This tells you the number of protons, and hence the number of electrons. Arrange the electrons in levels, always filling up an inner level before you go to an outer one. The Periodic Table gives you the atomic number of 17. Therefore there are 17 protons and 17 electrons. The electronic arrangements of the first 20 elements After this the pattern alters as you enter the transition series in the Periodic Table.
Noble gases have full outer levels.
This generalisation will need modifying for A'level purposes. It is, however, a simplification and can be misleading. It gives the impression that the electrons are circling the nucleus in orbits like planets around the sun. As you will find when you look at the A'level view of the atom, it is impossible to know exactly how they are actually moving. The circles show energy levels - representing increasing distances from the nucleus.