Beyond Science returns with this A Level Atomic Structure blog, exploring atomic models. Here, we’ll cover each aspect of Atomic Models, so you can confidently tackle your A Level exams. We will explore a variety of different models including: the Solid Sphere Model, the Plum Pudding Model, the Nuclear Model and more.
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Solid Sphere Model
John Dalton, early 1800s
John Dalton suggested that substances were made of small hard spheres called atoms. He said that the atoms could not be divided into anything smaller.
New substances were formed by the addition of other atoms and from the rearrangement of
Dalton also hypothesised that the atoms of each element were different sizes and each with a specific atomic weight.
These ideas were based on Dalton’s observations that particles of gases combined in specific proportions.
Dalton’s ideas are still useful today. Scientists use a model similar to the solid sphere model
to describe the arrangement of particles in solids, liquids and gases.
Discovery of Radioactivity
Henri Becquerel, late 1800s
Henri Becquerel was awarded a Nobel prize, along with his students Marie Curie and Pierre Curie, for his work in discovering radioactivity.
Becquerel’s findings showed that atoms are not indivisible, as was previously thought. Particles are emitted from within unstable nuclei.
Becquerel detected radioactivity due to its effects on photographic film.
Marie Curie decided on the term radioactivity to describe their findings, after having studied the emitting properties of the element radium.
The Plum Pudding Model
Joseph J Thomson, early 1900s
Whilst carrying out extensive experiments involving cathode ray tubes, Joseph J Thomson discovered negative particles which he called electrons.
It was well known that atoms have no overall charge and so Thomson suggested that if atoms contain negatively charged particles, there must also be a positively charged component.
He modified the solid sphere model of the atom to a sphere of positive charge with negatively charged electrons embedded within it. This was named the plum pudding model, referencing the currants scattered throughout a Christmas pudding.
Ernest Rutherford, 1909 – 1911
Ernest Rutherford and his team, Hans Geiger and Ernest Marsden, tested out the plum pudding model. They fired positively charged alpha particles at thin pieces of gold foil. Most of the alpha particles went straight through with no deflection. However, a small number of them were scattered in different directions.
The experiment led to the conclusion that the mass of the atom is concentrated at the centre,
which they called the nucleus. It also told them that the nucleus has a positive charge. This nuclear model replaced the plum pudding model.
Later experiments determined that the positive charge of the nucleus is due to the presence of smaller particles called protons.
Niels Bohr, 1913
For many scientists, Rutherford’s nuclear model had flaws. The nuclear model did not explain
why the cloud of negatively charged electrons does not collapse into the positive nucleus.
Niels Bohr suggested that the electrons orbit the nucleus in levels (shells) and the levels are
at fixed distances from the nucleus; each level has a fixed quantity of energy.
Bohr also suggested that when atoms absorb or emit electromagnetic energy, electrons
would move from one level to another. The difference in energies between these levels
being equivalent to the energy of the absorbed or emitted electromagnetic wave.
These calculations came about after experiments observing light being emitted from
Modern Atomic Model
James Chadwick, 1932
Experiments by James Chadwick in 1932 provided evidence for the model that had already been accepted for nearly 20 years.
He proved that neutrons exist within the nucleus of the atom.
The discovery of the neutron accounted for discrepancies in the atomic weights of elements and explained why the positively charged protons in the nucleus of the atom do not repel and separate from each other.
This model, the atomic model, is the model that is mainly referenced today.
The Quantum Model
Schrödinger et al, 1926
The quantum model is the most accurate model of the atom we have to date. It explains some of the observations that Bohr’s model does not, such as electrons occupying the same level but having different quantities of energy.
The quantum model is not the result of work from any one scientist but combines the ideas of several: Louis de Broglie, Werner Heisenberg, Max Planck and Erwin Schrödinger to
name a few.
However, Schrödinger was the first to formulate an equation that calculates the probability of finding an electron in a particular position at any given time. He also used his now
famous thought experiment, Schrödinger’s cat, as a teaching tool to illustrate that particles affect each other.
The quantum model is complicated and very difficult to visualise and represent in diagrams. As a result, scientists tend to use whichever model is most suitable for their context and purpose.
Through various experiments, it has been shown that not all electrons in a particular energy level have the same quantity of energy.
According to the quantum model, electrons behave like waves, they can be diffracted just like light.
The quantum model describes each energy level as having sublevels and within each sublevel, there are orbitals.
An orbital is a space in which an electron with a particular quantity of energy is likely to be found at any given time.
We hope that’s demystified A Level Atomic Structure for you! You can read more of our blogs here.
Atomic Structure Questions
1. The model that describes the atom as a solid, positively charged sphere embedded with negatively charged electrons is called the:
- a) Christmas pudding model
- b) Plum pudding model
- c) Fig pudding model
2. Who suggested that electrons can only orbit the nucleus in levels at fixed distances from the nucleus?
- a) Neils Bohr
- b) Ernest Rutherford
- c) James Chadwick
3. Which discovery, in 1932, helped explain why positively-charged protons in the nucleus do not repel each other?
- a) Neutron
- b) Weak nuclear interaction
- c) Alpha radiation