Describe the structure of the atom. RADIOACTIVITY: ATOMIC STRUCTURE AND ISOTOPES By the end of this presentation you should be able to…… Describe the structure of the atom. Explain and recognise isotopes of an element. Simply state the origin of radioactivity. Recall the properties of the three sub-atomic particles that make up an atom.

What do atoms look like? They are very small!

Relative size of the atom and the nucleus. The ratio of the diameters is 10 000 : 1 ! If the diameter of a particular atom was 10 metres, its nucleus would be 1 millimetre across!!

Structure of the atom nucleus proton neutron electrons

The relative masses and charges of these particles are given below Proton 1 +1 Neutron Electron 1/ 2000 -1

Atoms of different elements are organised based on their behaviour, which is very closely related to the number of protons in their nuclei.

The periodic table shown in the previous slide does not show the information provided in the usual way. Usually each box looks like this…. (A) This is the “mass” or “nucleon number” and is equal to the number of protons + the number of neutrons in the nucleus. 14 N 7 NITROGEN Element symbol Element name (Z) This is the “atomic” or “proton number” and is used to identify the element.

BUT! They then found two atoms of the same element (like chlorine), that seemed to have different masses! They both looked like chlorine, smelled like chlorine, reacted in a similar way….so they must have the same number of protons and electrons. They must be the same element! They solved the dilemma by calling these special atoms “ISOTOPES”, and realised that the difference in the mass must be caused by the particle that contributes to mass, but is neutral, so does not affect reactions!

ISOTOPES Atoms that have the same number of protons (so represent the same element) , but different numbers of neutrons (so have different masses) Hydrogen has isotopes.

Lithium has isotopes

In fact, many elements have isotopes

and some of them are….UNSTABLE! This instability originates in the nucleus, and in an attempt to become more stable, the nucleus will literally “spit out” a particle or some energy. These unstable atoms are called “RADIOACTIVE” The stuff that is spat out is called “RADIATION” If the unstable atom is an isotope, it is called a “RADIOISTOPE”.