1. The Development of a New Atomic Model
Objectives
Compare and contrast the Bohr model and the quantum model of the atom.
Describe the electron configurations for Groups 1 and 2; and apply the electron configurations to the elements properties

2. Bohr Model of the Hydrogen Atom
The Development of a New Atomic Model
Bohr Model of the Hydrogen Atom
Niels Bohr proposed a hydrogen-atom model that linked the atom’s electron to photon emission.
According to the model, the electron can circle the nucleus only in allowed paths, or orbits.
The energy of the electron is higher when the electron is in orbits that are successively farther from the nucleus.

3. The Development of a New Atomic Model
Bohr Model of the Atom

4. Photon Emission and Absorption
The Development of a New Atomic Model
Photon Emission and Absorption

5. Bohr Model of the Hydrogen Atom, continued
The Development of a New Atomic Model
Bohr Model of the Hydrogen Atom, continued
When an electron falls to a lower energy level, a photon is emitted, and the process is called emission.
Energy must be added to an atom in order to move an electron from a lower energy level to a higher energy level. This process is called absorption.

6. Quantum Theory The Development of a New Atomic Model
Quantum theory describes mathematically the wave properties of electrons and other very small particles.
Electrons do not travel around the nucleus in neat orbits, as Bohr had postulated.
Instead, they exist in certain regions called orbitals.
An orbital is a three-dimensional region around the nucleus that indicates the probable location of an electron.

7. Electron Configurations
The Development of a New Atomic Model
Electron Configurations
The arrangement of electrons in an atom is known as the atom’s electron configuration.
The lowest-energy arrangement of the electrons for each element is called the element’s ground- state electron configuration.

8. Group 1 & 2 of the Periodic Table,
The Development of a New Atomic Model
Group 1 & 2 of the Periodic Table,
The elements of Group 1 of the periodic table are known as the alkali metals.
lithium, sodium, potassium, rubidium, cesium, and francium
In their pure state, all of the alkali metals have a silvery appearance and are soft enough to cut with a knife.
The elements of Group 2 of the periodic table are called the alkaline-earth metals.
beryllium, magnesium, calcium, strontium, barium, and radium
Group 2 metals are less reactive than the alkali metals, but are still too reactive to be found in nature in pure form.

9. Groups 1 & 2 of the Periodic Table, continued
The Development of a New Atomic Model
Groups 1 & 2 of the Periodic Table, continued
Hydrogen has an electron configuration of 1s1, but despite the ns1 configuration, it does not share the same properties as the elements of Group 1.
Hydrogen is a unique element.
Like the Group 2 elements, helium has an ns2 group configuration. Yet it is part of Group 18.
Because its highest occupied energy level is filled by two electrons, helium possesses special chemical stability.