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Periodic Trends in Ionisation Energies and Covalent Radii

Periodic Trends in Ionisation Energies and Covalent Radii. Trends in the Periodic Table and Bonding. Covalent Radii of Elements. The size of an atom is measured by it’s covalent radius, the distance between the nucleus and it’s outer electrons. nucleus. covalent radius. energy levels.

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Periodic Trends in Ionisation Energies and Covalent Radii

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  1. Periodic Trends in Ionisation Energies and Covalent Radii Trends in the Periodic Table and Bonding

  2. Covalent Radii of Elements The size of an atom is measured by it’s covalent radius, the distance between the nucleus and it’s outer electrons. nucleus covalent radius energy levels Values for covalent radii can be found on page 5 of the data book

  3. 3+ 9+ - - - - - - - - Looking across a period Across a period we can see the covalent radius decreasing. As we move left to right we are adding a proton to the nucleus and an electron to the outermost energy level. So, from lithium to fluorine: Lithium Atom Fluorine Atom

  4. 3+ 9+ 9+ - - - - - - - - - - - - - - - Looking across a period The lithium atom has a smaller nuclear charge than neon and so a larger covalent radius Fluorine’s greater nuclear charge pulls the outer energy level in closer. radius = 134pm radius = 71pm

  5. Cs Li - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Looking down a group The single electron in the outermost energy level is much further from the nucleus in caesium. The caesium atom also has many more electrons between the single outer electron and the nucleus. This causes the caesium atom to have a much larger covalent radius. This screening effect counteracts the attraction from the greater nuclear charge.

  6. Decreasing Atomic Size Atomic Size Summary Across a period from left to right atomic size decreases This is because of the atom having more electrons & protons and therefore a greater attraction which pulls the atom closer together hence the smaller size.

  7. Decreasing Atomic Size Increasing Atomic Size Atomic Size Summary Down a group atomic size increases This is because of the extra outer energy levels and the screening effect of the outer electrons.

  8. Ionisation Energy The ionisation energy is the energy required to remove one mole of electrons from one mole of atoms in the gaseous state. The first ionisation energy of magnesium: Mg (g) Mg+ (g) + e- 744 kJmol-1 Values for ionisation energies can be found on page 10 of the data book

  9. Ionisation Energy The second ionisation energy of magnesium: Mg+ (g) Mg2+ (g) + e- 1460 kJmol-1 The third ionisation enthalpy shows a massive increase because it requires an electron to be removed from magnesium’s second energy level. Mg2+ (g) Mg3+ (g) + e- 7750 kJmol-1

  10. Looking across a period From lithium to neon the first ionisation energy increases. Why? B Ne Li Be C N O F Li (g) Li+ (g) + e- 526 kJmol-1 Ne (g) Ne+ (g) + e- 2090 kJmol-1

  11. 3+ - An atom of Lithium The lithium atom has 3 protons inside the nucleus The outer electron is attracted by a relatively small nuclear charge Li (g) Li+ (g) + e- 526 kJmol-1

  12. 10+ - - - - - - - - An atom of Neon The neon atom has 10 protons inside the nucleus Each of neon’s eight outer electrons is attracted by a stronger nuclear charge Ne (g) Ne+ (g) + e- 2090 kJmol-1

  13. Looking down a group The first ionisation energy decreases down a group in the periodic table. Why? Li (g) Li+ (g) + e- 526 kJmol-1 Cs (g) Cs+ (g) + e- 382 kJmol-1

  14. Cs Li - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1. More Energy Levels 2. Screening Effect As we saw with atomic size, the single electron in the outermost energy level is much further from the nucleus in caesium than in lithium. Caesium’s attraction for it’s outer electron is lowered by the screening effect caused by all it’s other electrons.

  15. Increasing Ionisation Energy Ionisation Energy Summary Across a period from left to right ionisation energy increases This is due to the increase in atomic charge having a greater pull on the electrons and therefore more energy is required to remove electrons.

  16. Increasing Ionisation Energy Decreasing Ionisation Energy Ionisation Energy Summary Down a group ionisation energy decreases This is due to the outer electrons being further away from the nucleus and so the attraction is weaker and they are more easily removed.

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