Atomic Structure

• There are three fundamental particles – the proton, the neutron and the electron.
• Both protons and neutrons are found in the nucleus of an atom. Electron orbit the nucleus in atomic shells. 
• Electrons have a charge of negative 1, protons have a charge of positive 1 and neutrons have no charge.
• Electrons also have effectively no mass, it is so small it is considered 0. both protons and neutrons have a mass of 1. 
• Protons and neutrons are held in the nucleus by a force called the ‘strong nuclear force’.
• The strong nuclear force is much stronger than the ‘electrostatic forces’ that hold electrons and protons together in an atom. 
• The number of protons in the nucleus is called the atomic number, or the proton number. 
• The number of protons and electrons is equal, so the atom has an overall neutral charge. 
• The number of electrons in the outer shell of an atom determines its chemical properties. 
• All atoms of the same element have the same number of protons. Atoms of different elements have different atomic numbers. 
• The total number of protons and neutrons combined in the nucleus is the mass number. Mass number = protons + neutrons.
• Electron weigh almost nothing, so do not contribute to the mass number. 
• Atoms of the same element have the same number of protons, and thus also electrons, but the number of neutrons may vary. 
• Isotopes of the same element have the same number of protons and electrons, and so react the same way chemically. 
• Different isotopes of the same element may have different mass numbers as they have different numbers of neutrons. 
• The electron shell closest to the nucleus fills first and can hold up to 2 electrons. The second shell holds up to 8, and the third shell holds up to 18. 
• The mass spectrometer is useful for the accurate identification of relative atomic masses. 
• There are several types of mass spectrometer, though they all work on the principle of forming ions from the sample given and then separating the ions based on the ratio of their charge to their mass.  
• Time of Flight mass spectrometry has six stages; vacuum, ionisation, acceleration, ion drift, detection, data analysis. 
• Vacuum – the whole thing is kept in a high vacuum to prevent the ions that are produced from colliding with molecules in the air. 
• There are two types of ionisation: Electrospray ionisation and Electron Impact ionisation.
• In Electrospray ionisation, the sample is dissolved in a volatile solvent and forced through a fine needle that is connected to the positive side of a high voltage supply.
• This produces tiny positively charged droplets that have gained a proton from the solvent.
• The solvent evaporates into the vacuum and the droplets get smaller until they have no more than one positively charged ion.
• In Electron Impact ionisation, the sample is vapourised and high energy electrons are fired at it from an electron gun, which is a hot wire filament with a current running through it that emits a beam of high energy electrons.
• This usually knocks off one electron, with each particle forming a +1 ion.
• Acceleration – the positive ions are attracted towards a negatively charged plate and accelerate towards it. The lighter ions and more highly charged ions have a higher speed, so reach the plate first. 
• Ion Drift – the ions pass through a hole in the negatively charged plate, forming a beam and travel along the tube, called the flight tube, to a detector. It is in the ion drift that the ions travelling at different speeds separate and spread out. 
• Detection – when ions with the same charge arrive at the detector, the lighter ones are first as they have higher velocities. The flight times are recorded as the positive ions pick up an electron form the detector, which causes a current to flow. 
• Data Analysis – the signal from the detector is passed to a computer which generates a mass spectrum, a graph displaying the different isotopes present in the sample and the abundancies at which they were present at. 
• As it detects individual ions, isotopes are detected separately, as they have different masses. 
• Electrons can be removed from atoms and the energy it takes can be measured. This is called the ionisation energy because as the electrons are removed, the atoms become positive ions. 
• Ionisation energy (IE) is the energy required to remove a mole of electrons from a mole of atoms in the gaseous state and is measured in kJ mol^-1. 
• The energies of removing the electron form an atom one by one can be measured. Starting from the outer electrons and working towards the inner ones. 
• • The first electron needs the least energy as it is being removed from a neutral atom. This is the first IE. 
  • The second electron needs more energy than the first because it is being removed from a +1 ion. This is the second IE. 
  • The third electron needs more energy than the second because it being removed from a +2 ion. This is the third IE. 
  • The fourth needs more and so on. 
• These are called ‘successive ionisation energies’. For example, sodium; One electron is relatively easy to remove, then comes a group of eight that are more difficult to remove, and finally two that are very difficult to remove. 
• This suggests sodium has: one electron furthest away from the nucleus (easy to remove); eight electrons nearer to the nucleus (harder to remove); two electrons very close to the nucleus (very difficult to remove as they are nearest to the positive charge of the nucleus. 
• This tells us the number of in each main level/orbit; 2,8,1. We can tell this by looking at the jump in ionisation energies present in the table. 
• The trends in ionisation energies moving across -> a period in the periodic table can also give information about the energies of electrons in main levels and sub levels. Ionisation energies generally increase across a period because the nuclear charge is increasing, and this makes it more difficult to remove an electron. 
• The 1st IE for aluminium is lower than that of magnesium despite being further across the periodic table because (and increasing nuclear charge), the outer electron is in a further orbital, and less energy is needed to remove it.