Bohr + Chadwick

Bohr’s

Model of an Atom

The dual wave-particle model of light did not explain

why atomic emission spectra of elements were discontinuous rather than continuous.  

Bohr correctly predicted the

frequency lines in hydrogen’s atomic emission spectrum.  

The lowest allowable energy state of an atom is called its

 ground state.  

When an atom gains energy, it is in an

excited state. 

Bohr suggested that an electron in a hydrogen atom moves around the nucleus in

only certain allowed circular orbits. 

The number assigned to each orbit of an electron is a 

 

quantum number. 

A hydrogen atom is in the ground state wheni

its single electron is in the n = 1 orbit.  

When energy is added, the electron moves to a

higher-energy orbit, n = 2. 

Light - Visible Series

Balmer

Infrared Series

Pashcen

infrared

(of electromagnetic radiation) having a wavelength just greater than that of the red end of the visible light spectrum but less than that of microwaves. Infrared radiation has a wavelength from about 800 nm to 1 mm, and is emitted particularly by heated objects.

Ultraviolet Series

lyman

Ultraviolet

(of electromagnetic radiation) having a wavelength shorter than that of the violet end of the visible spectrum but longer than that of X-rays.

Amount of Atoms per shell=

2n squared

Limits of Bohr’s Model: Bohr’s model explained hydrogen’s spectral lines, but it 

failed to explain any other element’s lines.  

Limits of Bohr’s Model: The behavior of electrons is still not fully understood, but substantial evidence indicates 

they do not move around the nucleus in circular orbits. 

James Chadwick discovered

the neutron - sub-particle with no charge

How did Chadwick discover the neutron

Chadwick directed a beam of alpha particles from a polonium source at a thin sheet of beryllium.

unknown radiation

This bombardment produced a penetrating, neutral radiation that did not deflect in an electric or magnetic field, unlike charged particles. This was initially mistaken for gamma rays.

To explain how such a high-energy radiation could knock protons out of the paraffin, Chadwick used the laws of conservation of energy and momentum to calculate the mass of the particles in the radiation. His calculations showed that the mass was

approximately equal to that of a proton but without an electric charge, leading him to propose the existence of the neutron.