Electronic Structure Of The Atom

Question 1

What are the three types of atomic spectra?

Answer 1

• Absorption spectrum
• Emission spectrum
• Continuous spectrum

Question 2

What is the appearance of the absorption spectrum?

Answer 2

Appear as black lines against the coloured background of the visible spectrum

Question 3

Explain how the absorption spectrum is formed.

Answer 3

• Formed when white light is passed through a sample of gaseous atoms and the emerging light analyzed on a coloured background.
• The resulting spectrum appears as black lines on the colored background.

Explanation:
- The black lines represent the wavelengths of white light that have were absorbed by the gaseous atoms to cause some of their electrons to be excited and move to higher energy levels.

Question 4

What is the appearance of the emission spectrum?

Answer 4

Appear as a number of separate sets, or series, of narrow coloured lines on a black background.

Question 5

Explain how the emission spectrum is formed.

Answer 5

• Formed when an electric discharge is passed through a gas at low pressure in a gas discharge tube.
• The gaseous atoms absorb energy; their electrons get excited and move to higher energy levels.
• When they fall back to lower energy levels they emit electromagnetic radiation of certain wavelengths which appears as a spectrum of a series of coloured lines on a black background.

Question 6

How does a continuous spectrum appear?

Answer 6

It appears as a continuous series of colours with no gaps between them.

This is because it consists of all wavelengths in the visible region of the electromagnetic spectrum.

A rainbow is an example of a continuous spectrum. In the laboratory a continuous spectrum can be formed by passing white light through a triangular prism.

Question 7

How is the atomic emission spectrum of hydrogen formed?

Answer 7

• A hydrogen emission spectrum is formed by passing an electric current through a discharge tube containing hydrogen gas at low pressure.
• Some electrons in the individual atoms absorb a sufficient amount of energy of a specific wavelength and get excited/promoted to higher energy levels.
• The energy absorbed is equal to the energy difference between two the energy levels.
• When the excited electrons fall from higher energy levels to lower energy levels, they emit energy in form of electromagnetic radiation of definite wavelengths.
• This gives rise to an emission spectrum which can be recorded as a series of sharp colored lines on a black background on a photographic plate.
• Lyman series, Balmer series, Paschen series, Brackett series, Pfund series and Humphrey series are formed when excited electrons fall back from higher energy levels to lower energy levels n=1, n= 2, n= 3, n=4, n=5 and n=6 respectively.

Question 8

Describe the atomic emission spectrum of hydrogen

Answer 8

• The emission spectrum of hydrogen consists of a series of sharp continuous lines in the Ultraviolet, Visible and Infrared regions of the electromagnetic spectrum.
• Each line in the series represents radiation of definite wavelength or frequency.
• In each series, the lines become closer as the frequency increases (wave length decreases) and at very high frequencies, the lines converge to form a continuum.
• The series of lines in the ultraviolet region is called the Lyman series.
• The series of lines the visible region are the Balmer series
• The series lines in the Infrared region are the Paschen series, Brackett series, Pfund series and Humphrey series.

Question 9

Give evidence from the atomic hydrogen spectrum for existence of energy levels

Answer 9

1. A hydrogen atom consists of a single electron but its emission spectrum does not consist of a single line but different series of single lines close together.

This shows the single hydrogen electron can occupy different energy levels and within each energy level there are sub-energy levels.

2. The spacing between adjacent lines in each series of the spectrum is different and decreases with decreasing wavelength showing that the series of lines are produced from radiations of different wavelengths due to electrons falling from different energy levels to lower energy levels.
3. The existence of a continuum between the series of lines shows that there are distinct energy levels and the different lines between the continuums indicate different sub-energy levels.
4. The number of lines in the hydrogen spectrum increases with increasing resolving power.

Question 10

State the quantum theory

Answer 10

Matter cannot absorb or emit energy in continuous amounts, but only in small discrete packets/units called Quanta.