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Section 2.1 Atoms and Their Structures

Section 2.2 Electrons in Atoms

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Section 2.1

Atoms and Their Structures

• Relate historic experiments to the development of the modern model of the atom.

• Illustrate the modern model of an atom.

• Interpret the information available in an element block of the periodic table.

Section 2.1

Atoms and Their Structures

energy: the capacity to do work

Section 2.1

Atoms and Their Structures

atom

atomic theory

law of definite proportions

hypothesis

experiment

An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus.

theory

scientific method

scientific law

electron

proton

isotope

neutron

nucleus

atomic number

mass number

isotope

atomic mass unit

Section 2.1

Early Ideas About Matter

• Greek philosophers thought that matter was composed of four elements: earth, water, air, and fire.

Section 2.1

Early Ideas About Matter (cont.)

• Democritus (460–370 B.C.) first proposed that the world is made up of empty space and atoms, the smallest particle of a given type of matter.

• The idea that matter is made up of fundamental particles called atoms is known as the atomic theory of matter.

Section 2.1

• Antoine Lavoisier concluded that when a chemical reaction occurs, matter is neither created nor destroyed but only changed. This conclusion became known as the law of conservation of matter.

Modern Atomic Theory

Section 2.1

• In 1799, Joseph Proust proposed the law of definite proportions, or the principle that the elements that comprise a compound are always in a certain proportion by mass.

Modern Atomic Theory (cont.)

Composition of iron sulfide

Section 2.1

• Dalton’s Atomic Theory

– All matter is made of atoms.

– Atoms are indestructible and indivisible.

– All atoms of one element are exactly alike, but they are different from atoms of other elements.

Modern Atomic Theory (cont.)

Section 2.1

• In natural processes, atoms are not destroyed; they are recycled.

Modern Atomic Theory (cont.)

Section 2.1

• A hypothesis is a prediction that can be tested to explain observations.

• An experiment is an investigation with a control designed to test a hypothesis.

• A theory is an explanation based on many observations and supported by the results of many investigations.

Hypotheses, Theories, and Laws

Section 2.1

• The systematic approach scientists use to answer questions and solve problems is called the scientific method.

Hypotheses, Theories, and Laws (cont.)

Section 2.1

• A scientific law is a fact of nature that is observed so often that it becomes accepted as truth.

– The Sun rising in the east each morning is a scientific law.

Hypotheses, Theories, and Laws (cont.)

Section 2.1

• From Thomson’s experiments, scientists concluded that atoms were composed of electrically-charged particles.

• An electron is a negatively-charged particle.

• A proton is a positive-charged subatomic particle.

• A neutron is a subatomic particle that has a mass equal to a proton by no electrical charge.

The Discovery of Atomic Structure

Section 2.1

• Atoms of an element that are chemically alike but differ in mass are called isotopes.

The Discovery of Atomic Structure (cont.)

Section 2.1

• Scientists proposed different atomic models.

The Discovery of Atomic Structure (cont.)

Section 2.1

• Rutherford’s gold foil experiment revealed an atomic arrangement far different from earlier models.

The Discovery of Atomic Structure (cont.)

Section 2.1

• Rutherford’s team proposed a new model of the atom that included a nucleus, a small dense, positively-charged central core.

The Discovery of Atomic Structure (cont.)

Rutherford’s Model Modern Model

Section 2.1

• The atomic number refers to the number of protons in the nucleus of an atom.

• The mass number is sum of the protons and neutrons in the nucleus of an atom.

• Isotopes of an element have different mass numbers because they have different numbers of neutrons.

Atomic Numbers

Section 2.1

Atomic Numbers (cont.)

Section 2.1

• To easily compare the masses of individual atoms, scientists devised a different unit of mass, the atomic mass unit (u), which is approximately the mass of a single proton or neutron.

Atomic Mass

Section 2.1

Atomic Mass (cont.)

Section 2.1

• Each box of the periodic table contains several pieces of information about an element.

Atomic Mass (cont.)

Section 2.1

• To calculate the weighted average atomic mass, you first need to calculate the mass contribution of each isotope.

Atomic Mass (cont.)

Naturally Occurring Isotopes of Chlorine

Section 2.1

Section Assessment

The idea that matter is made up of fundamental particles called atoms is known as ___.

A. the law of conservation of matter

B. the law of conservation of mass

C. the atomic theory of mass

D. the atomic theory of matter

Section 2.1

Section Assessment

Isotopes of an element have different mass numbers because they have different numbers of ___.

A. protons

B. electrons

C. neutrons

D. nuclei

End of Section 2.1

Section 2.2

Electrons in Atoms

• Relate the electron to modern atomic theory.

• Compare electron energy levels in an atom.

• Illustrate valence electrons by Lewis electron dot structures.

Section 2.2

Electrons in Atoms

atom: smallest particle of a given type of matter

Section 2.2

Electrons in Atoms

electromagnetic spectrum

emission spectrum

energy level

electron cloud

valence electron

Lewis dot diagram

Each element has a unique arrangement of electrons.

