Daltons Atomic Theory and its Modifications

len Alfred Ajibola - Thu, 18th April, 2019 @ 11:26 AM

Topics in Chemistry

Le Chatelier's Principle: Changes in concentration and pressure in dynamic equilibrium Chemical Equilibrium: Dynamic Equilibrium in Chemistry Static and Dynamic Equilibrium explained with their Differences Chemistry Scheme of Work, SS1, First Term Chemistry Scheme of Work, SS1, Second Term Chemistry Scheme of Work, SS1, Third Term Compounds in Chemistry: Characteristics of Compounds Types of Mixture: Homogenous and Heterogeneous Mixtures Mixtures: Characteristics of Mixtures Physical Properties of Matter explained with Examples Chemical Properties of Matter explained with Examples Differences between Physical and Chemical Change Ionic Theory and Electrolysis Physical and Chemical Properties of Acids States of Matter: Characteristics of Solids Liquid State of Matter: Characteristics of Liquids Gaseous State of Matter: Characteristics of Gases Molar Mass, Worked Examples and Differences with Molecular Mass Elements: Characteristics of Elements Salts and Types of Salt in Chemistry

Academic Questions in Chemistry

Please check out our Test Your Knowledge page to see all Questions and Answers

Which of the following isn't an element of the periodic table.

  • A. J

  • B. Y

  • C. W

  • D. B

  • E. U

  • F. K

According to the periodic table, elements in the same group have got the same number of shells.

  • A. True

  • B. False

The periodic table of elements is also called the Mendeleev's table.

  • A. True

  • B. False

The simplest formula of a compound is termed _____.

  • A. Emperical formula

  • B. Molecular formula

  • C. Avogadro's number

  • D. Mass formula

  • E. Molar mass formula

  • F. Mole ratio

A compound composed of iron (Fe) and oxygen (O) was analyzed and found to contain 69.94% iron and 30.06% oxygen. Find the empirical formula of the compound. (Molar mass of Fe=55.85, O=16)

  • A. FeO

  • B. Fe2O3

  • C. Fe3O4

  • D. FeO2

  • E. Fe3O6

  • F. Fe2O4

Which of the following statement is false concerning hydrocarbons?

  • A. Hydrocarbons can be aromatic

  • B. Hydrocarbons are made up of carbon and hydrogen only

  • C. Alkynes are aliphatic hydrocarbons

  • D. Alkenes have a carbon-carbon double bond in their structure

  • E. Benzene ring has a carbon-carbon triple bond in its structure

  • F. Hydrocarbons are the main constituents of petroleum and natural gas

The ability of carbon to form long chain of itself is called _____.

  • A. Carbon chain reaction

  • B. Catenation

  • C. Carbontion

  • D. Carbontion

  • E. Organic chemistry

  • F. Carbon moleculation

Calculate the relative molecular mass of methane. (Carbon = 12.001, H = 1.00794).

  • A. 13.00894 amu

  • B. 11.00206 amu

  • C. 12.00794 amu

  • D. 14.001 amu

  • E. 15.001 amu

  • F. 16.033 amu



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Dalton’s Atomic Theory:

John Dalton, an English chemist is considered the father of modern chemistry because of the understanding he brought into the composition of matter and the basis for their chemical reactions.

Recall that chemistry is the study of matter and the changes they undergo.

Please read on the concept of matter here.

Some of the atomic theory which he postulated in 1808 had to be modified based on the discoveries of other scientists.

In this article, we will look into these atomic theories of Dalton and their modifications.


Atomic Theory 1

  • All matter consists of tiny indivisible particles called atoms.

Modification of Atomic theory 1

Based on the discovery made by Rutherford, atoms can no longer be regarded as indivisible because they can be further divided into the followings:

  • Protons: They are positively charged particles in the nucleus of an atom.

  • Neutrons: These particles have no charge (neutral) and are found in the nucleus of an atom.

  • Electrons: They are negatively charged particles that revolve round the shell of atoms.

  • Quarks: A quark is a fast moving energy and is comprised of both the protons and neutrons in the nucleus of an atom.

  • Please read on elements and characteristics of elements here.


Atomic theory 2

  • Atoms of the same element are identical to each other in every aspect because they have the same shape and mass, while atoms of different elements are different in all respect.

Modification of Atomic theory 2

The discovery of isotopes has shown that not all atoms of the same elements are similar in every aspect.

Isotopy is a phenomenon whereby atoms of the same element have the same atomic number but different mass number. As a result, the same element can have a different mass and density.

Note: The difference in the mass (mass number) of isotopes is a result of the varying number of neutrons in the nucleus of their atoms.

The mass number of an atom is defined as the sum of protons and neutrons in the nucleus of the atom.

You can read on worked examples of molar mass here.

Based on the above explanations, isotopes have the same number of protons (atomic number) but different number of neutrons.

The mass spectrometry experiment conducted on (isotopic) atoms had given a confirmation to the existence of isotopes.

Some elements exhibiting isotopy include:

Also, the discoveries of isobars have shown that the atoms of different elements are not completely different.

Isobar is a phenomenon whereby atoms have different atomic numbers but the same mass number. For instance:

  • 147N and 146C are isobars: These are different elements (nitrogen and carbon) with the same mass number. Both elements will have the same weight under a specific gravitational force.
    Please read on mass versus weight here.
  • 2411Na and 2412Mg are isobars: These are different elements (sodium and magnesium) with the same mass number.
    You can read more on gravity here.


