MODULE 1B Atoms Molecules and Ions

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Chemistry lesson number 02; introduction about atoms, molecules, and ions, etc.


The Chemical Formulas


Daltons Atomic Theory (1808)


1. An element is composed of extremely small, indivisible particles called atoms.

2. All atoms of a given element have identical properties that differ from those of other elements.

3. Atoms cannot be created, destroyed, or transformed into atoms of another element.

4. Compounds are formed when atoms of different elements combine with one another in small whole-number ratios.

5. The relative numbers and kinds of atoms are constant in a given compound.

Fundamental Laws of Matter

1. Law of Definite Proportion

Different samples of the same compound always contain its constituent elements in the same proportion by mass.

2. Law of Multiple Proportion

If two elements can combine to form more than one type of compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.

3. Law of Conservation of Mass

Matter can be neither created nor destroyed.

The Structure of the Atom

Fundamental Particles:

ParticleMass (amu)Charge

Electron (e-)0.00054858-1

Proton ( p+)1.0073+1

Neutron (no)1.00870

The Electrons

Humphrey Davy in the early 1800s passed electricity through compounds and noted:

that the compounds decomposed into elements.

Concluded that compounds are held together by electrical forces.

Michael Faraday in 1832-1833 realized that the amount of reaction that occurs during electrolysis is proportional to the electrical current passed through the compounds

Cathode Ray Tubes experiments performed in the late 1800s & early 1900s.

Consist of two electrodes sealed in a glass tube containing a gas at very low pressure.

When a voltage is applied to the cathodes a glow discharge is emitted.

These rays are emitted from cathode (- end) and travel to anode (+ end).

Cathode Rays must be negatively charged!

J.J. Thomson modified the cathode ray tube experiments in 1897 by adding two adjustable voltage electrodes.

Studied the amount that the cathode ray beam was deflected by additional electric field.

- Thomson used his modification to measure the charge to mass ratio of electrons.

e/m = -1.75881 x 108 coulomb/g of e-- Named the cathode rays electrons.

Robert A. Millikan won the 1st American Nobel

Prize in 1923 for his famous oil-drop


- In 1909 Millikan determined the charge and

mass of the electron.

- Millikan determined that the charge on a

single electron = -1.60218 x 10-19


Using Thomsons charge to mass ratio, we

get that the mass of one electron is 9.10 x

10-28 g.

e/m = -1.76 x 108 coulomb

e = -1.6022 x 10-19 coulomb

Thus m = 9.10 x 10-28 g


- 1895, Wilhelm Konrad Roentgen, discovered


- Marie Curie, suggested the name radioactivity,

spontaneous emission of particles and/or


Three types of rays:

1. Alpha rays (() positively charged particles2. Beta rays (() are electrons3. Gamma rays (() high-energy rays; no chargeThe Proton Eugene Goldstein noted streams of positively

charged particles in cathode rays in 1886.

Particles move in opposite direction of cathode rays.

Called Canal Rays because they passed through holes (channels or canals) drilled through the negative electrode.

Canal rays must be positive.

Goldstein postulated the existence of a positive fundamental particle called the proton.

The Nucleus

Ernest Rutherford directed Hans Geiger and

Ernst Marsdens experiment in 1910.

(- particle scattering from thin Au foils

Gave us the basic picture of the atoms structure.

In 1912 Rutherford decoded the (-particle

scattering information.

Explanation involved a nuclear atom with electrons surrounding the nucleus .

Rutherfords major conclusions from the (-particle scattering experiment

The atom is mostly empty space.

It contains a very small, dense center called the nucleus.

Nearly all of the atoms mass is in the nucleus.

The nuclear diameter is 1/10,000 to 1/100,000 times less than atoms radius.

The Neutron

James Chadwick in 1932 analyzed the results of (-

particle scattering on thin Be films.

Chadwick recognized existence of massive

neutral particles which he called neutrons.

Atomic Number

The atomic number is equal to the number of protons in the nucleus.

Sometimes given the symbol Z.

On the periodic chart Z is the uppermost number in each elements box.

In 1913 H.G.J. Moseley realized that the

atomic number determines the element .

The elements differ from each other by the number of protons in the nucleus.

The number of electrons in a neutral atom is also equal to the atomic number.

Mass Number

Mass number is given the symbol A.

A is the sum of the number of protons and neutrons.

Z = proton number

N = neutron number

In general, the mass number is given by


Isotopes are atoms of the same element but with different neutron numbers. Isotopes have different masses and A values but are the same element.

One example of an isotopic series is the hydrogen isotopes.

1H or protium is the most common hydrogen

isotope (one proton and no neutron).

2H or deuterium is the second most abundant hydrogen isotope (one proton and one neutron).

3H or tritium is a radioactive hydrogen isotope (one proton and two neutrons).

A common symbolism used to show mass and proton numbers is

Mass number

Atomic Number

Thus for the isotopes of hydrogen, we write

protium deuterium tritium

Molecules A molecule is an aggregate of at least two atoms in a definite arrangement held together by chemical forces (also called chemical bonds)

Examples of molecules:

O2, H2, Cl2 diatomic molecules

H2O, C12H22O11 polyatomic molecules


An ion is an atom or a group of atoms that has

a net positive or negative charge.

Two basic types of ions:

1. Positive ions or cations - one or more electrons

less than neutral

Na+, Ca2+, Al3+ - monoatomic cations

NH4+ - polyatomic cation

2. Negative ions or anions - one or more electrons

more than neutral

F-, O2- , N3- - monoatomic anions

SO42-, PO43- - polyatomic anions

Mass number = number of protons + number of neutrons

Mass Number = atomic number + number of neutrons

Practice Exercise 2.1: Indicate the number of protons, neutrons, and electrons in each of these species:

a. EMBED Equation.3 b. EMBED Equation.3 c. EMBED Equation.3 d. EMBED Equation.3

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Module 2: Atoms, Molecules and Ions Page 3 of 4