Electromagnetic Radiation and Atomic Spectra

Question 1

What is the speed of light?

Answer 1

3 x 10^8 m/s. All electromagnetic radiation travels at the same constant velocity (speed of light) in a vacuum and velocity has the symbol c.

Question 2

What happens to the wavelength as the energy increases?

Answer 2

The wavelength gets smaller and the frequency increases.

Question 3

Between what two types of radiation does visible light lie in the electromagnetic spectrum?

Answer 3

Ultraviolet and infrared radiation.

Question 4

Describe wavelength.

Answer 4

Wavelength is the distance between two crests on a wave and is usually measured in meters or nano meters (one nano meter is 10^-9 m) The symbol for wavelength is lambda.

Question 5

What is frequency?

Answer 5

This is the number of wavelengths that pass a certain point in one second. The symbol for frequency is f and is measured in Hertz (s^-1)

Question 6

What is the relationship between frequency, velocity and wavelength?

Answer 6

f=c/lambda

Question 7

What do we mean by electromagnetic radiation has a dual nature?

Answer 7

Electromagnetic radiation can behave as:

• A stream of particles called photons when electromagnetic radiation is absorbed or emitted.
• A wave motion with characteristic properties such as wavelength and frequency.

Question 8

Do photons in high frequency radiation transfer greater amounts of energy than photons in low frequency radiation?

Answer 8

Yes, which means that energy (E) carried by a photon is related to its frequency.

Question 9

What is the relationship between Energy and frequency?

Answer 9

E=hf, where h is Planck’s constant and has the value 6.63x10^-34. The energy calculated using this relationship is in Joules (J).

Question 10

How do you obtain mol^-1 in units of J mol^-1?

Answer 10

Use the relationship E=Lhf, where L is Avogadro’s constant with the value 6.02x10^23 mol l^-1. Avogadro’s constant is the number of formula units in 1 mole of a substance. So the number of photons in one mole of photons is Avogadro’s constant.

Question 11

What equation do you use to calculate the energy value in kJ mol^-1?

Answer 11

E=Lhf/1000

Question 12

What happens when white light is passed through a prism?

Answer 12

A rainbow effect is seen known as a continous spectrum. It includes all colours in the visible part of the electromagnetic spectrum (400 to 700nm).

Question 13

What happens when energy in the form of high voltage electricity is passed through a tube of gas?

Answer 13

Coloured light is produced which can be analysed by passing it through a prism or using spectroscopy. When this is done, a series of coloured lines is seen instead of a continous spectrum. These lines correspond to specific parts of the elctromagnetic spectrum and is called the atomic emission spectrum for that element. If these lines are in the visible part of the spectru we can see them using a spectroscope but if they lie outwith the visible region we can use other instruments to hep see them.

Question 14

What happens when energy is transferred to atoms?

Answer 14

Electrons become excited and move to higher energy levels.

Question 15

What causes Atomic Emission Spectra?

Answer 15

When excited electrons move back down to lower energy levels, the energy lost is released as photons. Each line in an Atomic Emissin Spectra represents the energy given out when an excited electron moves to a lower energy level.

Question 16

What are the Balmer, Lyman and Paschen Series?

Answer 16

The Balmer series is the series where lines are seen in the visible region and the excited electrons fall down to energy level 2 (n=2).
The Lyman series is the series where the lines are seen in the ultraviolet region and the excited electrons fall down to energy level 1 (n=1).
The Paschen series is the series where the lines are seen in the infrared region and the excited electrons fall down to energy level 3 (n=3).

Question 17

How can the energy difference between energy levels be calculated?

Answer 17

Using the frequency of each line. Either E=Lhf/1000 (for kJ per mole) or E=hf (for J) or E=Lhf (for J per mole)

Question 18

What is AES and AAS used for?

Answer 18

To identify and quantify elements in a sample.

Question 19

What is AES?

Answer 19

Atomic Emission Spectroscopy measures the radiation emitted when excited electrons fall back to lower energy levels to produce and emission spectrum.
It studies the transitions between electronic energy levels in atoms or ions. These energy differences are usually in the visible region of the electromagnetic sprectrum, but can someimes lie in the ultraviolet region.

Question 20

How is AES done?

Answer 20

A sample is converted to a gas and then the electrons are excited by high temperatures using a flame or electricity.
When the excited electrons fall back down to lower energy levels, photons are emitted and radiation of different wavelengths is given out, producing an emission spectrum.
The intesnsity of lines at definite wavelengths can then be measured to determine which elements are in the sample and how much of each element there is.

Question 21

What is AAS?

Answer 21

Atomic Absoption Spectroscopy measures the absorbed radiation needed to promote electrons to higher energy levels, to produce an absoption spectrum.
AAS is when electrons are promoted to higher energy levels by absorbing energy. An absoption spectrum is then produced by measuring how the transmission of light by the sample varies with wavelength.
The wavelength of the absorbed light is specific to a particular element. Therefore this technique is used to identify which elements and how much of tem are in a sample.

Question 22

What is the special relationship between an Emissin Spectrum and an Absoption Spectrum?

Answer 22

The lines in the Emission Spectrum can be super-imposed onto the Absoption Spectrum to create a continous spectrum.

Question 23

What is a calibration graph and what is it used for?

Answer 23

A calibration graph is prepared using samples of known concentraions of the element of interest. The radiation absorbed by these samples is plotted against the concentration.
Therefore when a sample of unknown concentration is analysed by measuring its absorbance, the concentration can be read from the calibration graph.