Question

Scientists such as Dalton, Thomson, and Rutherford proposed important models, which were ultimately challenged by later technology. What do we know to be false in Dalton's atomic model? What was missing in Thomson's model of the atom? What was Rutherford's experiment that led to the current model of the atom?

Short answer

Dalton's model incorrectly stated that atoms are indivisible. Thomson's model lacked a nucleus. Rutherford's gold foil experiment revealed the nucleus.

Step-by-step solution

- Identify False Statements in Dalton's Atomic Model

Dalton proposed that atoms are indivisible and indestructible. We know now that this is false because atoms can be divided into subatomic particles (protons, neutrons, and electrons) and they can also be split in nuclear reactions.

- Identify What Was Missing in Thomson's Model

Thomson's model, known as the 'plum pudding' model, proposed that electrons were embedded within a positively charged 'pudding'. What was missing in this model was a nucleus. Thomson did not account for the existence of a central positively charged nucleus in which most of the atom's mass is concentrated.

- Explain Rutherford's Experiment

Rutherford conducted the gold foil experiment, where he directed alpha particles at a thin sheet of gold foil. Most particles passed through, but some were deflected at large angles. This led him to conclude that the atom has a small, dense, positively charged nucleus at its center, with electrons orbiting around it. This overturned the previous models and led to the current understanding of the atomic structure.

Key concepts

Dalton's Atomic Model

John Dalton proposed one of the earliest models of the atom in the early 19th century. His model stated that atoms are indivisible and indestructible particles, and that all atoms of a given element are identical in mass and properties. However, we now know that this is not true.

Atoms can be split into smaller particles called subatomic particles. These include protons, neutrons, and electrons. This discovery contradicted Dalton's belief that atoms are indivisible. We can also alter atoms in nuclear reactions, further proving they are not indestructible.

The discovery of isotopes showed that not all atoms of a given element are identical in mass. Isotopes are atoms of the same element that have different numbers of neutrons, and therefore different masses. Therefore, while Dalton's model laid important groundwork, modern science has revised many of his ideas.

Thomson's Plum Pudding Model

J.J. Thomson's plum pudding model was proposed in 1904. In this model, the atom was described as a 'pudding' made of positive charge with negatively charged 'plums' or electrons embedded within it. This was a significant step forward as it introduced the existence of electrons.

However, Thomson's model had its shortcomings. It failed to recognize the existence of a central nucleus. Later experiments showed that most of the atom's mass and all of its positive charge are concentrated in a small, central nucleus, which Thomson's model did not account for.

The absence of a nucleus meant Thomson's model could not explain why atoms are mostly empty space and why electrons orbit around a nucleus rather than being embedded in a 'pudding'. Despite its flaws, the plum pudding model was crucial in advancing the understanding of atomic structure.

Rutherford's Gold Foil Experiment

Ernest Rutherford's gold foil experiment, conducted in 1909, was a groundbreaking study that dramatically altered our understanding of the atom. In the experiment, Rutherford and his team directed a beam of alpha particles at a very thin sheet of gold foil. They observed that most of the particles passed through the foil, but some were deflected at very large angles.

This observation was surprising because, according to the plum pudding model, the alpha particles should have passed through with very little deflection. Rutherford concluded that the atom must have a small, dense, positively charged nucleus at its center. This nucleus contained most of the atom's mass. The electrons orbit this nucleus, similar to planets orbiting the sun.

This led to the modern atomic model in which the atom is mostly empty space with a tiny, dense nucleus. Rutherford's model was a significant leap forward and laid the foundation for future discoveries in atomic physics.