PDF Electronic Structure - Chemistry Past Paper

# Getting started Scientists working in a hospital laboratory use a range of core scientific principles. Write a list of core scientific principles you think they might need and why they are useful. Remember these may be to do with physics, chemistry or biology. When you have completed this unit, s...

# Getting started Scientists working in a hospital laboratory use a range of core scientific principles. Write a list of core scientific principles you think they might need and why they are useful. Remember these may be to do with physics, chemistry or biology. When you have completed this unit, see if you can add any more principles to your list. # Periodicity and properties of elements ## A1 Structure and bonding in applications in science ### The electronic structure of atoms You should already know about the structure of an atom. The nucleus contains positive protons and neutral neutrons. Surrounding the nucleus are energy shells containing negative electrons. You should also know that protons and neutrons both have a relative mass of 1 and that the relative mass of an electron is almost 0. Lab technicians need to understand the electronic structure of atoms. They can use this knowledge to predict how chemical substances will behave and react. The protons and the neutrons are found in the nucleus at the centre of an atom. The electrons are in shells or energy levels surrounding the nucleus. Each shell can hold electrons up to a maximum number. When the first shell is full, electrons then go into the second shell and so on. The maximum number of electrons in each shell is shown in Table 1.1. | Electron shell | Maximum number of electrons | |---|---| | 1 | 2 | | 2 | 8 | | 3 | 18 | | 4 | 32 | | 5 | 50 | A sodium atom containing 11 electrons has an electron arrangement of 2, 8, 1. This can be represented by a simple Bohr diagram, as shown in Figure 1.1. This is the simple version of electron structure you will have seen at Key Stage 4. Under Bohr's theory, an electron's shells can be imagined as orbiting circles around the nucleus. However, it is more complicated than this. Electrons within each shell will not have the same amount of energy and so the energy levels or shells are broken down into sub-shells called orbitals. These are called s, p, d and f orbitals. The orbitals have different energy states. The Aufbau principle states that electrons fill the orbital with the lowest available energy state in relation to the proximity to the nucleus before filling orbitals with higher energy states. This gives the most stable electron configuration possible. Electrons have the same charge and so repel each other, so if there is more than one orbital in an energy level (sub-shell) they will fill them singly until all the orbitals in that sub-shell have an electron in them and then they will pair up. Figure 1.2 shows the energy levels of the shells, sub-shells and orbitals for an atom. **Key term** * **Electron configuration** - the distribution of electrons in an atom or molecule. * **Spin** - electrons have two possible states, 'spin up' and 'spin down'. In an orbital, each electron will be in a different 'spin state'. **Step by step: Electron structures** When writing out electron structures, you should follow these rules. Half arrows are used to represent each electron in the orbitals. They are drawn facing up and down as each electron in an orbital will have a different spin. 1. The electrons sit in orbitals within the shell. Each orbital can hold up to two electrons. 2. The first shell can hold two electrons in an s-type orbital. 3. The second shell consists of one s-type orbital and three p-type orbitals. This diagram represents lithium. 4. The third shell consists of one s-type orbital, three p-type orbitals and five d-type orbitals. 5. Electrons fill the lowest energy level orbitals first. 6. Where there are several orbitals of exactly the same energy, for example, the three 2p orbitals in the second shell, then the electrons will occupy different orbitals wherever possible. 7. So the electronic structure of nitrogen (which has 7 electrons) is: 8. And the electronic structure of a sodium atom (which has 11 electrons) becomes: ## Assessment practice 1.1 Copy out the following table and complete the electronic structures for the elements. Three have been done for you. | Element | Number of electrons | Electron structure | |---|---|---| | Hydrogen | 1 | 1s¹ | | Helium | | | | Lithium | | | | Boron | 6 | 1s² 2s² 2p² | | Carbon | | | | Oxygen | 8 | 1s² 2s² 2p⁴ | | Magnesium | | | | Chlorine | | | | Calcium | | | **PAUSE POINT** Try explaining what you have learned so far. **Hint** Close the book and write out all the key concepts you have learned so far. What do you know about electronic structure? Could you draw the electronic structure for calcium? What is new compared to what you learned at level 2 about electronic structure?