The Concept Of Atoms And Molecules
The Concept Of Atoms And Molecules
Long ago, ancient Greek philosophers had pondered on the idea that matter could be composed of very small bits called atoms. They however did not go beyond this. In the early part of the nineteenth century (1808), an English chemist named John Dalton came up with a set of testable suggestions about the atom, and it became known as
Dalton’s atomic theory.
The ideas about the theory states as follows:
1. Matter is made up of small, indivisible particles called atoms.
2. Atoms can neither be created nor destroyed.
3. Atoms of a particular element are all exactly alike in every way and are different from atoms of all other elements.
4. Chemical combination is the union of atoms in small whole numbers.
This theory stood for about a century
before certain faults were discovered in
most of its ideas in the early part of the
twentieth century . We shall highlight
these faults in this lecture.
Distinction between Atoms and Molecules
The atomistic philosophers made no clear distinction between atoms and molecules because they did not know about the chemical elements.
The distinction between atoms and molecules became important when the concept of chemical elements was accepted (this concept was proposed by Robert Boyle, and it states that all materials are composed of a small number of elements).
Based on the later development of the molecular theory (also known as the kinetic theory), a clear distinction between atoms and molecules has been established.
An atom can be defined as the smallest particle of an element, which can take part in chemical change or reaction and still retain the identity of the element. Notice that the atom is not the smallest particle of an element, as there are electrons, protons and neutrons which are smaller, but, it is the smallest which can be involved in chemical change and still retain the chemical properties of the element.
Electrons, protons and neutrons are fundamental particles of matter, they are found alike in all substances. Hence, they do not retain the identities of their respective elements when they are involved in chemical change.
A molecule can be defined as the smallest, uncharged particle of an element or compound, which moves about as a whole, such that its component parts do not become detached during the motion.
In other word, a molecule is the smallest part of a substance, which has a separate existence.
A molecule of a compound consists of more than one kind of atom (this is because compounds are formed between two or more elements). E.g. a molecule of water consists of two atoms of hydrogen and one atom of oxygen. A molecule of an element consists of one or more atoms of the same kind.
E.g. a molecule of argon (Ar) and helium (He) consist of one atom of argon and one atom of helium respectively, while a molecule of hydrogen (H2), chlorine (Cl2) and ozone (O3) consist of two, two and three atoms of hydrogen, chlorine and oxygen respectively.
Note: - All gases consist of molecules. This
is why it is possible for a quantity of a gas to expand and occupy every
space of the container in which it is kept (irrespective of the size of the container)
- Since every gas becomes a liquid when cooled
sufficiently, and every liquid turns to solid when cooled to its freezing point,
it is therefore not only gases, but at least some liquids and solids also
consist of molecules.
The Concept of The Atomic Model
The electrons and protons haven been found as the fundamental particles of an atom, it was now left to describe their arrangement. J.J. Thomson described a model of an atom which suggested that an atom is a uniform sphere of about 10-8 cm radius, made up of even distribution of protons, with the electrons embedded, such as to give the most stable electrostatic arrangement.
This arrangement was found to be Inadequate,
and was totally discarded in the light of a classic experiment performed by
Rutherford. Rutherford directed an alpha particle (α- Helium ion, He2+) onto a thin foil of metal. He observed that most of the particles passed through the foil either undeflected or deflected by small angles (300). A few of the particles were however deflected by very large angles (up to 1800) along their original paths.
The explanation of this observation is given thus: The fact that most of the
α-particles passed through the foil either undeflected or deflected by small angles, suggests that the mass and the protons of an atom are not uniformly distributed (as proposed by J.J. Thompson). Otherwise, the
α-particles would have encountered large repulsions uniformly distributed throughout the metal.
The large deflections observed for a few of these very energetic
α-particles suggests that there must be a seat in the atom of concentration of large electrical force, which must be produced by a body of same charge as the
α-particles and which is of significant mass - not electrons (electrons are very light and would be swept away by the heavier
For the fact that only a few of the
α-particles are deflected suggest that the large electrical force must be confined to a very small region of space.
Therefore, Rutherfold suggested a model for the atom as consisting of a dense central core or nucleus, in which all the positive charges, (i.e. protons) and almost all the mass of the atom are concentrated; and around which equal number of negatively charged electrons move in regular, defined orbitals, completely balancing the positive charges of the nucleus - this is the modern atomic model.
The Atomic Number of an Element
The atomic number of an element, i.e., the number of protons present in the atom was determined by H.G. Moseley. He compared the wavelenghts of the characteristic X-ray emitted by a number of elements and thus arrived at values for the nuclear charges (produced by the protons contained) of the atoms.
