Models of the Atom
This is part of preliminary HSC Chemistry course under the topic of Atomic Structure and Atomic Mass
HSC Chemistry Syllabus
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Model the atom’s discrete energy levels
Models of the atom
Nuclear (Rutherford) Model
In 1911, eminent physicist Ernest Rutherford introduced a groundbreaking model to illustrate the structure of an atom. Drawing upon experimental evidence, Rutherford proposed that an atom comprises a densely concentrated nucleus, possessing a positive charge and housing the majority of the atom's mass. This nucleus, he hypothesised, was encircled by a cloud of orbiting electrons.
To validate his theory of a tiny yet heavy nucleus at the heart of the atom, Rutherford devised an ingenious experiment. He used the alpha particles emitted by a radioactive source to bombard a thin gold foil. Surrounding the gold foil was an object coated with a zinc sulfide (ZnS) sheet that would emit a flash of light whenever an alpha particle passed through. Contrary to the anticipated scattering of alpha particles, Rutherford observed that the majority passed through the gold foil with minimal deflection, confirming his theory of a compact, centrally-located nucleus.
Rutherford's atomic model bears resemblance to a miniaturised solar system, with the positively-charged nucleus at the center and electrons orbiting it in various directions. These positively-charged particles in the nucleus are known as protons, which, despite carrying an equal but opposite charge to electrons, are considerably larger and heavier. As a result, atoms maintain electrical neutrality—the number of protons matches the number of electrons, resulting in a net charge of zero.
In 1932, James Chadwick further enhanced our understanding of atomic structure with his discovery of the neutron. This neutral particle, found in the nucleus alongside protons, offered an explanation for the cohesiveness of the atom's dense nucleus
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Bohr Model
In 1913, Bohr and Rutherford introduced an updated atomic model that, like Rutherford's model, depicted a small, dense nucleus enveloped by orbiting electrons. However, Bohr proposed a significant deviation from the planetary model: he asserted that electrons resided in 'quantised' shells, thereby providing a stable orbit around the nucleus. This quantisation implied that electrons could only occupy specific energy levels within the atom.
The Rutherford model had a significant flaw in that the motion of electrons was unstable. According to classical mechanics and electromagnetic theory, an electron would release electromagnetic radiation and lose energy while orbiting the nucleus, ultimately spiraling inwards and colliding with the nucleus.
Bohr's model rectified this instability by explaining that electrons could jump between energy levels by emitting or absorbing energy in fixed quanta (units of energy). For instance, an electron moving to an orbital closer to the nucleus would emit energy equivalent to the difference in energy levels. Conversely, to leap to a higher orbital, an electron would absorb light energy equal to the energy difference.
Schrödinger Model
In 1926, Erwin Schrödinger advanced a new atomic model that better explained the behaviour of electrons around the atomic nucleus. Using mathematical equations, Schrödinger described the probability of locating an electron in a certain position around the atom. This quantum mechanical model of an atom, rather than detailing the exact path of an electron, predicted the probability of finding an electron in a specific location. This model is typically represented as an electron cloud surrounding a nucleus.
Summary
- An atom consists of a small, dense nucleus composed of protons (which carry a positive charge) and neutrons (which are neutral).
- Orbiting the nucleus is a 'cloud' of negatively charged electrons.
- An atom maintains electrical neutrality due to the balance between the number of electrons and protons.
- The electrostatic attraction between the negatively charged electrons and the positively charged nucleus holds the atom together, preventing its disintegration.