2.4 - Estudo do Átomo - Modelo Atômico de Thomson (Modelo do Pudim de Passas)

Summary

This lesson explores Thomson's atomic model, which revised Dalton's theory by introducing the electron as a subatomic particle. Through experiments with cathode rays, Thomson discovered that these rays consisted of negatively charged particles, leading him to propose the 'plum pudding' model. In this model, electrons were embedded in a positively charged sphere, balancing the atom's overall charge. Thomson also established the relationship between the charge and mass of electrons. Though his model was later refuted by further discoveries, Thomson’s work was instrumental in advancing our understanding of atomic structure and the electron's role within it.

Takeaways

• 😀 Dalton's atomic model proposed that atoms were indivisible spheres, but this was later proven incorrect by the discovery of the electron.
• 😀 J.J. Thomson conducted cathode ray experiments and discovered that cathode rays were negatively charged particles, later identified as electrons.
• 😀 Thomson's discovery of the electron challenged Dalton's model, as he demonstrated that atoms contain smaller subatomic particles.
• 😀 The electron was found to be a universal particle, present in all materials, and had a mass-to-charge ratio much smaller than the lightest known atom, hydrogen.
• 😀 Thomson proposed a new atomic model, known as the 'pudding model,' where atoms were spheres of positive charge with electrons embedded within them.
• 😀 In Thomson's model, the positive charge was assumed to be evenly distributed throughout the atom, with electrons arranged to minimize repulsion.
• 😀 Thomson's model suggested that the atom was mostly empty space, with electrons spread out homogeneously within a positively charged mass.
• 😀 He proposed that the total positive charge in the atom balanced the total negative charge of the electrons, making the atom electrically neutral.
• 😀 Thomson calculated the mass-to-charge ratio of the electron, which was later refined by other scientists, including Millikan in his oil drop experiment.
• 😀 Subsequent experiments by Rutherford and others refuted Thomson’s model, showing that the positive charge is concentrated in a nucleus and that electrons are organized in distinct orbits.

Q & A

What was the key difference between Dalton's and Thomson's atomic models?

Dalton's model proposed that atoms were indivisible spheres, while Thomson's model introduced the idea of atoms being made up of a positive sphere with negatively charged electrons embedded in it.

How did Thomson demonstrate the existence of the electron?

Thomson demonstrated the existence of the electron through his experiments with cathode rays, where he observed that the rays were deflected by electric and magnetic fields, indicating they carried a negative charge.

What was the 'pudding model' of the atom proposed by Thomson?

Thomson's 'pudding model' described the atom as a sphere of positive charge with negatively charged electrons embedded within it, much like raisins in a pudding.

What were the three key conclusions that Thomson made from his experiments?

Thomson concluded that cathode rays were negatively charged, that they were particles (not radiation), and that these particles were universal, present in all materials.

How did Thomson explain the neutral nature of the atom in his model?

Thomson hypothesized that the positive charge in the atom balanced out the negative charge of the electrons, making the atom overall neutral.

What was the significance of the charge-to-mass ratio of the electron discovered by Thomson?

Thomson discovered that the charge-to-mass ratio of the electron was significantly smaller than that of a hydrogen atom, suggesting the electron was a much smaller particle present in all atoms.

Why did Thomson propose that the positive charge in the atom was 'permeable'?

Thomson proposed the positive charge in the atom was 'permeable' because he couldn't identify a solid, massive positive particle and speculated that the positive charge was a soft, jelly-like substance through which electrons could pass.

What did Thomson mean by the atom being 'neutral' in his model?

In Thomson's model, 'neutral' meant that the positive charge of the atom was balanced by the negative charges of the electrons, so the atom had no overall charge.

What was the flaw in Thomson's idea of the atom’s 'permeable' positive charge?

Later studies, such as Rutherford's, showed that the positive charge is not 'permeable' but concentrated in a dense nucleus, disproving Thomson’s assumption of a diffuse positive charge.

How did Thomson's work contribute to the discovery of the electron's charge and mass?

Thomson's work on the charge-to-mass ratio of the electron provided a foundation for later experiments, such as those by Millikan, who was able to determine the exact charge of the electron, allowing for a calculation of its mass.