Biology-Chemistry

• An atom is the smallest unit of matter that retains all of the chemical properties of an element.
• The number of protons in the nucleus determines which element an atom.
• The number of electrons surrounding the nucleus determines which kind of reactions the atom will undergo.
• Protons and neutrons do not have the same charge, but they do have approximately the same mass.
• A single neutron or proton has a weight very close to 1 amu.
• In uncharged, neutral atoms, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus.
• Protons, neutrons, and electrons are very small, and most of the volume of an atom—greater than 99 percent—is actually empty space.
• The number of protons determines what atom we are looking at.
• The number of protons in an atom is called the atomic number.
• The number of neutrons for a given element can vary.
• Forms of the same atom that differ only in their number of neutrons are called isotopes.
• Mass number = protons + neutrons.
• The relative atomic mass (atomic weight) is an average of the atomic masses of all the different isotopes in a sample, with each isotope's contribution to the average determined by how big a fraction of the sample it makes up.