WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.2 Basic ideas about atoms

Atomic structure is not specifically mentioned in the specification. However, since all learners are expected to demonstrate knowledge and understanding of standard content covered at GCSE level, pages 17–18 give a recap on the minimum knowledge that is required about the structure of the atom and how elements and ions are represented. Atomic structure Atoms are made up of three fundamental particles: the proton, the neutron and the electron.

Electrons surrounding the nucleus

Most of the volume of the atom is empty

Key terms Atomic number  (Z) is the number of protons in the nucleus of an atom. Mass number  (A) 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. Ion  is a particle where the number of electrons does not equal the number of protons.

The nucleus is made up of protons and neutrons. Nearly all the atom’s mass is here

The masses and charges of these particles are very small and so are inconvenient, therefore we call the mass of a proton 1, its charge +1 and we describe the other particles relative to these values.

Particle

Relative mass

Relative charge

proton

1 1

+1

neutron electron

0

Negligible (1/1840) −1 An atom is electrically neutral because the number of negative electrons surrounding the nucleus equals the number of positive protons in the nucleus.

Exam tip

Representing elements and ions All atoms of the same element contain the same number of protons. The number of protons in the nucleus of an atom determines the element to which the atom belongs and is known as the atomic number . It is also useful to have a measure for the total number of particles in the nucleus of an atom. This is called the mass number . The full symbol for an element incorporates the atomic number, mass number and symbol e.g. mass number →  23 Na ←  symbol atomic number →  11 Atoms of the same element are not all identical. They always have the same number of protons, but they can have different numbers of neutrons. Such atoms are called isotopes . Most elements exist naturally as two or more different isotopes. For example, 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. A particle where the number of electrons does not equal the number of protons is no longer an atom but is called an ion and has an electrical charge. 17 DRAFT 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. Study point It is incorrect to state that atomic number = the number of protons and electrons. Isotopes of an element have the same chemical properties. Knowledge check Give the number of protons, neutrons and electrons in the two main isotopes of copper: Cu-63 and Cu-65. 1