Atomic Structure

How the Modern Model of the Atom Developed

Atomic Theory

Evidence for Smaller Particles Inside Atoms

Dalton's atomic theory laid the foundation for nineteenth-century chemistry. However, by the early twentieth century, experimental evidence began to challenge some of his key ideas. Results showed that atoms were not indivisible, but could be broken down into smaller particles.

J. J. Thomson demonstrated that when metals are heated strongly, they emit tiny negatively charged particles. These particles could be deflected by electric and magnetic fields, showing they carried charge. He concluded that they originated from within atoms themselves.

Further research established that these particles, later called electrons, have a mass of around 1/2000 of a hydrogen atom. At the same time, the work of Marie Curie and Pierre Curie on radioactivity showed that some atoms could emit heavier, positively charged alpha particles, providing further evidence of internal atomic structure.

J. J. Thomson

The Scientist Linked to the Discovery of the Electron

J. J. Thomson's work helped show that atoms were not indivisible. His experiments showed that tiny negatively charged particles could be emitted from matter and deflected by electric and magnetic fields.

These particles were later called electrons. Their discovery provided a major challenge to Dalton's original idea of the atom as a solid, indivisible particle.

Ernest Rutherford

The Scientist Who Changed the Model of the Atom

Ernest Rutherford was born near Nelson in New Zealand, one of twelve children. From early on, he showed strong academic ability, studying at Nelson College where he was also active in rugby and later became head boy. He went on to graduate from Canterbury College in Christchurch.

Although he briefly considered becoming a schoolteacher, he instead moved to England to pursue scientific research. His talent was quickly recognised. In 1908, Rutherford was awarded the Nobel Prize in Chemistry, although he reportedly found it frustrating that it was not in physics, which he viewed as the more fundamental science.

He is often quoted as saying that "all science is either physics or stamp collecting," a statement that reflects both his confidence and his perspective on scientific disciplines.

Despite this, Rutherford's impact on science was immense. He is widely regarded as one of the greatest experimental physicists and chemists in history. Albert Einstein referred to him as "the second Newton," highlighting the scale of his contribution to our understanding of the atom.

He was knighted in 1914, became Director of the Cavendish Laboratory in Cambridge in 1919, and was later given the title Lord Rutherford of Nelson in 1931.

Gold Foil Experiment

Rutherford's Nuclear Model

Ernest Rutherford carried out an experiment in which alpha particles were directed at thin gold foil. Most of the particles passed straight through, while a small number were deflected and a very few bounced back.

These observations showed that the atom is not a uniform solid. Instead, it contains a small, dense central nucleus, with most of the atom consisting of empty space. The surrounding region is so diffuse that many particles pass through without interaction.

From this, Rutherford proposed a new model of the atom, developed through his work with a team of scientists in Cambridge.

Electron Energy Levels

From Rutherford to Bohr and Quantum Theory

Ernest Rutherford could not explain how electrons remained in orbit around the nucleus. Classical physics suggested they should be pulled in by the strong attraction of the nucleus. Building on this, Niels Bohr proposed a more detailed atomic model with defined electron energy levels.

It was later recognised that, although Bohr's model captured an important idea, electron behaviour was more complex. Electrons showed properties of both particles and waves, making them difficult to describe using a single model.

A clearer explanation was developed through the work of Erwin Schrödinger, based on ideas first suggested by Louis de Broglie. Light was already known to display unusual behaviour, sometimes acting like particles and at other times like waves.

For example, light can behave like a stream of particles when reflected from a surface, but can also spread out like waves, similar to ripples in water. De Broglie proposed that this dual behaviour also applies to electrons, forming the basis of modern quantum theory.

Niels Bohr

The Scientist Who Proposed Defined Electron Energy Levels

Niels Bohr was born into a wealthy family in Copenhagen, with a home overlooking Christiansborg Castle, where the Danish parliament once met. At school, he was considered a good but not exceptional student.

Outside the classroom, he was a strong football player, although he did not reach the same level of recognition as his brother Harald, who played for Denmark and won a silver medal at the 1908 Olympics.

