Chapter 4Chapter 4

Atomic StructureAtomic Structure

Before the atom….Before the atom….

Many cultures believed that all Many cultures believed that all things were composed of the things were composed of the classical elements: classical elements: Earth, Wind, Water, and FireEarth, Wind, Water, and Fire

Much of their understanding came Much of their understanding came from things that they could see – from things that they could see – they could not see atoms.they could not see atoms.

4.1 Atoms4.1 Atoms

The discovery and study of atoms The discovery and study of atoms ((the fundamental unit of all the fundamental unit of all mattermatter) has been done through ) has been done through indirect observations, logic, and indirect observations, logic, and scientific deduction.scientific deduction. These particles can only be viewed with These particles can only be viewed with

the most powerful telescopes.the most powerful telescopes.

4.1 Atoms4.1 Atoms

Atoms were first proposed to exist by Atoms were first proposed to exist by a Greek philosopher, Democritus, a Greek philosopher, Democritus, more than 2000 years ago.more than 2000 years ago. Democritus thought that atoms were Democritus thought that atoms were

invisible, indestructible, fundamental invisible, indestructible, fundamental units of matter.units of matter.

His ideas agreed with later scientific His ideas agreed with later scientific theories, but lacked experimental theories, but lacked experimental support because scientific experiments support because scientific experiments were unknown in his time.were unknown in his time.

4.1 Atoms4.1 Atoms

2200 years after Democritus, John 2200 years after Democritus, John Dalton (1766-1844), proposed an Dalton (1766-1844), proposed an atomic theoryatomic theory.. Dalton, who studied chemistry very Dalton, who studied chemistry very

differently than Democritus, performed differently than Democritus, performed experiments to arrive at his atomic experiments to arrive at his atomic theory.theory.

4.1 Daltons Atomic 4.1 Daltons Atomic TheoryTheory

1. All elements are composed of 1. All elements are composed of submicroscopic, indivisible particles submicroscopic, indivisible particles called atoms.called atoms.

2. Atoms of the same element are 2. Atoms of the same element are identical.identical. The atoms of any one element are The atoms of any one element are

different from those of any other different from those of any other element.element.

4.1 Dalton’s Atomic 4.1 Dalton’s Atomic TheoryTheory

3. Atoms of different elements can 3. Atoms of different elements can physically mix together or can physically mix together or can chemically combine with one another chemically combine with one another in simple whole-number ratios to form in simple whole-number ratios to form compounds.compounds.

4. Chemical reactions occur when atoms 4. Chemical reactions occur when atoms are separated, joined, or rearranged.are separated, joined, or rearranged. However, atoms of one element are never However, atoms of one element are never

changed into atoms of another element as changed into atoms of another element as a result of a chemical reaction.a result of a chemical reaction.

4.1 Atoms4.1 Atoms

atomatom – the smallest particle of an – the smallest particle of an element that retains the properties element that retains the properties of that element.of that element. Example: Pure copper “penny” would Example: Pure copper “penny” would

contain approximately 2.4 x 10contain approximately 2.4 x 102222 copper copper atoms – that is 3.5 trillion times the atoms – that is 3.5 trillion times the number of people on this planet.number of people on this planet.

4.1 Atoms4.1 Atoms

Even though atoms are so small, we can Even though atoms are so small, we can see images from them with the right see images from them with the right technology.technology.

Scanning Tunneling Microscopes (STMs) Scanning Tunneling Microscopes (STMs) and Atomic Force Microscopes (AFMs) and Atomic Force Microscopes (AFMs) can be used to visualize individual atoms.can be used to visualize individual atoms.

TEAM MicroscopeTEAM Microscope

TEAM Published ArticleTEAM Published Article

4.1 Concept Practice4.1 Concept Practice

1. Democritus and Dalton both 1. Democritus and Dalton both proposed that matter consists of proposed that matter consists of atoms. Explain how their approaches atoms. Explain how their approaches to reaching the same conclusion to reaching the same conclusion differed?differed?

A: Democritus approached the subject A: Democritus approached the subject logically, but without experimental logically, but without experimental evidence; Dalton based his ideas on evidence; Dalton based his ideas on an analysis of data obtained by an analysis of data obtained by experiments.experiments.

4.1 Concept Practice4.1 Concept Practice

2. Which of these statements would Dalton 2. Which of these statements would Dalton have agreed with (use Dalton’s atomic have agreed with (use Dalton’s atomic theory)?theory)?

a. Atoms are the smallest particles of mattera. Atoms are the smallest particles of matter

b. The mass of an iron atom is different from b. The mass of an iron atom is different from the mass of a copper atomthe mass of a copper atom

c. Every atom of silver is identical to every c. Every atom of silver is identical to every other atom of silverother atom of silver

d. A compound is composed of atoms of two or d. A compound is composed of atoms of two or more different elements more different elements

A: Dalton would have agreed with all of them.A: Dalton would have agreed with all of them.

4.2 Electrons, Protons, 4.2 Electrons, Protons, NeutronsNeutrons

Most of Most of Dalton’s Atomic TheoryDalton’s Atomic Theory is is accepted today – one important revision accepted today – one important revision is that atoms are not indivisible (they is that atoms are not indivisible (they can be broken down)can be broken down)

ProtonsProtons, , neutronsneutrons, and , and electronselectrons are are three three subatomic particlessubatomic particles that make that make up an atom.up an atom. There are dozens of subatomic particles, but There are dozens of subatomic particles, but

we will only be studying these three we will only be studying these three subatomic particles in chemistry.subatomic particles in chemistry.

4.2 Electrons4.2 Electrons

The The electronelectron is the smallest is the smallest subatomic particle with a mass subatomic particle with a mass 1/1840 the size of the proton.1/1840 the size of the proton.

