Problem 79 - Chemistry: The Central Science

Question

Describe a major contribution to science made by each of the following scientists: (a) Dalton, (b) Thomson, (c) Millikan, (d) Rutherford.

Step-by-Step Solution

Answer

(a) Dalton's major contribution to science was the development of modern atomic theory, where he proposed that all matter is made up of indestructible atoms that form compounds and that all atoms of a particular element have the same properties. (b) J.J. Thomson discovered the electron by conducting experiments with cathode rays, leading to the understanding that atoms were not indivisible and composed of smaller particles. (c) Robert Millikan precisely measured the electrical charge of the electron through his oil-drop experiment, which allowed for a greater understanding of atomic structure and electron behavior. (d) Ernest Rutherford, through his gold foil experiment, proposed the nuclear model of the atom, where the majority of the atom's mass and positive charge are concentrated in a central nucleus, paving the way for the development of nuclear physics.

Step 1: 1. John Dalton (a)

John Dalton was an English chemist, physicist, and meteorologist. His most significant contribution to science was the development of the modern atomic theory. Dalton suggested that all matter is made up of indivisible and indestructible atoms, which come together to form compounds. He also proposed that all atoms of a particular element have the same mass, size, and chemical properties. This groundbreaking idea laid the foundation for our understanding of the chemical behavior of elements and the composition of matter.

Step 2: 2. J.J. Thomson (b)

Joseph John (J.J.) Thomson was an English physicist, responsible for the discovery of the electron. Thomson conducted a series of experiments using cathode rays, which led him to conclude that they were composed of negatively charged particles, which he called "corpuscles" (later known as electrons). This discovery was revolutionary, as it proved that atoms were not indivisible and were composed of smaller particles. Thomson's work paved the way for the development of the electron's charge and mass and further understanding of atomic structure.

Step 3: 3. Robert Millikan (c)

Robert Millikan was an American physicist who is best known for his oil-drop experiment (1909-1913) in which he precisely measured the electrical charge of the electron. The experiment involved balancing the gravitational force on oil droplets with an electric force applied between two parallel plates. By carefully manipulating the droplets and voltages, Millikan was able to determine the charge on individual droplets and ultimately found that the charge was always a whole number multiple of a specific value, now known as the elementary charge. This discovery allowed for a greater understanding of the structure of atoms and the behavior of electrons, which was essential for the development of modern physics and chemistry.

Step 4: 4. Ernest Rutherford (d)

Ernest Rutherford was a New Zealand-born British physicist who is known for his work on the structure of the atom. In his famous gold foil experiment (1911), Rutherford bombarded very thin gold foil with alpha particles and observed their scattering patterns on a screen. Contrary to the expectation based on the then-current plum pudding model of the atom, some alpha particles were scattered at large angles or even bounced back, indicating the presence of a much denser and positively charged region in the atom. Based on these results, Rutherford proposed the nuclear model of the atom, in which the majority of the atom's mass and positive charge are concentrated in a small central nucleus. The nuclear model paved the way for the development of nuclear physics and our understanding of atomic structure.

Key Concepts

John Dalton

John Dalton, an English educator born in the 18th century, forever changed science with his atomic theory. Before Dalton, the concept of the atom was vague. It lacked a solid scientific foundation. Dalton's atomic theory proposed that all matter is composed of tiny, indivisible particles, called atoms.
He suggested that atoms of a given element are identical in mass and properties.
This was an essential step forward. It formed the basis for understanding why elements interact the way they do in chemical reactions.

• Dalton asserted that atoms from different elements could combine to form compounds.
• He proposed that chemical reactions involve the rearrangement of these indivisible atoms.

This groundbreaking theory was pivotal in transforming chemistry into a modern science.

J.J. Thomson

J.J. Thomson, an English scientist, made a pivotal discovery in the late 19th century: the electron.
Using cathode ray tubes, he observed rays that were deflected by electrical and magnetic fields. This deflection indicated that the rays were composed of negatively charged particles, which he called "corpuscles." Later, these particles were renamed electrons.

• Thomson's work challenged the then-prevailing notion that atoms were indivisible.
• He demonstrated that atoms contain smaller, subatomic particles.
• This discovery spurred further research into the atom's internal structure.

Thanks to Thomson, scientists could explore atomic structure more deeply, leading to today’s understanding of electricity and chemical bonding.

Robert Millikan

American physicist Robert Millikan is renowned for his meticulous determination of the electron's charge.
His famous oil-drop experiment, conducted between 1909 and 1913, played a key role in this discovery.
By balancing the gravitational force on tiny oil droplets with an applied electric force, Millikan could measure the charge of individual oil droplets.

• This experiment showed that the charge was quantized, meaning it could only occur in whole-number multiples.
• Millikan's work provided the exact value of the elementary charge of an electron.
• Determining the electron's charge was crucial for advancing our understanding of atomic physics.

His discoveries helped confirm the discrete nature of electric charge, which became a foundational concept in modern physics.

Ernest Rutherford

Ernest Rutherford, a physicist from New Zealand, is celebrated for his gold foil experiment.
Conducted in 1911, this experiment radically changed how scientists viewed the atom.
By directing alpha particles at a thin sheet of gold foil and observing their scattering, Rutherford unearthed surprises. Many particles passed straight through, but some were deflected at unexpected angles.

• The results suggested a dense, positively charged center within the atom, which Rutherford called the nucleus.
• Rutherford’s nuclear model introduced the idea that most of an atom’s mass is in its nucleus.
• This finding was instrumental in developing nuclear physics and understanding atomic structure.

His model laid the groundwork for further research on nuclear reactions and atomic energy.