses Atomic Mass

The average atomic mass for the naturally occurring element expressed in atomic mass units (amu).

Atomic Mass Unit

Equal to exactly one-twelfth the mass of a carbon-12 atom

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2.5 Periodic Table of the elements A tabular arrangement of elements in rows and

columns, highlighting the regular repetition of properties of the elements

A period consists of the elements in one horizontal row.

A group consists of the elements in one vertical column.

Groups are numbered using two systems:

IUPAC Numbers 1 through 18

Older system Roman numerals I–VIII and the letters A and B

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Give one example for each of the following.a) a main-group element in the second period

b) an alkali metal

c) a transition element in the fourth period

d) a lanthanide element

Periodic Table of the elements-exercise

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Metal A substance that has a characteristic shine and is generally a good conductor of heat and electricity

Example: aluminium, copper, iron, tin, gold, lead

Nonmetal An element that does not exhibit the characteristics of a metal

Example: Oxygen, nitrogen, carbon

Metalloid An element having characteristics of both metals and nonmetals.

Example: silicon, germanium

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Metal, nonmetal, semimetal-exercise

determine whether the element is a metal, nonmetal or metalloid.a. S b. Fe c. Ba d. Cu e. Ne

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2.6 chemical formulas; molecular & ionic substances

Chemical formula A notation that uses atomic symbols with numerical subscripts to convey the relative proportions of atoms of the different elements in the substance.The chemical formula for glucose is C6H12O6

Molecular Formula A formula giving the exact number of different atoms of an element in a molecule.The molecular formula for glucose is C6H12O6

Empirical formula Is the simplest positive integer ratio of atoms present in a compound

The empirical formula for glucose is CH2O

Ion an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing one or more electrons.

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cation is a positively charged ion formed by losing one or more electrons.Example: Al3+, Mg2+, Na+

anion is a negatively charged ion formed by gaining one or more electrons.Example: Cl-, Br-,I-

ionic compound is composed of cations and anions.

Example: NaCl

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[1]Write the formula for the compound of each of the following pairs of ions.

a) Fe3+ and CN-

b) b. K+ and SO42-

c) c. Li+ and N3-

d) d. Ca2+ and P3-

chemical formulas; molecular & ionic substances-exercise

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2.7 Organic compounds Organic Compounds

An important class of molecular substances; they contain carbon combined with other elements – notably hydrogen, oxygen, and nitrogen.

Hydrocarbons contain only carbon and hydrogen

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2.8 Naming simple compounds Monatomic ions: Ions formed from a single atom.

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Polyatomic Ions: An ion consisting of two or more atoms chemically bonded together and carrying an electrical charge

phosphate PO43-

monohydrogen phosphate HPO42-

dihydrogen phosphate H2PO4-

carbonate CO32-

hydrogen carbonate (bicarbonate) HCO3-

sulfateSO42-

hydrogen sulfate (bisulfate) HSO4-

sulfite SO32-

hydrogen sulfite (bisulfite) HSO3-

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Naming simple compounds-exercise

[1]Give systematic names to the following binary compounds.

a) N2O

b) b. P4O10

c) c. AsCl3

d) d. Cl2O7

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[2]Write the formulas of the following compounds.

a) nitrogen tribromide

b) xenon tetroxide

c) oxygen difluoride

d) dichlorine pentoxide

Naming simple compounds-exercise

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2.9 Writing chemical equations A chemical equation is the symbolic representation of a chemical reaction in terms of chemical formulas.

For example: 2Na + Cl2 2NaCl

Reactants are the starting materials; they are written on the left of the equation.

Products are the materials at the end of the reaction; they are written on the right of the equation.

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Writing chemical equations-exercise

[1] For the balanced chemical equation Pb(NO3)2 +K2CO3 PbCO3 + 2KNO3, how many oxygen atoms are on the left side?

[2] In the equation 2PbS + O2 2PbO + 2SO2, howmany oxygen atoms are there on the right side? Is the equation balanced as written?

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2.10 Balancing chemical equations

balancing the equation to ensure that the same number of each atom is present in both the reactants and the products

Na + Cl2 2NaCl

Na are not balanced To balance Na, we insert the coefficient “2” before Na

on the reactant side.

2Na + Cl2 2NaCl

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Balancing chemical equations -exercise

a. Cl2O7 + H2O HClO4

b. MnO2 + HCl MnCl2 +Cl2 + H2O

c. Na2S2O3 + I2 NaI + Na2S4O6

d. Al4C3 +H2O Al(OH)3 +CH4

e. NO2 +H2O HNO3 +NO

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Checking outcome To list the postulates of atomic theory To describe the structure of the atom To write the nuclide symbol for a given nuclide To identify groups and periods on periodic table To determine chemical formulas, molecular and ionic

substances To write chemical equations

Thank you