Unit 1: Section 1 - Particles and Radiation

Unit 1: Section 1 – Particles and Radiation Atomic Structure Atoms are made up of protons, neutrons and electrons. The nucleus contains protons and neutrons known as nucleons. Orbiting the nucleus are the electrons.   Each atom is about 1x10-10m in diameter. Although the nucleus is only small, the majority of the diameter arises from the vast distances the electrons orbit at.   Proton number   The proton number is the number of protons in the nucleus. Usually the proton number is referred to by the letter Z and is found on bottom when written in standard notation.   It is the proton number that defines an element; no two elements have the same number of protons. If an atom is described as being neutral, the number of electrons orbiting the nucleus equals that of the number of protons found in the nucleus. The number of electrons in an atom determines the way an element reacts and their chemical behaviour and therefore the proton number can tell you a large amount about an elements chemical property.   Nucleon Number   The nucleon number is also known as the mass number, and is represented by the symbol A. It describes the number of protons and neutrons found in the nucleus.   Isotopes   “Atoms with the same number of protons but different number of neutrons”   A difference in the number of neutrons affects the stability of the nucleus, usually the higher the number of neutrons in comparison to the number of protons, the greater the level of instability.

Stable and Unstable nuclei   Strong Nuclear force  The nucleus is held together by the strong nuclear force. It must therefore be an attractive that is stronger than the electromagnetic force, which repels like charged atoms. The Strong nuclear force has a very short range and is only able to hold nucleons together when they are separated by about as far as 3 femtometres (1fm = 1x10-15m) the size of a nucleus. At very small distances however, the strong force has an opposite effect and becomes repulsive, this prevent the nucleus from collapsing into itself.  α Emission  Alpha emission occurs when the nucleus of an atom is too big for the strong nuclear force to keep them stable. As a result an alpha particle is emitted. The proton number decreases by two and the nucleon number decreases by four.  β Emission  Beta-minus decay is the emission of an electron from the nucleus with an antineutrino. It happens when there are too many neutrons in comparison to the number of protons in the nucleus. The proton number increases by one and the nucleon number remains the same. 

Particles and Antiparticles  Every particle has its own antiparticle with the same mass but with opposite charge, baryon, lepton and strangeness numbers.

Atomic Structure

Stable and Unstable Nuclei

Particles and Antiparticles