Chapter 2 Atoms, Molecules, and Ions

semester 2/2013

2.2 The Structure of Atom

2.3 Atomic Number, Mass Number, and Isotopes

2.4 The Periodic Table

2.5 Molecules and Ions

2.6 Chemical Formulas

Ref: Raymond Chang/Chemistry/ Eleventh Edition

Prepared by A. Kyi Kyi Tin

2.2 The Structure of Atom

Atom..the basic unit of an element that can enter into chemical combination (extremely small and indivisible)

Three subatomic particles…electrons , protons, and neutrons.

The Electron (Joseph John Thomson (1856-1940)

Negatively charged particles

Source: (from cathode ray tube experiment, forerunner of the television tube)

The cathode ray is attracted by the plate bearing positive charges and repelled by the plate bearing negative charges, it must consist of negatively charged particles.

Cathode Ray Tube

J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics)

Radiation…emission and transmission of energy through space in the form of waves..

Radioactivity (Antoine Henri Becquerel(1852-1908)

spontaneous emission of particles and / or radiation /

Any element that spontaneously emits radiation is “Radioactive”.

(Uranium compound)2.2

Three types of rays produced by decay of radioactive substances such as “Uranium”..

(i) Alpha ( ) rays..positively charged particles ( ) particles.. deflected by positively charged plate

(ii) Beta () rays..electrons..deflected by negatively charged plate

(iii) Gamma () rays..high-energy rays..no charge and are not affected by an external field.

1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron (-)3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

The Proton and the Nucleus

1910,Ernest Rutherford’s experiment

(Refer text bk. Pg.47,48)

Nucleus…a dense central core within the atom

Proton…positively charged particles in the nucleus

Source: whenever “. Came close to a nucleus in the

scattering experiment, it experienced a large repulsive

and therefore a large deflection. (i.e nucleus is composed

of positively charged particles, which called proton)

The Neutron (James Chadwick, 1932)

-electrically neutral particles having a mass slightly greater than that of protons

Particle Mass (g) Charge unitElectron

Proton

Neutron

9.10 x 10 -28

1.67 x 10 -24

1.67 x 10 - 24

-1 +1

0

Source: When a thin sheet of beryllium is bombarded with -particles, a very high-energy radiation similar to - rays was emitted by the metal i.e Neutron

mass p = mass n = 1840 x mass e-

2.2

2.3 Atomic Number, Mass Number, and Isotopes

Atomic Number (Z)…the number of protons in the nucleus of each atom of an element.

Mass Number (A)… the total number of neutrons and protons present in the nucleus of an atom of an element.

Isotopes…atoms that have the same atomic number but different mass number.

Mass number

Atomic numberXAZ

tritiumdeuteriumhydrogen

HHH

............

......,................,......... 31

21

11

2.4 The Periodic Table

-A chart in which elements having similar chemical and physical properties are grounded together.

-Groups…Vertical Columns

-Periods….Horizontal Rows

-Metals….Good conductor of Heat & Electricity

-Nonmetals…Poor conductor of heat & electricity

-Metalloids….Intermediate b/w Metal & Non-metal

-Alkali metals(1A)/ Alkaline Earth metals(2A)/ Halogens( 7A)/ Noble gases (or) Rare gases (8A)

Period

Grou

p

Alk

ali Metal

Nob

le Gas

Halogen

Alk

ali Earth

Metal

2.5 Molecules and Ions

Molecule…an aggregate of at least two atoms in definite arrangement held together by chemical forces (Chemical bonds) SAME ELEMENTS (or) TWO (or) MORE ELEMENTS IN A FIXED RATIO

Diatomic molecules same elements (H2,O2,Cl2)

different elements (HCl, CO)

Polyatomic molecules more than two atoms (O3, H2O,NH3)

Ions..an atom or a group of atoms that has a net positive or negative charge.

Cations / Anions / monoatomic ions / polyatomic ions

2.6 Chemical Formulas

Molecular Formula…shows the exact number of atoms of each element in the smallest unit of a substance.

Allotrope..one of two or more distinct forms of an element (carbon..diamond and graphite)

Structural Formula…shows how atoms are bonded to one another in a molecule.

Empirical Formula..which elements are present and the simplest whole number ratio of their atoms..

Molecular

formula

Empirical

formula

H2O H2O

H2O2 HO

NH3 NH3

H2C2O4 HCO2

O3 O

Ionic compounds consist of a combination of cations and an anions. (example Na+Cl-)