3.1.1.2 Mass number and isotopes

Where is the mass of an atom concentrated and why?

• It’s concentrated in the nucleus because it contains the heaviest subatomic particles (neutrons and protons)

What are the charges of the the subatomic particles?

• proton; +1

• neutron; 0

• electron; -1

What is the mass of each subatomic particle?

• Proton; 1

• Neutron; 0

• Electron; negligible (0.0005)

How is mass number and atomic representing?

Mass Number;

• represented using A and can be calculated as the sum of protons and neutrons in an atom

• seen as bottom number in periodic table

Atomic Number;

• represented using Z and equal to number of protons in an atom

• seen as top number in periodic table

What is the Ar?

• Ar is the relative atomic mass

• Can be defined as the mean mass of an atom of an element, divided by 1/12th of the mean mass of an atom of the carbon-12 isotope.

What is an isotope?

• Isotopes are atoms of the same element with the same number of protons, but different number of neutrons

• Same atomic number, but different mass number

• How do isotopes differ?

• Neutral atoms of isotopes will react chemically in the same way as their proton number and the electron configuration is the same (sharing and transfer of electrons is unaffected)

• BUT, the different mass numbers mean they will have different physical properties.

How are ions formed?

When an atom loses or gains electrons, making it no longer neutral and therefore will have an overall charge.

What is mass spectrometry?

An analytical technique used to identify different isotopes and find the overall relative atomic mass of an element.

How does mass spec work?

1. The sample is vaporised into a gas

2. It is then put into the mass spectometer

3. The gas sample is immediately ionised.

4. The +ve ions are accelerated by an electric field. A -ve acceleration plate attracts the +ve ions on the back.

5. After acceleration, all ions have the same kinetic energy

6. The ions drift through

7. Ions are detected in detector

8. A mass spectrum is generated using the charged ions

What are the 4 main events in a TOF spectrometer?

1. Ionisation

2. Acceleration

3. Ion drift

4. Detection

How does electron impact work?

• An electron gun fires high energy electrons

• This knocks out the outermost electron to form a positive ion

• It is used for elements and substances with low formula mass

• It can cause larger organic molecules to fragment.

How does electrospray ionisation work?

1. The sample is dissolved in a volatile, polar solvent.

2. Then it’s injected through a small, fine needle, giving a fine mist or aerosol

3. The tip of the needle has a high voltage

4. At the tip, the sample molecule gains a proton from the solvent , forming MH+ (M(g) + H+ ==> MH+(g))

5. The solvent then evaporates away while the MH+ ions move towards a negative pole.