Module 2 Chemistry

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Chemical Bonding and energy

Radiation is emitted when an electron in an excited atom jumps to a lower energy level. (Light is emitted)

Energy of radiation is greater for shorter wavelengths.

Lines of emission spectrum of H atoms have wavelengths which can be predicted using Rydberg's EQ.

Because atoms give reproducible line spectra rather than continuous spectra, the changes in E level which cause the spectral lines must always be set values. (Only certain E values are possible for electrons in the H atom)

As light is emitted, the valence electrons jump to lower energy level and the atoms become more stable. We also perceive light waves as different colours.

Planck proposed a relationship between the energy (E) of radiation and its frequency (v).

2. ENERGY = Planck's constant (h) x frequency (v)

E=h x v

It can also be related to wavelength by replacing frequency (v)!

E= h x c/wavelength

This equation shows that E increases as wavelength decreases.

Energy is directly proportional to its frequency.

Bohr proposed a model for the H atom and its energy shells.

The Bohr's EQ gives the potential energy of the electron at the nth level.

En = -Rhc/n^2

R- Rydbergs constant= 1.097*10^7m-1

h- Planck's constant = 6.626* 10^-34 J.s

c- velocity of light= 2.998*10^8s-

You can simplify Bohr's equation to=

En = -2.179*10^-18 * 1/n2 (J/atom)

If the shell number is infinite, the interaction is called zero energy state.

You can calculate the difference in Energy between different electron shell levels!

A negative value for an energy change means that energy has been released or given off by the process.

As an electron is moving from n=2 to n=1, it is getting closer to the nucleus and going to a lower energy state.

The calculated value for change in energy would be negative if the electron jumps towards the nucleus.

To calculate the change in E, you take the final E value and subtract the initial E.