Presentation on theme: "Periodic Trends in Ionisation Energies and Covalent Radii Trends in the Periodic Table and Bonding."— Presentation transcript:
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 its covalent radius, the distance between the nucleus and its outer electrons. Values for covalent radii can be found on page 5 of the data book nucleus energy levels covalent radius
Looking across a period Across a period we can see the covalent radius decreasing. So, from lithium to fluorine: 3+ - 9+ - - - -- - - Lithium AtomFluorine Atom As we move left to right we are adding a proton to the nucleus and an electron to the outermost energy level.
Looking across a period The lithium atom has a smaller nuclear charge than neon and so a larger covalent radius Fluorines greater nuclear charge pulls the outer energy level in closer. 3+ - radius = 134pmradius = 71pm 9+ - - - -- - - - - - -- - -
Looking down a group The single electron in the outermost energy level is much further from the nucleus in caesium. Cs Li - - This causes the caesium atom to have a much larger covalent radius. The caesium atom also has many more electrons between the single outer electron and the nucleus. This screening effect counteracts the attraction from the greater nuclear charge. - - - -- - - - - - -- - - - - -- - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - -
Atomic Size Summary Decreasing Atomic Size 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.
Atomic Size Summary Increasing Atomic Size Down a group atomic size increases This is because of the extra outer energy levels and the screening effect of the outer electrons. Decreasing Atomic Size
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-e- 744 kJmol -1 Values for ionisation energies can be found on page 10 of the data book
Ionisation Energy The third ionisation enthalpy shows a massive increase because it requires an electron to be removed from magnesiums second energy level. Mg 2+ (g)Mg 3+ (g)+e-e- 7750 kJmol -1 Mg + (g)Mg 2+ (g)+e-e- 1460 kJmol -1 The second ionisation energy of magnesium:
Looking across a period From lithium to neon the first ionisation energy increases. Why? Li (g) Li + (g)+e-e- 526 kJmol -1 Ne (g) Ne + (g)+e-e- 2090 kJmol -1 LiBe B CNOF Ne
An atom of Lithium The lithium atom has 3 protons inside the nucleus Li (g) Li + (g)+e-e- 526 kJmol -1 3+ - The outer electron is attracted by a relatively small nuclear charge
An atom of Neon The neon atom has 10 protons inside the nucleus 10+ - - - - - - - - Ne (g) Ne + (g)+e-e- 2090 kJmol -1 Each of neons eight outer electrons is attracted by a stronger nuclear charge
Looking down a group The first ionisation energy decreases down a group in the periodic table. Why? Li (g) Li + (g)+e-e- 526 kJmol -1 Cs (g) Cs + (g)+e-e- 382 kJmol -1
1. More Energy Levels 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. Li - Caesiums attraction for its outer electron is lowered by the screening effect caused by all its other electrons. Cs - - - -- - - - - - -- - - - - -- - - - - - - - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - 2. Screening Effect
Ionisation Energy Summary Increasing Ionisation Energy 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.
Ionisation Energy Summary Decreasing Ionisation Energy 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. Increasing Ionisation Energy