WJEC Chemistry for AS: Study and Rev Guide

Atoms consist of a nucleus made up of positively charged protons and uncharged neutrons surrounded by shells containing negatively charged electrons moving non-stop. Almost all of the atom’s mass is in the nucleus. An atom has the same number of protons and electrons. Elements Each element has its own atomic number . We often incorporate the atomic number and the mass number into the symbol of an element, e.g. the full symbol of fluorine is 19 9 F. 9 is the atomic number and 19 is the mass number. Most elements exist naturally as a mixture of atoms which are only different in their mass numbers. These atoms are called isotopes . For example, naturally occurring chlorine consists of two isotopes, one having a mass number of 35 and one having a mass number of 37 or 35 17 Cl and 37 17 Cl. Ions Positive ions (or cations) form when an atom loses one or more electrons e.g. K K + + e – Negative ions (or anions) form when an atom gains one or more electrons e.g. F + e – F – In both examples the number of protons has not changed. Atomic structure 1.2 Basic ideas about atoms Pointer Although atomic structure is not specifically mentioned in the specification, prior GCSE knowledge about it is expected. Atomic number is the number of protons in the nucleus of an atom. Mass number is the number of protons + the number of neutrons in the nucleus of an atom. Isotopes are atoms having the same number of protons but different numbers of neutrons. Key Terms Grade boost Don’t forget, in any atom: The atomic number = the number of protons The number of protons = the number of electrons The number of neutrons = the mass number – the atomic number a. Give the number of protons, neutrons and electrons in the two main isotopes of zinc, Zn-64 and Zn-66. b. State the number of protons and electrons in (i) 16 O 2– (ii) 207 Pb 2+ . c. State the difference, if any, between the chemical properties of Zn-64 and Zn-66, giving a reason for your answer. QUICKFIRE QUICKFIRE QUICKFIRE 11 1.2 Basic ideas about atoms