Bohr's academic potential became more apparent when he travelled to England on a Carlsberg grant. He initially joined J. J. Thomson's research group in Cambridge, but soon moved to Manchester to work with Ernest Rutherford. This period was crucial in shaping his scientific thinking.

In 1916, he returned to Copenhagen as Professor of Physics and later founded the Institute of Theoretical Physics in 1921. Bohr became one of the leading figures in twentieth-century science and was awarded the Nobel Prize in Physics in 1922 for his contributions to atomic structure and quantum theory.

Quantum Model

Schrödinger's Electron Cloud Model

Erwin Schrödinger developed a mathematical model to describe the electronic structure of the hydrogen atom. His work showed that electrons can exist in a much wider range of energy states than previously thought and do not behave as fixed particles, but as a cloud of negative charge.

This electron cloud has a variable density and can take on different shapes. Because of this, the term "state" is used rather than "position," as the model does not link an electron's energy directly to a fixed distance from the nucleus.

Many such states are possible, and simple diagrams are no longer sufficient to describe the atom. Instead, these states are labelled using quantum numbers. The main energy levels are defined by principal quantum numbers (1, 2, 3, 4), with energy increasing as the number increases.

Within each level, there are further subdivisions known as orbitals, labelled s, p, d and f. However, not all orbitals are possible for every energy level, as there are specific restrictions based on the value of the principal quantum number.

Erwin Schrödinger

The Scientist Who Mathematically Described the Electron

Erwin Schrödinger, alongside Max Born, played a key role in developing the theoretical framework for understanding the nature of the electron. Although he once remarked that he disliked quantum physics and regretted being involved in it, he continued throughout his life to contribute significantly to the development of the theory.

Schrödinger openly opposed Nazi anti-Semitism and left Berlin in 1933, the same year he was awarded the Nobel Prize. He initially planned to settle in Oxford, but this did not materialise after university authorities disapproved of his unconventional personal life, including living with both his wife and a mistress.

By 1940, he had established himself in Dublin, where he remained for 17 years. During this time, he wrote several influential works, including What is Life?, which later inspired James Watson in his work on the structure of DNA.

Schrödinger also maintained a lifelong interest in Hindu philosophy, reflecting the breadth of his intellectual pursuits.

Quantum Numbers

Energy States, Orbitals and Electron Spin

Each energy state in a hydrogen atom corresponds to a solution of Erwin Schrödinger's equation. A single equation can produce multiple solutions, similar to how sin x = 0 gives several possible values of x. These solutions are represented using quantum numbers, with the principal quantum number acting as a key label.

Schrödinger's model describes the electron in three dimensions, requiring additional quantum numbers to define each state fully. These values together determine the energy and behaviour of the electron within the atom.

Wolfgang Pauli extended this idea by stating that only one electron can occupy a specific energy state. If two electrons share the same orbital, they must have opposite spins.

A complete and exact description of energy levels is only possible for hydrogen. For atoms with more than one electron, the equations become too complex to solve directly. Instead, the structure is based on experimental evidence, which shows that similar patterns apply across all elements.

Louis de Broglie

The Scientist Who Linked Electrons with Wave Behaviour

Louis de Broglie was born into an aristocratic French family and was initially expected to pursue a career in diplomacy. He graduated from the Sorbonne in 1910 with a degree in history. However, after considerable thought, he made the decision to move into theoretical physics, a choice that would define his scientific career.

His work had a profound impact, particularly in developing ideas that linked waves and particles, and he was awarded the Nobel Prize in 1929 in recognition of these contributions. De Broglie later became professor of theoretical physics at both the Henri Poincaré Institute and the Sorbonne.

He was a prolific writer, publishing 25 scientific works and exploring the philosophical aspects of modern physics in depth. In 1960, following the death of his elder brother, he inherited the title of the seventh Duc de Broglie, reflecting both his scientific and aristocratic legacy.

