Trends

• In studying the Periodic Table in Chapter 4 we learned that when elements are arranged in order of increasing atomic number, the properties of the elements recur periodically
• This means that the properties displayed by an element are repeated at regular intervals in other elements
• Atomic radius


  The distance from the nucleus of the atom to the outermost electrons
• Since the position of an electron cannot be precisely located (Heisenberg Uncertainty Principle), it is not possible for chemists to measure exactly where the electron cloud of an atom ends
• Covalent radius


  Half the distance between the nuclei of two atoms of the same element that are joined together by a single covalent bond
• Factors that determine the size of an atom

  • Number of orbits (shells) containing electrons
  • Electrostatic attraction between positively charged protons in the nucleus and negatively charged electrons in the orbits
• Atomic radius trends


  • Atomic radius increases down groups due to additional electrons going into a new energy level and screening effect of inner electrons
  • Atomic radius decreases across periods due to increase in effective nuclear charge and no increase in screening effect
• The values of the atomic radii decrease along any one period, e.g. from lithium to fluorine and from sodium to chlorine
• The values of the atomic radii increase down any one group, e.g. from lithium to sodium to potassium
• Effective nuclear charge


  The nuclear charge after subtracting the effect of the screening electrons
• First ionisation energy


  The minimum energy required to completely remove the most loosely bound electron from a neutral gaseous atom in its ground state
• Ionisation energy values decrease down the groups and increase along the periods in the Periodic Table
• Reasons for ionisation energy trends

  • Ionisation energy decreases down groups due to increasing atomic radius and screening effect of inner electrons
  • Ionisation energy increases across periods due to increasing effective nuclear charge and decreasing atomic radius
• The alkali metals have the lowest first ionisation energies, making them very reactive, while the noble gases have the highest first ionisation energies, making them very unreactive
• Ionisation energy


  The energy required to remove an electron from an atom or ion
• Decreasing atomic radius


  Ionisation energy values increase
• Evidence for the fact that electrons are arranged in energy levels of different energies is also provided by studying the values of a number of ionisation energies of any one particular element
• Atomic radius


  Half the distance between the nuclei of two atoms of the same element, which are joined together by a single covalent bond
• Atomic radius values


  Increase down any one group in the periodic table because the electrons are going into a new energy level that is further from the nucleus and there is an increase in the screening effect by inner electrons
• Atomic radius values


  Decrease from left to right across a period because of increasing effective nuclear charge and no increase in the screening effect
• Ionisation energy values


  Decrease down the groups because of increasing atomic radius and an increase in the screening effect of inner electrons
• Ionisation energy values


  Increase along the periods because of increasing effective nuclear charge and decreasing atomic radius
• Atoms with filled and half-filled outer sublevels have an increased stability leading to such elements having first ionisation energies that are much higher than expected
• Second ionisation energy


  The energy required to remove an electron from an ion with one positive charge in the gaseous state
• Electronegativity values


  Decrease down a group because of increasing atomic radius and an increase in the screening effect of inner electrons
• Electronegativity values

  Increase along the periods because of increasing effective nuclear charge and decreasing atomic radius
• The chemical properties of an element are determined by the number of electrons in the outermost energy level
• Within the alkali metals and halogen groups of the Periodic Table there are definite trends in the chemical and physical properties of the elements