PretestPretest

1. True or False: Compounds have fixed compositions

TRUE!

PretestPretest

2. What is an atom?

An atom is the smallest particle of an element.

PretestPretest

3. Which of the following units is a unit of mass?

a) mL

b) °C

c) g

d) cm

PretestPretest

4. Volume is

a) The straight-line distance between two pointsb) The quantity of matter in an objectc) The amount of space taken up by an objectd) A representation of an object or event

PretestPretest

5. What is density?

Density is the ratio of the mass of a substance to its volume.

D = M/V

PretestPretest

6. Which two of the following events can take place when a liquid absorbs energy?

a) The average kinetic energy of the particles in the liquid increasesb) The temperature decreasesc) The liquid freezesd) The liquid changes to a gas

Studying Atoms Studying Atoms

Chapter 4.1

Atomic Structure

I. Ancient Models of AtomsI. Ancient Models of Atoms

A. Democritus believed that all matter is made of extremely small particles called atoms, which cannot be divided.

B. Aristotle believed that there is no limit to the number of times matter can be divided.

II. Dalton’s Atomic TheoryII. Dalton’s Atomic Theory

A. Dalton found evidence for atoms by measuring the masses of elements that combine when compounds form.

B. By looking at the ratio of the masses of elements in the compound, he found that compounds have fixed compositions.

Dalton’s Theory StatesDalton’s Theory States

1.  All elements are composed of atoms, which cannot be divided.

2.  All atoms of the same element have the same mass, & atoms of different elements have different masses.

3.  Compounds contain different atoms of more than one element.

4. In a particular compound, atoms of different elements always combine in the same way

D. Over time, some of Dalton’s ideas were found to be incorrect and his theory was revised when new discoveries were made.

III. Thomson’s Model of the III. Thomson’s Model of the AtomAtom

A. Some materials have either a positive or a negative electric charge.

B. Objects with like charges repel or push apart.

  C. Objects with opposite charges attract or pull together.

D. When charged particles flow, it is called an electric current.

E. Thomson’s experiments with electric currents changed the way scientists thought of atoms. He showed evidence that atoms are made of even smaller particles.

F. Atoms are neutral. Thomson discovered negative particles in atoms, but to be neutral, he knew there must also be positive particles in an atom.

Thomson’s ExperimentThomson’s Experiment

G. His model of the atom showed that the negative particles are scattered through an atom full of positively charged matter. This model is called the plum pudding model.

IV. Rutherford’s Atomic IV. Rutherford’s Atomic TheoryTheory

A.     Rutherford did an experiment where he shot positive alpha particles at a sheet of gold. He thought the particles would go straight through, but some of them were deflected by the gold atoms.

Rutherford’s ExperimentRutherford’s Experiment

B. He concluded that they were deflected by a positively charged object in the center of the atoms, which he called the nucleus.

Review:Review:

What theory did Dalton propose about the structure of matter?

– All matter is composed of individual particles called atoms, which cannot be divided.

Review:Review:

What evidence did J.J. Thomson provide about the structure of an atom?

– He provided the first evidence that atoms are made of even smaller particles.

Review:Review:

What did Rutherford discover about the structure of an atom?

– All of the positive charge of an atom is concentrated in its nucleus.

Review:Review:

What evidence did Thomson have that his glowing beam contained negative particles?

– The beam was attracted to a positively charged plate and repelled by a negatively charged plate.

Review:Review:

Why was Dalton’s model of the atom changed after Thomson’s experiment?

– Dalton assumed atoms were solid, indivisible particles. Thomson had evidence that smaller particles existed inside atoms.

Review:Review:

Explain why scientists accepted Dalton’s atomic theory but not the idea of an atom proposed by the Greek philosophers.

– Dalton had data from experiments to support his theory, whereas the Greeks did not have data.

Review:Review:

If you observed a beam of particles being bent toward a negatively charged plate, what might you conclude?

– The particles have a positive charge.

Review:Review:

In the Rutherford experiment, why weren’t all the alpha particles deflected?

– The nucleus is small compared with the atom as a whole. Very few of the alpha particles came close enough to a gold nucleus to be deflected.

The Structure of an AtomThe Structure of an Atom

Chapter 4.2

Atomic Structure

I. Properties of Subatomic I. Properties of Subatomic Particles Particles

Rutherford found evidence for 2 kinds of subatomic particles and predicted the existence of a third kind.

A. ProtonA. Proton

A Proton is a positively charged subatomic particle that is found in the nucleus of an atom.

- Protons have a charge of 1+.

- They were first predicted to exist by Rutherford.

B. ElectronB. Electron

An Electron is a negatively charged subatomic particle that is found in the space outside the nucleus.

- Electrons have a charge of 1-.

- They were first predicted to exist by Thomson.

C. NeutronC. Neutron

A Neutron is a neutral subatomic particle that is found in the nucleus of an atom.

- Neutrons have a charge of 0 and have almost the same mass as protons.

- They were first predicted to exist by Chadwick.

