P4 Revision

P4 Revision

1. Atoms, isotopes and ions

   • The idea of the atom as the building block of matter has developed over time. What was thought of as a single particle about 1 × 10‾¹º m across is now known to be a collection of smaller particles.
     • Structure of the atom Protons, neutrons and electrons Atoms are very small, they have a radius of around 1 × 10-10 metres. The modern view of the atom is of a nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus.
     • Using atomic symbols Mass number and atomic number are two important pieces of information about an atom. An atom can be represented using the symbol notation: _{Z}^{A}\textrm{X} Where: A is the mass number Z is the atomic number X is the symbol
     • Chlorine atom with mass number 35 and atomic number 17. This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.

2. Models of the atom

   • Atoms are made up of protons, neutrons and electrons. Change the number of neutrons in an atom and it becomes an isotope, change the number of electrons, it becomes an ion.
     • An atom’s nucleus can only be stable if it has a certain amount of neutrons for the amount of protons it has. Elements with fewer protons, such as the ones near the top of the periodic table, are stable if they have the same number of neutrons and protons. For example carbon, carbon-12 is stable and has six protons and six neutrons. However as the number of protons increases, more neutrons are needed to keep the nucleus stable. For example lead, lead-206 has 82 protons and has 124 neutrons.
     • Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay by emitting radiation.
   • After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding. To explain the two types of static electricity, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time: solids cannot be squashed, therefore the atoms which make them up must be solid throughout rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide

3. Uses and dangers of radiation

   • The nuclei of atoms contain a large amount of energy. Releasing this energy would free the world from having to use fossil fuels. There are two methods of doing this: fission and fusion.
   • Irradiation Shining visible radiation from a torch beam onto a hand lights the hand up because the hand has been exposed to light. Exposing objects to beams of radiation is called irradiation. The term applies to all types of radiation including radiation from the nuclei of atoms. Irradiation from radioactive decay can damage living cells. This can be put to good use as well as being a hazard.
   • Irradiation for sterilisation Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source - typically cobalt-60. The gamma rays emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive.
   • Medical irradiation Doctors also use radioactive sources for a number of reasons, eg: sterilisation of surgical instruments beams of gamma rays, called a gamma knife, can be used to kill cancerous tumours deep inside the body These beams are aimed at the tumour from many different directions to maximise the dose on the tumour but to minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose - enough to kill the tumour, but not so much so that the healthy tissue is damaged.
   • Advantages and disadvantages of irradiation Advantages sterilisation can be done without high temperatures it can be used to kill bacteria on things that would melt Disadvantages it may not kill all bacteria on an object it can be very harmful - standing in the environment where objects are being treated by irradiation could expose people’s cells to damage and mutation

4. Radioactive Decay

   • With the wrong number of neutrons, nuclei can fall apart. A nucleus will regain stability by emitting alpha or beta particles and then ‘cool down’ by emitting gamma radiation.

5. balance charges in an atom

   • https://www.bbc.com/bitesize/guides/ztr8nbk/revision/1
     • P4 Revision

6. |Balances of charges

   • Atoms, the smallest particles of matter that retain the properties of the matter, are made of protons, electrons, and neutrons. ... When an atom has one or more missing electrons it is left with a positive charge, and when an atom has at least one extra electron it has a negative charge.