Atoms

• Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells
• The numbers of particles in an atom can be calculated from its atomic number and mass number
• Atom


  The smallest part of an element that can exist
• Nucleus


  The central part of an atom, containing protons and neutrons, and having most of the mass of the atom
• Electron


  Subatomic particle with a negative charge and a negligible mass relative to protons and neutrons, arranged in areas called shells
• The nucleus is very small compared to the atom as a whole, and electrons are even smaller, so most of an atom is empty space
• The diameter of a nucleus can be 100,000 times less than the diameter of an atom
• Subatomic particle


  Objects found in atoms, including protons, neutrons and electrons
• Proton


  Subatomic particle with a positive charge and a relative mass of 1
• Neutron


  Uncharged subatomic particle, with a mass of 1 relative to a proton
• Relative mass


  The number of times heavier a particle is, compared to another
• The mass of electrons is very small compared to protons and neutrons, so most of the mass of an atom is concentrated in its nucleus
• Protons and electrons have opposite electrical charges
• Subatomic particles


  • Proton
  • Neutron
  • Electron
• Protons are positive, and neutrons are neutral
• Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells
• Atomic number


  The number of protons in the nucleus of an atom. Also called the proton number.
• Atom


  The smallest part of an element that can exist.
• Proton


  Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1.
• Nucleus


  The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom.
• Element


  A substance made of one type of atom only.
• All the atoms of a given element have the same atomic number, and the atomic number of each element is unique - no two elements have the same atomic number
• Electron


  Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.
• Charge (electrical)


  An imbalance of electrons and protons in a material. An excess of electrons results in negative charge, a deficit of electrons results in positive charge.
• Neutral atom


  An atom in which the number of protons and electrons is the same.
• Mass number


  The number of protons and neutrons found in the nucleus of an atom.
• Subatomic particle


  Objects found in atoms, including protons, neutrons and electrons.
• Calculating numbers of subatomic particles

  1. number of protons = atomic number

  2. number of electrons = atomic number

  3. number of neutrons = mass number - atomic number
• To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number
• The atomic number of a chlorine atom is 17 and its mass number is 35. The number of protons is 17, the number of electrons is 17, and the number of neutrons is 18.