AS/A-level Chemistry - Atomic Structure

Mass number: The number of protons and neutrons in the nucleus of an atom.

Atomic number: The number of protons in an element.

Isotope: An element with the same number of protons and a different number or neutrons. This changes the mass but does not change the chemical properties of an element. 

📚 This definition always come up in AS Chemistry exam! ✏️

Please remember the process of TOF as it always come up in AS/A-level Chemistry! 👩‍🏫

Time of Flight Mass Spectrometry 

There are three main steps to this process which is used to find the relative atomic mass of an atom:

1. Ionisation

• A sample is dissolved and injected through a fine needle into a vacuum
• High voltage is applied which produces a positive charge
• Solvent evaporates leaving 1⁺ ions

1. Separation of Charged Ions

• Ions are accelerated using an electric field so they all have the same kinetic energy
• Ion drift occurs when they enter the flight tube (lighter ions travel faster taking less time to reach the detector)

1. Detection

• Negatively charged plate which produces a current when the positive ions hit the plate, the more ions the bigger the current
• Mass spectrum is used to display the results

Electron Arrangement

S-Block – Outer Electrons are in the S sub-level (Group 1 and Group 2 are s-block)

P-Block – Outer Electrons are in the P sub-level (Group 3 – Group 7)

D-Block – Outer Electrons are in the D sub-level (Transition Methatls)

Ionisation Energy

First Ionisation Energy: 

The enthalpy change when an electron is removed from a gaseous ion to form a uni-positive gaseous ion.

                                             E.g. Na _(g) - e ➔ Na⁺ _(g)

Second Ionisation Energy :

The enthalpy change when an electron is removed from a uni-positive gaseous ion to form a gaseous ion with a 2⁺ charge

                                                E.g. Na⁺ _(g) - e ➔ Na²⁺ _(g)

Third Ionisation Energy:

The enthalpy change when an electron is removed from a gaseous ion with a 2⁺ charge  to form a gaseous ion with a 3⁺ charge

                                               E.g. Na²⁺ _(g) - e ➔ Na³⁺ _(g)

Electron arrangement from Ionisation energies

• Decreasing Ionisation Energies down a Group
  • There are more shells in atoms lower in a group so the outer electrons are further away from the nucleus meaning there is more shielding and therefore weaker forces of attraction between the nucleus and electrons in the outer shell.
• Increasing Ionisation Energies across a Period
  • There is an increased nuclear charge due to more protons and all of the outer electrons are in the same shell meaning that they have similar shielding and stronger attraction to the nucleus.

Reference: 

2.2: Atomic Structure - Chemistry LibreTexts

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