Section 3

Modern Atomic Theory

Key Concepts

• What can happen to electrons when atoms gain or lose energy?

• What model do scientists use to describe how electrons behave in atoms?

• What is the most stable configuration of electrons in an atom?

Bohr’s Model of the Atom

• Niels Bohr(1885–1962), a Danish physicist who worked for a while with Rutherford.

• Bohr’s model focused on the electrons.

• A description of the arrangement of electrons in an atom is the centerpiece of the modern atomic model.

Bohr’s Model of the Atom

• Energy Levels

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

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

Evidence for Energy Levels

• Scientists can measure the energy gained when electrons absorb energy and move to a higher energy level.

• They can measure the energy released when the electron returns to a lower energy level.

Electron Cloud Model

• An electron cloud is a visual model of the most likely locations for electrons in an atom.

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

When the propeller ofan airplane is at rest, you can seethe locations of the blades. Whenthe propeller is moving, you seeonly a blur that is similar to adrawing of an electron cloud.

Atomic Orbitals

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

• An electron cloud is a good approximation of how electrons behave in their orbitals.

Electron Configurations

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

• An electron configuration is the arrangement of electrons in the orbitals of an atom.

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

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

Models of the Atom

1803 John Daltonpictures atoms astiny, indestructibleparticles, with nointernal structure.

Tiny,solid sphere

DALTONMODEL

1897 J. J. Thomson, a Britishscientist, discovers the electron,leading to his “plum-pudding”model. He pictures electronsembedded in a sphere ofpositive electric charge.

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THOMSONMODEL

The positive charge of thesphere balances the negativecharge of the electrons

Sphere withpositive chargethroughout

Negativelycharged particle(electron)

1911 New Zealander Ernest Rutherfordstates that an atom has a dense, Positively charged nucleus.Electrons move randomly in the spacearound the nucleus.

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RUTHERFORDMODEL

Path of amovingelectronNucleus

1913 In NielsBohr’s model, theelectrons move inspherical orbits atfixed distancesfrom the nucleus.

BOHR MODEL

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Electron

Nucleus

Electrons gain orlose energywhen they movebetween fixedenergy levels.

ELECTRON CLOUDMODEL

1932 JamesChadwick, a Britishphysicist, confirms theexistence of neutrons,which have no charge.Atomic nuclei containneutrons and positivelycharged protons.

1926 Erwin Schrödingerdevelops mathematicalequations to describe themotion of electrons inatoms. His work leads tothe electron cloud model.

The electron cloud isa visual model of theprobable locations ofelectrons in an atom.The probability offinding an electron ishigher in the denserregions of the cloud.

The nucleuscontains protonsand neutrons.

Reviewing Concepts

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

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

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

• 4. What did Bohr contribute to modern atomic theory?

• 5. What does an electron cloud represent?