Section 2.2

Electrons in Motion

• Niels Bohr (1885–1962) proposed that electrons must have enough energy to keep them in constant motion around the nucleus.

• Electrons occupy orbits of only certain amounts of energy.

Section 2.2

The Electromagnetic Spectrum

• High-voltage electricity or electromagnetic radiation, or radiant energy, can increase the energy of an electron.

• Electromagnetic radiation travels in the form of waves that have both electric and magnetic properties.

Section 2.2

The Electromagnetic Spectrum (cont.)

• The electromagnetic spectrum is the whole range of electromagnetic radiation.

– radio waves

– radiant energy

– visible light

• Higher-frequency electromagnetic waves have higher energy than lower-frequency waves.

Section 2.2

The Electromagnetic Spectrum (cont.)

Section 2.2

• Each element emits a different emission spectrum, or a spectrum of light released from excited atoms of the element.

Electrons and Light

Section 2.2

• Because electrons can have only certain amounts of energy, they can move around the nucleus only at distances that correspond to those amounts of energy.

Electrons and Light (cont.)

Section 2.2

• The regions of space in which electrons can move about the nucleus of an atom are called energy levels.

Electrons and Light (cont.)

Section 2.2

• The electron cloud is the space around the nucleus of an atom where the atom’s electrons are most likely to be found.

The Electron Cloud Model

Section 2.2

• Each energy level can hold a limited number of electrons.

– First energy level—2 electrons

– Second energy level—8 electrons

– Third energy level—18 electrons

The Electron Cloud Model (cont.)

Section 2.2

• The electrons in the outermost energy level are called valence electrons.

The Electron Cloud Model (cont.)

Section 2.2

• Many of the chemical and physical properties of an element are directly related to the number and arrangement of valence electrons.

The Electron Cloud Model (cont.)

Section 2.2

• A Lewis dot diagram is a diagram where dots are placed around the chemical symbol of an element to illustrate the valence electrons.

The Electron Cloud Model (cont.)

Section 2.2

Section Assessment

The first energy level of an atom holds a maximum of ___ electrons.

A. one

B. two

C. four

D. eight

Section 2.2

Section Assessment

In a Lewis dot diagram, each dot represents a(n) ___.

A. neutron

B. proton

C. electron

D. valence electron

End of Section 2.2

Resources Menu

Chemistry Online

Study Guide

Chapter Assessment

Standardized Test Practice

Image Bank

Concepts in Motion

Study Guide 1

Key Concepts

• Scientists make hypotheses based on observation.

• Dalton’s atomic theory states that matter is made up of indestructible atoms.

• Experiments in the late nineteenth and early twentieth centuries revealed that the mass of an atom is concentrated in a tiny nucleus.

• The number of protons in an atom’s nucleus is called the atomic number and equals the number of electrons in the atom.

• Atoms of the same element always have the same number of protons and electrons.

Study Guide 2

Key Concepts

• Electrons move around an atom’s nucleus in specific energy levels.

• Energy levels are spherical regions in which electrons are likely to be found.

• The greater the energy of the level, the farther from the nucleus the level is located.

• Electrons can absorb energy and move to a higher energy level.

• Lewis dot diagrams can be used to represent the valence electrons in a given atom.

Chapter Assessment 1

Early Greek philosophers proposed that matter was a combination of what four fundamental elements?

A. air, fire, wind, earth

B. fire, air, earth, water

C. hot, dry, cold, wet

D. water, wind, fire, ice

Chapter Assessment 2

What is an explanation based on many observations and supported by the results of many investigations?

A. theory

B. experiment

C. hypothesis

D. scientific law

Chapter Assessment 3

What is the systemic approach scientists use to answer questions and solve problems?

A. experiment

B. hypothesis

C. scientific law

D. scientific method

Chapter Assessment 4

What is the distance between corresponding points on two consecutive waves?

A. spectrum

B. rate

C. wavelength

D. frequency

Chapter Assessment 5

Atoms in group 2 of the periodic table have how many valence electrons?

A. 1

B. 2

C. 13

D. 18

STP 1

The mass of a proton is ___ the mass of an electron.

A. greater than

B. less than

C. equal to

STP 2

The mass of a neutron is ___ the mass of a proton.

A. greater than

B. less than

C. equal to

STP 3

The atomic number determines the identity of an element, as well as many of its chemical and physical properties.

A. true

B. false

STP 4

What kind of properties do electromagnetic radiation waves have?

A. electric

B. magnetic

C. nuclear

D. A and B

STP 5

The energy of higher-frequency electromagnetic waves is ____ lower-frequency waves.

A. higher than

B. lower than

C. the same as

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Rutherford’s Model Modern Model

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Naturally Occurring Isotopes of Chlorine

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CIM

Table 2.1 Particles of an Atom

Figure 2.4 The Nitrogen Cycle

Figure 2.9 Rutherford’s Experiment

Figure 2.20 The Emission Spectrum

Figure 2.21 Energy Levels in an Atom

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