Atomic Structure

Atomic Structure

1. In a neutral atom the number of protons must be equal to the number of electrons because they are opposite in charge

2. Atoms and electrons

   • Noble gasses are inert because they have a full outer shell of 8 shells
   • ions are formed by atoms losing or gaining electrons to obtain a full outer shell and therefore become stable
   • atoms can bond by sharing electrons to form a full outer shell of electrons

3. Mass number and isotopes

   • Atomic number Z- the number of protons
   • Mass number A- the number of protons and neutrons
   • Isotopes
     • isotopes are atoms of the same element with the same number of protons but different number of neutrons.
     • They have similar chemical reactions because they have the same number of electrons

4. Mass spectrometry

   • Ionisation- a beam of electrons from an 'electron gun' knocks out electrons from atoms so that they form positive ions
   • Acceleration- the positive ions are attracted towards negatively charged plates and are accelerated to a high speed. The lighter the faster they go
   • Deflection- the beam of ions move towards a magnetic field at right angles. The deflection depends on its m/z. Heavier ions are deflected less than lighter ions and 2+ ions are deflected twice as much as 1+ ions with the same mass. The deflection also depends on the magnetic field strength the stronger the field the greater the deflection
   • Detection- the magnetic field id gradually increased so that ions of increasing mass enter the detector one after the other. Ions strike the detecter accept electrons, lose their charge and create a current which is proportions to the ABUNDANCE of each ion.
   • Atoms lose an electron and become ions. Ions are deflected according to their masses and their charges and arrive at a detector.
     • the instrument is kept under a high vacuum do the ions do not collide with air molecules
     • the sample is vaporisd by heating to its gaseous state
   • Mass spectra
     • The larger the current the greater the abundance
     • Five decimal places- high resolution mass spectrometry and to the nearest whole number is low resolution mass spectrometry
   • Relative molecular mass
     • Many of these peaks result from fragmentation of the molecule but the peak with the largest m/z comes from the unbroken molecular ion, this is called the molecular ion peak. The molecular ion peak is the relative molecular mass of the molecule
     • The relative molecular mass of a molecule is obtained by looking at the peak in the spectrum with the largest m/z i.e. the peak furthest to the right

5. Electron arrangement

   • Orbitals
     • an electron fills a volume in space called its orbital
     • Different orbitals have different energies
     • Any single orbital can hold a maximum of two electrons
   • 4s is of lower energy level than 3d therefore it is filled before 3d
   • two electrons in the same orbital must have opposite spins
   • Atomic orbitals of lower energy are filled first
   • orbitals of the same energy are filled singly before pairing starts, this is because electrons repel each other
   • Ionisation energy
     • Electrons can be removed from atoms by hitting the atoms with a beam of electrons from an 'electron gun'
     • Ionisation energy is the energy required to remove a mole of electrons from a mole of atoms in the gaseous state.
     • The 1st electron needs the least energy to remove from a neutral atom. The 2nd electron needs more energy than the 1st because it is being removed from a 1+ ion. The 3rd electron needs even more energy to remove it because it is being removed from a 2
     • Trends in ionisation energies down a group
       • Ionisation energy decreases down a group because there is more shielding and the atomic radium increases therefore there is less attraction between the nucleus and the outer electron, so it is easier to remove the electron
     • Trends in ionisation energies across period 3
       • the ionisation energy for aluminium is lower because the outer electron is in a 3p orbital which is of slightly higher energy than the 3s orbital. it therefore needs less energy to remove it
       • The small drop for sulfur because there is a pair of electrons in the 3p orbital, the repulsion between the pair of electrons make it easier to remove one of the electrons

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