Wolfgang Pauli

The Scientist Behind the Pauli Exclusion Principle

Wolfgang Pauli was born in Vienna in 1900 and was quickly recognised as a prodigy. His academic progress was exceptional, and by the age of 21 he had already completed a doctorate, writing a thesis on quantum theory related to ionised hydrogen.

By 1928, he had become Professor of Theoretical Physics in Zurich, where he made major contributions to quantum mechanics, particularly in developing ideas around electron spin. Despite his theoretical strength, his personal life was less stable.

Following a brief and unhappy marriage, he experienced a nervous breakdown and later corresponded extensively with Carl Jung, exchanging ideas on science and psychology.

At the start of the Second World War, Pauli moved to the United States and became a naturalised citizen. He was awarded the Nobel Prize in 1945 for his contributions to physics.

Interestingly, his experimental skills were considered weak, and colleagues often joked about the "Pauli effect," where equipment would seem to fail simply when he was nearby.

Practice Question

Electron Shells and Orbitals

You have already learned that s-, p- and d-orbitals can hold two, six and ten electrons respectively.

The total number of electrons that can be held in the various shells is given by the formula 2n².

(a) Deduce the total number of electrons that can be held in the fourth and fifth shells.

(b) The extra numbers of electrons are possible because level 4 has f-orbitals and level 5 has f- and g-orbitals. How many different f- and g-orbitals are there?

Show Answer

Model Answer

Shell Capacity and Orbital Numbers

(a) Fourth and fifth shells

The maximum number of electrons in a shell is calculated using:

2n²

For the fourth shell, n = 4.

2 × 4² = 2 × 16 = 32

Therefore, the fourth shell can hold a maximum of 32 electrons.

For the fifth shell, n = 5.

2 × 5² = 2 × 25 = 50

Therefore, the fifth shell can hold a maximum of 50 electrons.

(b) Number of f- and g-orbitals

Each orbital can hold a maximum of 2 electrons.

An f-subshell can hold 14 electrons.

14 ÷ 2 = 7

Therefore, there are 7 different f-orbitals.

A g-subshell can hold 18 electrons.

18 ÷ 2 = 9

Therefore, there are 9 different g-orbitals.

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Practice Question

Einsteinium Isotopes

Einsteinium has atomic number 99 and isotopes with mass numbers 253 and 254.

(a) Give the numbers of protons, neutrons and electrons in an atom of einsteinium-253.

(b) What is the difference in the number of particles present in the atoms of einsteinium-253 and einsteinium-254?

(c) Use your answer to part (a) to decide how many 5f-electrons there are in an atom of einsteinium.

Show Answer

Model Answer

Subatomic Particles and Electron Configuration

(a)

The atomic number of einsteinium is 99, so it has:

99 protons and 99 electrons.

Number of neutrons = mass number − atomic number

253 − 99 = 154 neutrons

(b)

The difference between mass numbers 254 and 253 is 1.

This corresponds to a difference of one neutron.

(c)

Einsteinium is in the f-block.

Its electron configuration includes filling of the 5f subshell.

The number of 5f-electrons in einsteinium is 11.

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Practice Question

Alpha Decay of Einsteinium

All elements with very high atomic numbers have nuclei that break down spontaneously. This is the cause of radioactivity.

In the first step of its decay, einsteinium releases alpha radiation, which consists of particles containing two protons and two neutrons.

(f) Suggest what happens to an atom of einsteinium when it loses an alpha particle.

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Model Answer

Effect of Losing an Alpha Particle

An alpha particle contains 2 protons and 2 neutrons.

When einsteinium loses an alpha particle, its nucleus loses 2 protons.

Therefore, its atomic number decreases by 2.

Einsteinium has atomic number 99, so the new atomic number becomes:

99 − 2 = 97

The element with atomic number 97 is berkelium.

The mass number also decreases by 4, because the alpha particle contains a total of four nucleons.

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Further Reading

How X-Rays Reveal the Structure of Metal Crystals

Want to see how atomic structure links to real experimental evidence? Read this related guide on X-ray diffraction and how scientists use X-rays to investigate the structure of metal crystals.

Read the Metal Crystals Blog