Atomic structure

• Atom

  Electrically neutral because the number of positive charges balances the number of negative charges (the number of protons = the number of electrons in a neutral atom)
• Atoms are electrically neutral
• Sub-atomic particles

  • Proton
  • Neutrons
  • Electrons
• Neutron

  Relative Mass: 1, Relative Charge: 0, Found in: Nucleus
• Electronic structure


  • Maximum number of electrons in the first shell: 2
  • Maximum number of electrons in the second/third shell: 8
  • 2.8.8.2 rule (up to proton no. 20, calcium)
• Valence electrons


  Electrons in the outermost shell, Determines the chemical properties of the element, Atoms with the same number of valence electrons are found in the same group in the Periodic Table
• Shells


  The number of shells indicates the period number in the Periodic Table
• Sodium atom


  • Sodium atom has 1 electron in the valence (outermost) shell. (i.e. sodium atom has 1 valence electron)
• Proton (atomic) number

  The number of protons in an atom
• Mass (nucleon) number

  The total number of protons and neutrons in the nucleus of an atom
• Sodium atom


  • Number of protons = 11, Number of neutrons = 23 - 11 = 12, Number of electrons = 11

  
• An atom is the smallest particles that has the chemical characteristics of an element
• Isotopes

  Atoms of the same element with the same number of protons but different number of neutrons
• Isotopes

  • Chlorine-35 and chlorine-37
• Properties of Isotopes

  • Isotopes of the same element undergo the same types of chemical reactions (same number of protons = same number of electrons, same number of valence electrons, chemical reactions involve only the electrons)
  • Isotopes of the same element may have different physical properties (different number of neutrons = different relative atomic masses, e.g. melting and boiling points, density)
• Ion

  The particle formed when an atom gains or loses electrons, the number of protons and neutrons remains the same
• Ca atom and Ca2+ ion

  • Ca atom loses 2 electrons to form Ca2+ ion, thus it has 20 - 2 = 18 electrons
• F atom and F- ion


  • F atom gains 1 electron to form F- ion, thus it has 9 + 1 = 10 electrons
• Atoms of metals lose electrons to form positively charged ions
• Atoms of non-metals gain electrons to form negatively charged ions
• Neutrons have no charge and are also located in the nucleus.
• Protons are positively charged particles found in the nucleus.
• Electrons are negatively charged particles that orbit around the nucleus at different distances from the center.
• Protons

  • Relative charge: +1, relative mass: 1
  • Found in: nucleus
• Electrons

  • Found in: the outer shell
  • Relative mass: 1/1836, relative charge: -1