Atomic Structure 3

The study of atomic structure forms a large part of GCSE Chemistry. This is the last in our series of three quizzes on the topic and it looks not only at the numbers and arrangement of protons, neutrons and electrons in atoms, their atomic numbers and their atomic mass, but also at the numbers of electrons in the different shells of atoms and how the electrons in an atom’s outer shell are affected when ions are formed.

One of the biggest hurdles to understanding chemistry is the idea of scale. With atomic structure, we are dealing with processes and objects that we cannot see directly which is why it was not until well into the 20th century that protons and neutrons were identified. Our knowledge of what lies inside an atom has been discovered by using indirect observations, for example, the scattering of alpha particles as they passed through a thin piece of gold foil demonstrated that the mass of an atom was concentrated into a very small area at the centre - the nucleus.

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The first of the sub-atomic particles that were identified was the electron. Experiments using 'cathode rays' in 1897 by J.J. Thompson showed that they were the same no matter what element they came from. He guessed that they were some kind of fundamental particle that was a structural part of all atoms. They had a negative charge and in 1904 he suggested that an atom consisted of these 'corpuscles' whizzing round in a 'sea' of positive charge. We now know them as electrons which do move round in atoms but they do so in well defined areas known as 'shells' or 'energy levels'. They remain part of the atom because of the attraction of the nucleus. We also now appreciate that each of the 'shells' contains a specific maximum number of electrons. You only need to know how to work out the arrangement of electrons in each of the first 20 elements and their ions for the GCSE.

For the exam, you need to be able to recognise elements from their electron arrangements and state what happens to electrons and the electron structure of atoms when they bond with other atoms, including the formation of ions.

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  1. An atom contains 9 electrons. How many neutrons are there in the nucleus of this atom?
    Fluorine has 9 electrons around the nucleus. The question tells you it is an atom not an ion so it must have 9 protons and therefore the atomic number of 9. From the Periodic Table, the mass number of fluorine is 19 so 19 - 9 = 10 neutrons
  2. An atom of which element has 11 electrons?
    The question specifically tells you that the element is an atom and not an ion, so it must be sodium
  3. How many electrons are in the first shell of a carbon atom?
    Whatever the element, the first shell can ONLY ever contain 2 electrons maximum
  4. How many electrons are in the first shell of a hydrogen atom?
    Hydrogen is the simplest element with one proton and one electron
  5. Which element has 2 electrons, 2 neutrons and 2 protons?
    There are two protons in the nucleus so it has the atomic number of 2. When you look that up on the Periodic Table, you find that it corresponds to helium. The first shell is complete which is why helium is an unreactive gas
  6. How many electrons are in the outer shell of a nitride 3- ion?
    A nitrogen atom has 5 electrons in its outer shell. A nitride 3- ion must therefore have 8
  7. An element has 2 electrons in its inner shell, 8 in its next shell and 1 in its outer shell. What type of ion would it form?
    This atom will lose an electron to form a 1+ ion
  8. An atom has 7 electrons - 2 in its inner shell and 5 in its outer shell. What group of the periodic table is this element found in?
    There are different ways of numbering the groups of the periodic table. Most UK exam boards still follow the convention that the number of electrons in the outer shell = number of group. If you study chemistry to higher levels, you will find that this is no longer the case other than for groups 1 and 2.
  9. A fluorine atom has 9 electrons - 7 in its outer shell and 2 in its inner shell. How many electrons would be in the outer shell if it were to become a fluoride ion?
    Fluorine gains one electron to become a fluoride ion. It then has 10 electrons but only 9 protons, so overall, the fluoride ion has a charge of -1
  10. An atom has 2 electrons, 2 neutrons and 2 protons. What is the atomic number of this atom?
    The atom is helium. The number of protons is equal to the atomic number

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