Atomic radius

• The?atomic?radius?of an element is a measure of the size of an atom
• It is?half?the distance between the two?nuclei?of two covalently bonded atoms of the same type

The atomic radius of a hydrogen atom is determined by halving the distance between the nuclei of two hydrogen atoms covalently bonded

• Atomic radii show predictable patterns across the Periodic Table
  • They generally?decrease?across each Period
  • They generally?increase?down each Group
• These trends can be explained by the?electron?shell?theory
  • Atomic radii decrease as you move across a Period as the atomic number increases (increased positive?nuclear?charge) but at the same time extra electrons are added to the same?principal?quantum?shell
  • The larger the nuclear charge, the greater the pull of the nuclei on the electrons which results in smaller atoms
  • Atomic radii increase moving down a Group as there is an increased number of shells going down the Group
  • The electrons in the?inner?shells?repel the electrons in the?outermost shells,?shielding?them from the positive nuclear charge
  • This weakens the pull of the nuclei on the electrons resulting in larger atoms

Trends in the atomic radii across a period and down a group

• The diagram shows that the atomic radius increases sharply between the?noble?gas?at the end of each period and the?alkali?metal?at the beginning of the next period
• This is because the?alkali metals?at the beginning of the next period have?one?extra principal quantum shell
  • This increases shielding of the outermost electrons and therefore increases the atomic radius

Ionic radius

• The?ionic?radius?of an element is a measure of the size of an ion
• Ionic radii show predictable patterns
  • Ionic radii?increase?with increasing?negative charge
  • Ionic radii?decrease?with increasing?positive charge
• These trends can also be explained by the?electron?shell?theory
  • Ions with?negative?charges are formed by atoms?accepting?extra electrons while the?nuclear?charge?remains the same
  • The?outermost?electrons are further away from the positively charged nucleus and are therefore held only?weakly?to the nucleus which increases the ionic radius
  • The greater the negative charge, the larger the ionic radius
  • Positively?charged ions are formed by atoms?losing?electrons
  • The?nuclear charge?remains the same but there are now fewer electrons which undergo a?greater electrostatic force of attraction?to the nucleus which decreases the ionic radius
  • The greater the positive charger, the smaller the ionic radius

Trends in the ionic radii across a period and down a group