Question

What contribution did these scientists make to atomic models of the atom? (a) Dalton (b) Thomson (c) Rutherford

Short answer

Dalton proposed that atoms are indivisible particles, Thomson discovered electrons and the 'plum pudding' model, while Rutherford introduced the nuclear model of the atom with a central nucleus and electron orbits.

Step-by-step solution

Understanding the contributions of Dalton

John Dalton, in the early 19th century, proposed the first modern atomic theory. He suggested that all matter is made up of tiny, indivisible particles called atoms, which cannot be created or destroyed. Dalton's theory also stated that all atoms of a given element are identical in mass and properties, and that compounds are combinations of different types of atoms.

Evaluating Thomson's input

J.J. Thomson discovered the electron in 1897 through his cathode ray tube experiments. His atomic model, known as the 'plum pudding' model, described the atom as a positively charged sphere with negatively charged electrons scattered throughout, like plums in a pudding. This model was the first to incorporate subatomic particles and introduced the concept of the atom as a divisible entity.

Examining Rutherford's discovery

Ernest Rutherford, through his gold foil experiment in 1909, refuted Thomson's model by showing that atoms have a small, densely packed nucleus with a positive charge, surrounded by a cloud of electrons. His findings led to the Rutherford model of the atom, also known as the nuclear model, which depicts the atom as having a central nucleus around which electrons orbit.

Key concepts

Dalton's Atomic Theory

John Dalton, regarded as the father of modern atomic theory, revolutionized our understanding of the small-scale structure of matter in the early 19th century. Before Dalton, the idea of atoms was more philosophical than scientific, but he provided the first scientific theory of atoms.

Dalton's atomic theory laid out several key postulates:

• All matter is composed of indivisible particles called atoms.
• Atoms of the same element are identical in size, mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed when combined or rearranged in chemical reactions.
• Compounds are formed by the combination of two or more different kinds of atoms.

While some aspects of Dalton's theory have been refined, his contribution built the foundation for modern chemistry and atomic physics. His work influenced subsequent models and established the law of multiple proportions in chemical reactions.

Thomson's Cathode Ray Experiment

J.J. Thomson's groundbreaking experiment with cathode rays led to the discovery of the electron and consequently transformed the scientific community's comprehension of atomic structure. Conducted in 1897, Thomson's cathode ray experiment involved passing an electric current through a gas that was under low pressure, using a cathode ray tube.

Thomson observed that:

• Cathode rays were deflected by a magnetic field, suggesting that they were charged particles.
• The rays always traveled from the cathode to the anode, thus they were composed of negatively charged particles, which he named electrons.
• Electrons were much lighter than atoms and a part of them, indicating that atoms could in fact be divided into smaller parts.

Thomson's findings contradicted Dalton's model of the indivisible atom and led to the proposal of the 'plum pudding' model. In this model, he visualized the atom as a sphere of positive charge with electrons ('plums') scattered throughout, like raisins in a pudding. This new atomic model opened the way for further developments in the field.

Rutherford's Gold Foil Experiment

Ernest Rutherford's gold foil experiment, conducted in 1909, profoundly altered the then-accepted 'plum pudding' model of the atom. He did this using a thin sheet of gold foil and bombarding it with alpha particles, which are positively charged and relatively heavy.

Rutherford's observations were surprising:

• Most alpha particles passed straight through the foil, suggesting that atoms are mostly empty space.
• Some alpha particles were deflected at small angles, and a few even bounced back, demonstrating that there must be a concentrated positive charge causing this repulsion.

Rutherford concluded that the atom's positive charge was not spread out evenly as Thomson had suggested but was concentrated in a tiny, dense nucleus at the center. This nucleus was surrounded by the electron cloud, with electrons orbiting around it. Rutherford's model thus depicted the atom as having a small central nucleus with a vast empty space for electrons, analogous to planets orbiting the sun. This nuclear model of the atom laid the groundwork for future research, leading to the quantum mechanical model.