what is an element
Element is a pure substance that cannot be broken down into simpler substances by any chemical methods.
what are elements made of
It is made up of very tiny invisible particles (or species) which are called atoms
how can we see atoms
a high-resolution scanning electron
microscope can generate pictures of a single heavy atom resting on a thin carbon substrate.
who first proposed the idea of atoms
Democritus - a Greek philosopher who lived in the fifth century BC
what did democritus propose
everything was made of indivisible small pieces of matter - “atomos” (Greek word
for indivisible)
who proposed a modern atomic model based on the Democritus’ idea of atom
John Dalton, in 1800
Dalton’s theory
- All matters are made up of indestructible atoms.
- The atoms of a given element are identical; the atoms of different elements are different in some fundamental way(s).
- Chemical compounds are formed when atoms combine with each other. A given compound always has the same whole number ratio and types of atoms.
- Chemical reactions involve rearrangement of the atoms. The atoms themselves are not changed in a chemical reaction.
Limitations of Dalton’s theory
- With the discovery of sub-atomic particles, e.g., electrons, neutrons and
protons, the atom can no longer be considered indivisible. - Discovery of isotopes indicated that all atoms of the same element are not
perfectly identical. At least, they differ in their masses. - Dalton’s atomic theory could not explain why certain substances, with atoms of the same element, differ in their properties. For example, graphite and diamond are made up of only carbon atoms, but their properties are quite different
what did J. J. Thomson discover and suggest
J. J. Thomson in an experiment discovered that different metals produce a stream of negatively charged particles when a high voltage is applied
across two electrodes. He suggested that these particles are part of the atoms.
what did Thomson picture
Thomson pictured the structure of an atom as a “plum pudding”, with the
negatively charged electrons scattered in a positively charged sponge-like substance.
who and what was done to test Thomson’s model
Rutherford and his research team
tested Thomson’s model by firing
alpha particles, which consists of
two protons and two neutrons, at a
piece of gold foil.
what was hypothesized to happen in the experiment vs what happened
He thought that if Thomson’s model was valid, the alpha particles should either pass straight through or get stuck in the positive “sponge”.
It was observed that most of the particles did pass straight through the foil, but a very small number were repelled and bounced back.
what did Rutherford’s experiment show
A large number of undeflected paths imply that the atom is mainly an empty space Large extent of
deflections occurs when the positively charged alpha particles collide with and are repelled by a positively
charged nucleus. Only a small number of alpha particles bounce back suggests that the nucleus is very
small. Based on the results of the Rutherford’s experiment, atoms are in fact made up of smaller
particles or sub-atomic particles
Limitations of Thomson’s model
- Thomson’s model failed to explain the results of the alpha particle
scattering experiment carried out by Rutherford - the majority of the alpha
particle passes through the gold foil while a small number of them
bounced back. - There was no experimental evidence provided by Thomson. This was one
of the major drawbacks of his model.
Limitations of Rutherford’s model
- One of the drawbacks of the Rutherford model was that he did not say
anything about the arrangement of electrons in an atom which made his
theory incomplete. - Although the early atomic models were inaccurate and failed to explain
certain experimental results, they were the base for future developments
of the theories in atoms and sub-atomic particles.
who proposed the atom model in 1923
Niels Bohr
What was Niels Bohr’s atom like
It was seen as a small “solar system” with electrons moving in an orbit or energy level around the positively charged nucleus of one proton. The electrostatic force of attraction between the oppositely charged subatomic particles prevents the electrons from leaving the atom. Most of the volume of the atom is empty space.
Limitations of Niels Bohr’s model
● Given its mass and wavelike nature, the electron in the hydrogen atom
could not possibly orbit the nucleus in a well-defined circular path as
predicted by Bohr.
● The position (an orbit of a particular radius) and the energy of the
electron could not be specified.
