Mass Number and Isotopes

Question 1

What is the atomic (proton) number?

Answer 1

The atomic number of an element is the number of protons in the nucleus

Question 2

What is the mass number of an element?

Answer 2

The total number of protons and neutrons in the nucleus

Question 3

What are isotopes? What is example using chlorine?

Answer 3

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

E.g: 35Cl and 37Cl

Question 4

What is mass spectrometry?

Answer 4

Mass spectrometry is a very powerful instrumental technique which is used to find the relative atomic mass of elements and compounds

Question 5

What is a real life example of using mass spectrometry?

Answer 5

Forensic scientists use mass spectrometers to help identify substances such as illegal drugs

Question 6

How do mass spectrometers work?

Answer 6

Mass spectrometers work by forming ions from a sample and then separating the ions according to the mass/charge ratio of the atoms

Question 7

How many stages is mass spectrometry split into? What is each stage named?

Answer 7

Mass spectrometry is split into four stages. In order: ionisation stage, acceleration stage, drift stage and lastly detection stage

Question 8

The first method of mass spectrometry is ionisation. There are two methods of ionisation. What are they both called?

Answer 8

Electron impact ionisation and electrospray ionisation

Question 9

What occurs during electron impact ionisation?

Answer 9

In electron impact ionisation, the sample is vaporised and high energy electrons are fired towards it. An electron is knocked of from each particle form +1 ions

Question 10

What occurs during electrospray ionisation?

Answer 10

The sample is dissolved into a volatile solvent and forced through a fine, hollow needle. A very high voltage is applied to the end of the needle, which makes the particles gain a proton and become ions as a fine mist. The solvent evaporates leaving +1 ions

Question 11

What is the word equation for electron impact ionisation? What is the word equation for electrospray ionisation?

Answer 11

Electrospray ionisation:
X(g) + H+ (g) = XH+ (g)

Electron impact ionisation:
X(g) = X+ (g) + e-

Question 12

What do you need to remember when using electrospray ionisation?

Answer 12

When using/calculating electrospray ionisation, at the end of the test, one electron must be taken away from the total

Question 13

The second stage of mass spectrometry is the acceleration stage. What occurs in this stage?

Answer 13

The positive ions are attracted towards a negatively charged plate and accelerate towards it. This is done through an electric field so that all the ions have the same kinetic energy.

Question 14

What type of ions will achieve higher speeds?

Answer 14

Lighter ions and more highly charged ions will achieve higher speeds

Question 15

The third stage of mass spectrometry (drift stage) is the separation of the charged ions. What occurs in this stage?

Answer 15

Ions enter the flight tube. Ions with different masses have different time of flights. Therefore the lighter ions travel faster and take less time to reach the detector

Question 16

The fourth and last stage (detection stage) is detection of ions. What occurs in this stage?

Answer 16

The ions hit the detector (a negatively charged plate) and produce a current. The more ions that hit the detector, the bigger the current produced.

Question 17

How is the signal that is produced by the ions recieved and interpreted?

Answer 17

The signal from the detector is passed to a computer which generates a mass spectrum

Question 18

Outline the full process of mass spectrometry:
a. discuss both types of ionisation

Answer 18

1. Electrospray ionisation - the sample is dissolved into a volatile solvent and injected into the vacuum through a fine, hollow needle. The end of the needle is charged with a very high voltage, allowing the particles to gain a proton and making ions. The solvent dissolves leaving the +1 ions

Electron impact - High energy electrons are fired at the sample which knock off one electron from each atom to form a +1 ion

2. The ions are then attracted to a negatively charged plate and accelerate towards it. This is done through an electric field so the ions achieve the same kinetic energy. However light ions or more highly charged ions achieve higher speeds.
3. The ions enter the flight tube. They have different masses which lead to different time of flights. As a result lighter ions travel faster and take less time to reach the detector.
4. The ions hit the detector (a negatively charged plate) and produce a current. The more ions that hit the detector, the bigger the current will be
5. The signal from the detector is passed to a computer which generates a mass spectrum

Question 19

Isotopes of an element have the same chemical properties. Why is this?

Answer 19

Isotopes of the same elements have the same chemical properties because they have the same electronic configurations

Question 20

What is the definition of the term “relative atomic mass”?

Answer 20

The term relative atomic mass refer to the average mass of an atom relative to 1/12 of a carbon 12 atom

Question 21

How do you calculate the mass of an atom, in grams?

Answer 21

relative atomic mass divided by avogadro’s number

Question 22

How do you calculate the mass of an atom, in kgs? (normally)

Answer 22

relative atomic mass divided by (avogadro’s number multiplied by 1000