These values became the atomic number of the respective element.
Moseley’s discovery provided proof that there is in the atom a fundamental quantity which increases by regular steps as we pass from one element in the periodic system to the next. This quantity is the charge of the nucleus, i.e., the number of protons in the nucleus.
The Discovery of Neutrons
Observing that a helium atom, with atomic number 2 (containing 2 protons) actually have an atomic weight of approximately 4 amu instead of 2 amu expected of a 2 proton atom. It was believed that there must be some other fundamental unit in the atom whose weight must be adding to the total atomic weight of the element, but do not contribute anything to the electric charge.
Rutherford suggested it to be an electron/proton pair in the nucleus that is very much joined together that it acts like a single particle of no charge – the particle was called a neutron.
However, James Chadwick, a young scientist working in Rutherford’s laboratory, later showed neutrons not to be as described by Rutherford, but as single discrete particles with no charge, but with a mass of 1.0087 amu, almost equal to that of the proton. Therefore, the mass of an atom is the sum of its number of protons and its number of neutrons.
This Post? Please Share!!!
Recommended: Chemistry Materials for Success
chemistry tutorial and quiz software package that boosts learning of
chemistry and raises grades and test scores.
Designed as an
interactive learning material, it covers high school chemistry up to
first year of college.
has been found to be highly effective for preparing students for
finals, and for chemistry tests and exams in the following: SAT II,
AP, and state proficiency exams.
The Chemistry 1 Tutor: Volume 1 - 10
Solving Chemistry problems successfully
takes practice to learn the skills needed to be able to logically
tackle the problems and get at the solution quickly.
The Chemistry 1
Tutor DVD set has been proven by users to be the extra practice that
you need to achieve success in Chemistry.
Whether you are a
high school or first year college student or you are just taking a
refresher Chemistry course after twenty years of graduation from
college, this material will make a huge difference in your
The Chemistry 1 Tutor: Volume 1 - 10 Hour Course!
A very effective Chemistry review book
specially created to make the student feel they have a personal
private tutor helping them out with their Chemistry work at home.
are clearly explained for most students to easily understand, while
solutions to problems are provided in a step-by-step format that
makes the learning of Chemistry a breeze to its readers.
Rated highly by
most students, this book treats all Chemistry topics in a normal
one-year Chemistry course and is suitable for preparing for
standardized Chemistry tests, such as the SAT-II.
Homework Helpers: Chemistry
The Central Science
This is a great General Chemistry book
by all standards, attested to by millions of students who have found
strongest edge includes unparalleled problems, clarity, and
Most students find
this book very thorough, easy to read and understand.
Chemistry: The Central Science
Outline of College Chemistry:
1,340 Solved Problems + 23 Videos
This is a complete package as long as
helping you succeed in Chemistry is concerned - over 1,300
completely solved problems, examples, and practice tests to enable
you develop great problem solving skills and be able to tackle
Chemistry problems effectively.
Also, in this package is access to 23 detailed videos tutorial
specially made to explain commonly tested Chemistry concepts in
clearer format than most books, helping the student to develop
confidence, knowledge, and skills needed to make high scores in
Schaum's Outline of College Chemistry: : 1,340 Solved Problems + 23 Videos
1,001 Practice Problems For Dummies
This material gives
students in high schools and universities the opportunity to deepen
their understanding of Chemistry by taking 1001 well selected
practice problems base on what they learn in class.
It helps you to
understand Chemistry concepts through the supplementary explanations
it provides, and to develop skills to solving problems speedily.
Chemistry: 1,001 Practice Problems For Dummies
Concepts and Problems: A Self-Teaching Guide
If you are looking for a fast and easy
way to master the fundamentals of Chemistry, then you need this
book. It does not only contain tutorials, but also lots of problems
to solve, and answers to the problems with explanations and
Well loved by
students, this book seeks to get the student involved in the work
process and so make their learning deeper.
It covers high
school Chemistry course and up to first year university program, and
effectively prepares students for standardized tests.
Chemistry: Concepts and Problems: A Self-Teaching Guide
- Raymond Chang
This book is
specially designed for the two-semester general Chemistry course.
book has helped millions of students to easily grasp the fundamental
concepts of Chemistry through its clear and concise presentation
It also provides
the student with effective strategies to solving problems.
In all, this book has been praised by most student users as being
highly effective in helping them make good grades in Chemistry.
Chemistry - Raymond Chang