An electron has a An electron has a negative charge negative charge (exactly 1-)(exactly 1-)..

English Physicist Sir J.J. Thompson English Physicist Sir J.J. Thompson discovered the electron in 1897 discovered the electron in 1897 using the using the cathode ray tubecathode ray tube. (see p. . (see p. 86-87)86-87)

4.2 Protons4.2 Protons

The The protonproton is a subatomic particle that is a subatomic particle that has a has a positive charge (exactly 1+).positive charge (exactly 1+).

A proton is a much more massive A proton is a much more massive subatomic particle, when compared to subatomic particle, when compared to the electron, but it is still very small.the electron, but it is still very small.

E. GoldsteinE. Goldstein discovered the proton in discovered the proton in 1886 using a cathode ray tube and 1886 using a cathode ray tube and canal rays (a beam of protons.canal rays (a beam of protons.

4.2 Neutron4.2 Neutron

The The neutronneutron is a subatomic is a subatomic particles that have particles that have no charge no charge (neutral).(neutral).

Neutrons have a mass that is about Neutrons have a mass that is about the same as a proton.the same as a proton.

An English physicist, An English physicist, Sir James ChadwickSir James Chadwick, confirmed the , confirmed the existence of the neutron in 1932.existence of the neutron in 1932.

4.2 Electrons, Protons, 4.2 Electrons, Protons, NeutronsNeutrons The The fundamental building blocks fundamental building blocks of atoms of atoms

are the are the electronelectron, the , the protonproton, and the , and the neutronneutron..

Table 4.1 Properties of Subatomic ParticlesTable 4.1 Properties of Subatomic Particles

ParticlParticlee

SymbSymbolol

RelativRelative e ElectriElectrical cal ChargeCharge

Approx. Approx. Relative Relative Mass Mass (amu*)(amu*)

Actual Actual Mass Mass (g)(g)

ElectroElectronn

ee-- 1-1- 1/18401/1840 9.11x19.11x100-28-28

ProtonProton pp++ 1+1+ 11 1.67x11.67x100-24-24

NeutroNeutronn

nn00 00 11 1.67x11.67x100-24-24

*1 amu = 1.66x10*1 amu = 1.66x10-24-24 g g

4.2 Simple Rules about 4.2 Simple Rules about Matter and Electric Matter and Electric

ChargesCharges1. Atoms have no electric charge – they are 1. Atoms have no electric charge – they are

electrically neutral.electrically neutral.2. Electric charges are properties of 2. Electric charges are properties of

particles.particles. Electric charges are carried by particles of Electric charges are carried by particles of

matter.matter.

3. Electric charges exist in a single unit or 3. Electric charges exist in a single unit or multiples of a single unit.multiples of a single unit. The are no fractions of charges.The are no fractions of charges.

4. Electric charges cancel when equal 4. Electric charges cancel when equal number of positively charged particles are number of positively charged particles are balanced by negatively charged particles.balanced by negatively charged particles.

4.2 Concept Practice4.2 Concept Practice

3. Since all atoms have negatively charged 3. Since all atoms have negatively charged electrons, shouldn’t every sample of electrons, shouldn’t every sample of matter have a negative charge? Explain.matter have a negative charge? Explain.

A: No. The (-) charge is cancelled by the A: No. The (-) charge is cancelled by the (+) charge.(+) charge.

4. What experimental evidence did 4. What experimental evidence did Thompson have for the following ideas?Thompson have for the following ideas?

a. Electrons have a negative charge.a. Electrons have a negative charge.

b. Atoms of all elements contain b. Atoms of all elements contain electrons.electrons.

4.3 The Structure of the 4.3 The Structure of the Nuclear AtomNuclear Atom

At first (before the discovery of the At first (before the discovery of the neutron) scientists thought that neutron) scientists thought that protons and neutrons were evenly protons and neutrons were evenly distributed throughout the atom.distributed throughout the atom.

4.3 The Structure of the 4.3 The Structure of the Nuclear AtomNuclear Atom

In 1911, In 1911, Ernest Rutherford Ernest Rutherford and his and his co-workers tested this theory of co-workers tested this theory of atomic structure by firing a beam of atomic structure by firing a beam of alpha particles at a thin gold sheet.alpha particles at a thin gold sheet. From this experiment Rutherford From this experiment Rutherford

proposed that almost all of the mass proposed that almost all of the mass and all of the positive charge are and all of the positive charge are concentrated at a small region at the concentrated at a small region at the center of an atom – he called this region center of an atom – he called this region the the nucleusnucleus..

4.3 The Structure of the 4.3 The Structure of the Nuclear AtomNuclear Atom

nucleusnucleus – the central core of an atom, – the central core of an atom, composed of composed of protonsprotons and and neutronsneutrons.. Almost all of mass in an atom is contained in a Almost all of mass in an atom is contained in a

tiny nucleus which is extremely dense – if it was tiny nucleus which is extremely dense – if it was the size of a pea it would have a mass of 250 the size of a pea it would have a mass of 250 tons!tons!

The nucleus has a positive charge and The nucleus has a positive charge and occupies a very small volume of the atom – occupies a very small volume of the atom – the rest of the atom is the rest of the atom is more or less empty space more or less empty space in which the in which the negatively charged electrons are found.negatively charged electrons are found.

4.3 Concept Practice4.3 Concept Practice

5. How did the results of Rutherford’s 5. How did the results of Rutherford’s gold foil experiment differ from his gold foil experiment differ from his expectations?expectations?

A: He thought A: He thought αα particles would pass particles would pass directly through, however some were directly through, however some were deflected.deflected.

6. What is the charge, positive or 6. What is the charge, positive or negative, on the nucleus of every atom?negative, on the nucleus of every atom?

A: PositiveA: Positive