Atomic Theory

Atomic Theory

Middle School

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Learning Objectives

• Understand what the modern atomic theory says about atoms.

• Explore how our ideas about atoms have changed over a long time.

• Learn about the important discoveries made by different scientists.

• Compare different models of the atom, like the Plum Pudding and Nuclear models.

Key Vocabulary

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Atomos

Democritus believed matter was made of tiny, indivisible, solid spheres he called atomos.

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Atom

The smallest particle of an element, with electrons around a nucleus of protons and neutrons.

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Molecule

A particle made of two or more atoms chemically bonded together, which may be atoms of the same element or different elements.

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Nucleus

The massive, positively charged center of an atom, containing both protons and neutrons.

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The Earliest Idea of Atoms

• The first concept of the atom came from the Greek philosopher Democritus.

• He theorized that matter is made of tiny, indivisible spheres called '''atomos.'''

• The word '''atomos''' is Greek for '''uncuttable,''' reflecting this early idea.

• Different types of matter were thought to be made of different atoms.

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Multiple Choice

What does the term 'atomos,' coined by Democritus, translate to?

• Moves through space

• A small seashell

• A tiny sphere

• Uncuttable or indivisible

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Dalton's Atomic Theory

• Dalton's Solid Sphere Model states that all matter consists of indivisible atoms.

• Atoms of the same element are identical in their mass and properties.

• Compounds are formed when atoms of different elements combine in fixed, simple ratios.

• Chemical reactions involve the rearrangement of atoms, not their creation or destruction.

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Multiple Choice

According to Dalton's Atomic Theory, what happens to atoms in a chemical reaction?

• They are destroyed and new atoms are created.

• They change their mass and size.

• They are broken down into smaller particles.

• They are rearranged, but never created or destroyed.

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Thomson's Plum Pudding Model

• J.J. Thomson discovered the electron, proving atoms have smaller parts.

• ​He proposed the Plum Pudding Model to describe this new atomic structure.

• The model described the atom as being a sphere of positive charge.

• Electrons were embedded in the sphere, much like plums in a pudding.

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Multiple Choice

How did J.J. Thomson's Plum Pudding Model describe the arrangement of charges in an atom?

• A positive nucleus was surrounded by negative electrons.

• The atom was a solid, indivisible sphere with no charge.

• Positive and negative charges were in separate, concentric orbits.

• Negative electrons were embedded within a sphere of positive charge.

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The Nuclear and Planetary Models

Nuclear Model (1911)

• Ernest Rutherford discovered that atoms have a tiny, dense, positively charged center called the nucleus.

• His model showed that electrons move around this nucleus in mostly empty space (not in fixed paths).

• This conclusion came from his gold foil experiment, which disproved the Plum Pudding model.

Planetary Model (1913)

• ​Niels Bohr improved on Rutherford's model by suggesting electrons follow specific orbits around the nucleus.

• ​​He proposed that electrons are arranged in concentric circular paths, much like planets orbiting the sun.

• ​According to this model, electrons can only exist in these fixed energy levels, not in between.

Multiple Choice

What was the key difference between Rutherford's Nuclear Model and Bohr's Planetary Model?

• Rutherford's model embedded electrons in a positive sphere.

• Bohr's model placed electrons in specific, concentric circular orbits.

• Rutherford's model included a nucleus, while Bohr's did not.

• Bohr's model described a 'cloud' of electrons.

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The Modern Quantum Model

• Erwin Schrödinger introduced the Quantum Model of the atom in 1926.

• ​This model says we cannot know the exact path of an electron.

• An electron's location is described as a "cloud" around the nucleus.

• The nucleus has protons and neutrons, surrounded by an electron cloud.

Multiple Choice

According to Erwin Schrödinger's Quantum Model, how is the location of an electron described?

• As a particle that does not move.

• As a fixed, predictable path like a planet's orbit.

• As being embedded within the nucleus itself.

• As a 'cloud' representing a probable area around the nucleus.

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Clearing Up Common Misconceptions About Atoms

Misconception	Correction
Atoms are flat, two-dimensional shapes.	Atoms are actually three-dimensional spheres.
Electrons orbit the nucleus in fixed paths.	Electrons exist in a "probability cloud" around the nucleus.
Atoms are solid, indivisible objects.	Atoms are mostly empty space and are divisible.
All atomic models created are equally correct.	Models show a progression of scientific understanding over time.

Multiple Choice

What is the main difference between Dalton's Solid Sphere Model and Thomson's Plum Pudding Model?

• Dalton's model described orbits for electrons.

• Dalton's model included a nucleus, while Thomson's did not.

• Thomson's model included subatomic particles (electrons), while Dalton's did not.

• Thomson's model stated that atoms were indivisible.

Multiple Choice

Why was the discovery of the nucleus by Ernest Rutherford so significant for the evolution of the atomic model?

• It confirmed that electrons were embedded within a positive sphere.

• It established the exact, planet-like orbits of electrons around the nucleus.

• It introduced the idea that atoms were indivisible solid spheres.

• It proved that the atom was mostly empty space with a dense, positive center, disproving the Plum Pudding model.

Multiple Choice

Based on the progression of atomic models, what would be the logical next discovery after Bohr's model, which placed electrons in specific orbits?

• A discovery of a new particle embedded in the positive charge.

• A discovery that atoms of the same element can have different masses.

• A discovery that these orbits are not fixed paths, but areas of probability.

• A discovery that atoms are actually solid spheres.

Multiple Choice

What was the biggest change in how scientists thought about the atom, from early ideas to modern models?

• Changing the number of known elements from four to over one hundred.

• Realizing that chemical reactions happen when atoms are rearranged.

• Thinking of the atom as a solid ball that can't be broken, to an object made of even smaller, moving parts.

• The idea that everything is made of tiny particles called atoms.

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Summary

• All matter is made of atoms, and our understanding of them has evolved.

• Democritus imagined atoms as spheres, and Dalton gave them basic properties.

• Thomson discovered the electron, and Rutherford discovered the dense nucleus.

• Bohr placed electrons in orbits, but the modern model uses electron clouds.

Poll

On a scale of 1-4, how confident are you about the evolution of the atomic theory?

• 1

• 2

• 3

• 4