Atomic theory 3

  • Atoms are indestructible and can neither be created nor destroyed.

Modification of Atomic theory 3

With regards to ordinary chemical reaction, this statement remains valid.

After the discovery of radioactivity, the various radioactive elements have shown that atoms can be created (through a process called nuclear fusion) and destroyed (through a process called nuclear fission).

Example of radioactive elements are Thorium-232 and Uranium-238

You can read on the periodic table of elements here.


Atomic theory 4

  • Atoms of different elements can combine with each other in simple whole number ratios to form compounds.

Modification of Atomic theory 4

This statement is true for simple inorganic chemical reaction. With regards to complex organic compounds or molecules, the statement becomes false because they don’t combine in simple whole numbers; rather, they do so in complex numbers. For instance, glucose molecule has a chemical formula of C12H22O11.

Starch is another organic compound that contains many repeating units of simple sugars and with a chemical formula of (C6H10O5)n, where "n" is a large whole number.

Please read on sugars and carbohydrates here.


Atomic theory 5

  • Atoms of the same element share similar physical and chemical properties. They can also combine in more than one ratio to form two or more compounds.

Modification of Atomic theory 5

This theory cannot explain allotropy.

Allotropy is phenomenon whereby atoms of the same element exist in different forms but in the same physical state.

For instance, carbon in its solid state can exist in four different forms. These are:

The above forms of carbon have got different physical properties or characteristics. For instance, graphite is soft while diamond is very tough (hard).

You can read on the physical properties of metals here.


Advantages of Dalton’s Atomic Theory

1. His theory gave a clearer explanation to the laws of chemical combination which includes:

  • Law of constant composition
  • Law of multiple proportions

2. Dalton created the distinctions between atoms, molecules and compounds.
You can read on physical and chemical changes here

3. His work paved way for the discovery of atomic bomb.

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 10 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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Amazing facts in Chemistry

The only letters that failed to appear on the periodic table are letters:

J     &     Q

Gold and copper are the only two non-silvery colored metals.

Copper is the only metal that is naturally antibacterial. For this reason, some children utilize 'copper water bottles' in schools

Water freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre


In chemistry, hydrocarbons can be classified as either aliphatic or aromatic. Recently, both classifications of hydrocarbon were based on their structure rather than their origin.

Aliphatic hydrocarbons are put into three main groups according to the types of bonds they possess. These are:

  1. Alkanes

  2. Alkenes

  3. Alkynes

They are shown in the image below:

Alkanes, Alkenes and Alkynes - Len Academy

It's important to note the followings:

  • Alkanes have single bonds (only) in their structures.

  • Alkenes always have a carbon-carbon double bond present in their structure.

  • Alkynes always have a carbon-carbon triple bond present in their structure.


Aromatic hydrocarbons are classified into:

  • Arenes: They contain benzene ring as a structural unit. Below is the structure of a benzene ring.

Benzene Ring - Len Academy


  • Nonbenzenoid aromatic hydrocarbons: They possess special stability but lack a benzene ring as a structural unit.

Organic chemistry is the study of carbon and it's compounds.

Carbon is the focus of organic chemistry because it has a wide chemical diversity in the sense that it can combine with other carbon atoms to form a long chain of carbon molecule. This ability and process whereby carbon can form a long chain of itself is called catenation.

Please read the introduction to organic chemistry here

A major challenge encountered when calculating molecular mass is that it becomes difficult or impossible to calculate especially when the relative molecular mass of large molecules, polymers and macromolecules are involved.

Examples of large molecules (with indefinite molecular masses) include carbohydrates, cellulose and complex sugars.

The large molecules (above) have no specific chemical formula throughout their volume.

Please read more on carbohydrates and sugars here

Understand that Relative Molecular Mass prove to be useful only when we calculate substances with small and definite molecular sizes. This was proven through the modifications of Dalton's atomic theory.

Please read on Dalton's atomic theory and its modifications here.

Calculate the Relative Molecular Mass of methane (CH4). (Carbon=12.001 and H=1.00794)

  • Molecular Mass = Atomic mass of carbon + Atomic mass of Hydrogen

  • Methane is composed of one atom of Carbon and 4 atoms of Hydrogen.

  • 12.001 + (1.00794 x 4)

Molecular Mass of Methane = 16.033 amu

Please read more on molecular mass here

Calculate the Relative Molecular Mass of water (H2O). (Oxygen=15.9994 and Hydrogen=1.00794)

  • Molecular Mass = Atomic mass of Hydrogen + Atomic mass of Oxygen

  • Water contains 2 atoms of Hydrogen and 1 atom of oxygen.

  • (1.00794 x 2) + 15.9994

Molecular Mass of Water = 18.015 Da

Please read on emperical formula and its calculations here

Gold - Len Academy

Below are some facts about gold:

  • It is the only metal that has a gold (golden) color

  • Gold is present inside the earth crust in all the seven continents

  • Pure gold is soft and very stretchable (It can even be stretched into threads)

  • The Latin name of Gold is aurum which means 'shining dawn'. For this reason, its chemical symbol is 'Au'

  • Gold is a very good conductor of electricity. This is why it’s sometimes used in making USB and audio cables

  • Gold is non toxic and can be eaten with food. It is sometimes added to desserts in some expensive restaurants

  • The term Gold originated from an ancient English word 'geolu' which means yellow

  • 24karat gold is the purest gold element

Please read more interesting facts about gold here