Chemistry 1.1 Notes

Who is credited with the first atomic model?

Democritus (460–370 BCE) is credited with the first atomic model.

What did Democritus say matter is made of?

Matter is made of tiny, solid, indivisible particles which he called atoms.

How did Democritus describe different kinds of atoms?

Different kinds of atoms have different sizes and shapes, therefore different properties.

How did Democritus’ peers react to his atomic ideas?

His ideas were widely rejected by his peers, most notably Aristotle, who rejected his idea.

What did Aristotle believe everything was composed of?

Aristotle believed everything was composed of four elements: earth, air, fire, and water.

Why were Aristotle’s ideas accepted instead of Democritus’ ideas?

Democritus’ ideas could not be tested experimentally at the time, so Aristotle’s ideas were accepted to be true.

Until when were Aristotle’s ideas accepted?

Aristotle’s ideas were accepted until John Dalton revived Democritus’ ideas in the early 19th century.

Who revived Democritus’ ideas and developed the first modern atomic theory?

John Dalton (early 19th century) revived Democritus’ ideas and developed the first modern atomic theory.

What is Dalton’s first main point of his atomic theory?

All matter is composed of extremely small particles called atoms.

What is Dalton’s second main point of his atomic theory?

All atoms of one element are identical but differ from those of any other element.

What is Dalton’s third main point of his atomic theory?

Atoms of one element combine in simple whole number ratios to form compounds during chemical reactions.

How did Dalton describe the way atoms combine to form compounds?

Atoms combine in simple whole number ratios to form compounds

What is Dalton’s fourth main point of his atomic theory?

Atoms are indivisible; they cannot be created or destroyed, only rearranged during reactions.

How is Dalton’s model of the atom often sketched

Dalton’s model is sketched as a tiny, solid sphere representing an indivisible atom.

What was Dalton’s incorrect statement about atoms of one element?

Dalton said all atoms of one element are identical—this is FALSE.

What are isotopes?

Isotopes are atoms of the same element which have different numbers of neutrons, therefore they have different masses.

What was Dalton’s incorrect statement about atoms being indivisible?

Dalton said atoms are indivisible—this is FALSE

Into what smaller particles can atoms be divided?

Atoms are divisible into protons, neutrons, and electrons.

Who performed many experiments involving cathode rays

J.J. Thomson (1856–1940) performed many experiments involving cathode rays.

What did Thomson place around the cathode ray in his experiment?

Thomson placed oppositely charged electric plates around the cathode ray

What did J.J. Thomson know about electric charges?

He knew that opposite charges attract, like charges repel, and he knew the charges on each plate.

What did J.J. Thomson NOT know before his experiment?

He did NOT know what cathode rays were made of and did NOT know the subatomic particles of the atom.

In Thomson’s experiment, toward which charged plate does the cathode ray beam bend?

The beam bends toward the positively charged plate.

Based on the beam bending toward the positive plate, what did Thomson conclude about the charge in the beam?

He concluded the charge in the beam must be negative.

Which charged plate does the beam move away from?

The beam moves away from the negatively charged plate.

What does the beam moving away from the negative plate tell us about the charge in the beam?

It shows the beam contains negatively charged particles (negative repels negative).

What subatomic particle did Thomson discover from his cathode ray experiments?

Thomson discovered the electron.

What is the charge of the electron?

The electron has a negative charge.

What did Thomson observe when he repeated cathode ray experiments with many different elements?

He saw the same results each time.

What did Thomson conclude from seeing the same results with different elements?

He concluded that electrons are a part of all atoms of elements.

What atomic model did Thomson develop after his experiments?

Thomson developed the plum pudding model.

How does the plum pudding model describe the distribution of electrons and positive charge?

It shows evenly distributed negative electrons embedded in a uniform positively charged matrix of material.

How is Thomson’s model often sketched?

As a circle with small dots representing electrons scattered throughout a positively charged “pudding.”

Who was Ernest Rutherford in relation to J.J. Thomson?

Ernest Rutherford was a protégé (student) of J.J. Thomson.

What did Rutherford study to explore the properties of atoms?

Rutherford studied radioactive particles, specifically alpha particles, to explore atomic properties.

