The smallest particle of an element that still retains the properties of that element. Made of two regions Nucleus In center of atom Extremely small Majority of Mass of Atom Protons/Neutrons Electron Cloud Contains electrons
Cathode Rays move away from a negative charge Cathode Rays move away from a magnetic field in the same manner as a wire carrying a negative charge Found the Cathode Rays have mass – could move a paddle wheel in their path Charge to mass ratio of Cathode Rays the same regardless of Type of metal used in electrodes Type of gas used in tube
Cathode Rays are negatively charged Cathode Rays must be a fundamental particle of matter Calculated the charge to mass ratio of the particles Particles later named electrons
Determined the fundamental charge on matter (the electron) Used Thomson’s charge to mass ratio to determine the mass of an electron Verified that electrons are negative
Atom is divisible One of the basic subatomic particles is the negatively charged electron Atom is electrically neutral, so it must contain positive charges to balance out the electrons Electrons have an extremely small mass, therefore there must be other massive particles in the atom
Nucleus existed at the center of the atom Small – extremely small Massive Densely Packed Positive Electrons are in orbit around the nucleus.
All nuclei (execpt H) have protons and neutrons Proton – positive Charge equal and opposite the electron Mass about the same as a neutron 1.673 x 10 -27 kg (1836 x the mass of an electron) Neutron – electrically neutral 1.675 x 10 -27 kg Atom neutral so # protons = # electrons The number of protons determines the identity of the element
When two protons are extremely close to each other, there is a strong attraction between them. A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together. The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces.
The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud. Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms. This unit is the picometer, pm.