Comparing Subatomic Comparing Subatomic ParticlesParticles

Properties of Subatomic Particles

Particle Sym-bol

Relative Charge

Relative Mass (proton = 1)

Actual Mass (g)

Electron e- 1- 1/1836 9.11 x 10-28

Proton p+ 1+ 1 1.674 x 10-24

Neutron N 0 1 1.675 x 10-24

Protons, electrons, & neutrons can be distinguished by mass, charge, & location.

III. Atomic Number & Mass III. Atomic Number & Mass NumberNumber

A.     The Atomic Number of an element equals the number of protons in an atom of that element.

- Atoms of different elements have different numbers of protons.

- Each positive charge in an atom is balanced by a negative charge because atoms are neutral.

- The number of protons is the same as the number of electrons.

B. The Mass Number of at atom is the sum of the protons and neutrons in the nucleus of that atom.

- If you know the atomic number & the mass number, you can subtract to find the number of neutrons in an atom.

IV. IsotopesIV. Isotopes

A. Every atom of a given element does have the same number of protons & electrons but they do not have the same number of neutrons.

 B. Isotopes are atoms of the same element that have

different numbers of neutrons and different mass numbers.

 C. Isotopes of an element have the same atomic number,

but different mass numbers, because they have different numbers of neutrons.

Review:Review:

Name three subatomic particles:

– Protons– Electrons– Neutrons

Review:Review:

Name three properties you could use to distinguish a proton from an electron:

– Mass– Charge– Location in an atom

Review:Review:

Which characteristic of an atom always varies among atoms of different elements?

– The atoms of any element have a different number of protons than the atoms of all other elements.

Review:Review:

How are the isotopes of an element different from one another?

– Isotopes of an element have the same atomic number but different mass numbers because they have different numbers of neutrons.

Review:Review:

What do neutrons and protons have in common?

– They have almost the same mass and are both in the nucleus.

How are they different?

– Protons are positively charged and neutrons are neutral.

Review:Review:

How can atoms be neutral if they contain charged particles?

– The positive charges and negative charges balance each other out.

Review:Review:

What property do protons and electrons have that neutrons do not?

– They are charged particles and neutrons are neutral.

Review:Review:

Explain why it is impossible for an atom to have a mass number of 10 and an atomic number of 12.

– An atom with an atomic number of 12 has 12 protons. Because the mass number is the sum of protons and neutrons, the mass number must be at least 12.

Review:Review:

What is the difference between atoms of oxygen-16 and oxygen-17?

– Oxygen-17 atoms have one more neutron than atoms of oxygen-16.

Modern Atomic TheoryModern Atomic Theory

Chapter 4.3

Atomic Structure

I. Bohr’s Model of the AtomI. Bohr’s Model of the Atom

A. Bohr worked with Rutherford, and agreed with the atomic model of a nucleus surrounded by a large volume of space.

B. Bohr’s model focused on the electrons.

C. In his model, electrons move with constant speed in fixed orbits around the nucleus (like planets around a sun).

D. The possible energies that electrons in an atom can have are called energy levels.

Concept CheckConcept Check

When is an electron in an atom likely to move from one energy level to another?

– An electron in an atom can move from one energy level to another when the atom gains or loses energy.

Concept CheckConcept Check

What evidence is there that electrons can move from one energy level to another?

– Light, Heat, Sound

II. Electron Cloud ModelII. Electron Cloud Model

A. Bohr’s model was correct in assigning energy levels to electrons, but he was incorrect in assuming that electrons move like planets in a solar system.

B. Today, scientists know that electrons move in a less predictable way.

Concept CheckConcept Check

What model do scientists use to describe how electrons move around the nucleus?

– Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus.

Electron Cloud ModelElectron Cloud Model

III. Atomic OrbitalsIII. Atomic Orbitals

A. An electron cloud represents all the orbitals in an atom.

B. An orbital is a region of space around the nucleus where an electron is likely to be found.

C. The lowest energy level has only one orbital. Higher energy levels have more than one orbital.

D. Each orbital can contain no more than two electrons.

IV. Electron ConfigurationsIV. Electron Configurations

A. A configuration is an arrangement of objects in a given space.

B. Some configurations are more stable than others and are less likely to change.

C. The arrangement of electrons in the orbitals of an atom is the electron configuration.

Concept CheckConcept Check

Describe the most stable configuration of the electrons in an atom.

– The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies.

D. When all the electrons in an atom have the lowest possible energies, the atom is said to be in its ground state.

E. An excited state is less stable than the ground state.

Review:Review:

When is an electron in an atom likely to move from one energy level to another?

When atoms gain or lose energy

ReviewReview

What model do scientists use to describe how electrons move around the nucleus?

The electron cloud model

ReviewReview

Describe the most stable configuration of the electrons in the atom.

The one in which the electrons are in orbitals with the lowest possible energy.

ReviewReview

What did Bohr contribute to modern atomic theory?

The idea that electrons have energy levels with specific amounts of energy.

ReviewReview

What does an electron cloud represent?

The most probable locations of an electron in an atom.

ReviewReview

A boron atom has 2 electrons in the first energy level and 3 in the second energy level. Compare the relative energies of the electrons in these 2 energy levels.

The electrons in the second energy level will have more energy than the electrons in the first energy level.