What are alpha particles in Rutherford’s experiments?

Alpha particles are positively charged nuclei produced from the radioactive decay of radium salts.

How were alpha particles directed in Rutherford’s experiment?

Alpha particles were focused by a lead box and streamed toward a sheet of extremely thin gold foil.

How thick was the gold foil used in Rutherford’s experiment?

The gold foil was about 1,000 atoms thick.

What model of the atom did Rutherford compare his expectations to?

He compared his expectations to Thomson’s Plum Pudding Model.

What path did Rutherford expect the alpha particles to take according to the Plum Pudding Model?

He expected the alpha particles to pass straight through the gold foil with only slight deflections, since the positive charge was thought to be spread out.

What charge do alpha particles have?

Alpha particles have a positive charge.

What surprising result did Rutherford observe in his gold foil experiment?

A small percentage of alpha particles (less than 0.01%) were deflected at angles larger than 90 degrees back toward the particle emitter.

Where do most of the alpha particles hit in Rutherford’s diagram (diagram a)?

Most of the alpha particles hit straight through the gold foil.

What does diagram b show about the path of most alpha particles?

Diagram b shows that most alpha particles go straight through the foil, with only a few being deflected.

If some alpha particles are deflected back, what must they be hitting?

They must be hitting something positively charged (since positive charges repel positive charges).

What does the deflection of only a few alpha particles tell us about the size of what they are hitting?

It tells us that the positively charged region they hit is very small compared to the overall atom.

If only a few alpha particles are deflected back, what does this tell us about the atom as a whole?

It tells us that most of the atom is empty space.

If most alpha particles go straight through, what does this indicate about the atom’s structure?

It indicates that the atom’s volume is mostly empty space, with a small dense center.

What did Rutherford conclude about the center of the atom?

He concluded the atom has a dense, positively charged structure in its center that is very small compared to the total volume of the atom.

What did Rutherford discover in the center of the atom?

He discovered the nucleus of the atom.

What did Rutherford conclude about the volume of the atom?

He concluded the volume of the atom is mostly empty space.

What additional particle did Rutherford later discover using alpha particles?

Rutherford later discovered the proton using alpha particles.

Who discovered the neutron, and where is it located?

James Chadwick, Rutherford’s student, discovered the neutron, which is located in the nucleus.

Where are protons and neutrons located in the atom?

Protons and neutrons are located in the nucleus.

How is Rutherford’s atomic model often sketched?

As a small central nucleus with electrons shown around it, often with elliptical paths.

Who developed the Bohr Model of the atom?

Niels Bohr (early 20th century) developed the Bohr Model.

What did Bohr’s theory state about electrons and energy?

Bohr’s theory stated that electrons are restricted to specific energies.

According to Bohr, how do electrons move around the nucleus?

Electrons follow paths called orbits at a fixed distance from the nucleus.

What everyday system is Bohr’s model compared to?

It is compared to the way planets orbit the sun.

What do we now know about electron orbits compared to Bohr’s original idea?

We now know electrons do not have neat, planet-like orbits.

How is Bohr’s model often sketched?

As a nucleus with electrons on a single circular orbit or several distinct circular orbits.

What is the current model of the atom called?

The current model is the Quantum Mechanical Model, also known as the Electron Cloud model.

When was the Quantum Mechanical Model developed and by whom?

It was developed in the 1920s by many scientists, including de Broglie, Schrodinger, Pauli, and Heisenberg.

How does the Quantum Mechanical Model describe where electrons exist?

Electrons exist in regions of space around the nucleus called orbitals.

How are the paths of electrons described in the Quantum Mechanical Model?

The paths of electrons are random and cannot be predicted exactly.

What can we talk about instead of exact electron paths in the Quantum Mechanical Model?

We can talk about the probability of an electron being in a certain region, often likely near the nucleus.

What is Heisenberg’s Uncertainty Principle?

It states that you cannot simultaneously know both the exact position and the precise momentum of a quantum particle.

How is the Quantum Mechanical (electron cloud) model often sketched?

As a nucleus surrounded by a shaded, fuzzy region representing the electron cloud (orbitals).