- The nucleus contains almost all of the mass of an atom, as it cotains protons and neutrons.
- The nucleus of an atom contains all of the positive charge.
- The electrons orbit outside the nucleus.
- Electrons are negatively charged.
- Atomic number = number of protons in the nucleus.
- Mass number = number of protons and neutrons in the nucleus.
- Number of neutrons = mass number - atomic number.
- Number of electrons = number of protons (in a neutral atom)
Isotopes and Ions
- Isotopes are atoms with the same number of protons but different number of neutrons.
- Isotopes of an element have the same chemical properties, as they have the same electron arangement.
- Ions are formed when atoms gain or lose elctrons.
- E.g. An atom Cl has 17 electrons, a Cl- ion has gained one electron.
Relative Atomic Mass and Relative Isotopic Mass
Relative Atomic Mass
- Average mass of an atom relative to 1/12 of the mass of a carbon-12 atom.
- To calculate relative atomic mass, add together (mass number x percentage/100).
- 75% of Cl atoms have a mass number of 35
- 25% of Cl atoms have a mass number of 37
- Average mass of a Cl atom = (mass no x percent/100) + (mass no x percent/100)
= (35 x 75/100) + (37 x 25/100)
Relative Isotopic Mass
- Mass of a particular isotope relative to 1/12 of the mass of a carbon-12 atom.
- An orbital is a region that can hold up to two electrons with opposite spins.
- Orbitals have different shapes called - s, p, d and f.
- S orbitals are spherical in shape and come in sets of one.
- P orbitals are hour - glass shaped and come in stes of three.
- D orbitals come in sets of five which can hold up to 10 electrons.
E.g. Iron - 26 electrons - 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Successive Ionisation Energies
Successive Ionisation Energies
- Energy change when one mole of electrons is removed from one mole of gaseous positively charged ions.
- In an element, successive ionisation energies get bigger because the emaining electrons are held more tightly by the unchanged nuclear charge.
First Ionisation Energy
- Energy change when one mole of electrons is removed from one mole of gaseous atoms.
- M(g) -----> M+(g) + e-
- Ionisation is easier if :
- The nuclear charge is smaller.
- The electron is further away from the nucleus.
- There is more shielding.
First Ionisation Energies - First 20 Elements
- First ionisation energy shows periodicity.
Factors affecting the size of ionisation energy
- The size of that attraction will be governed by:
- The charge on the nucleus.
- The distance of the electron from the nucleus.
- The number of electrons between the outer electrons and the nucleus (shielding).
- Whether the electron is on its own in an orbital ( paired electrons are easier to remove).
The Pattern in Periods 2 and 3
- Why the ionisation energy increases:
- There is an increasing number of protons in the nuceus.
- The increasing nuclear charge, drags the outer electrons in closer to the nucleus.
Trends in ionisation energy down a group
- There is a bigger number of protons.
- But, the shielding and nuclear radius overcomes the nuclear charge.
- Li -1s22s11st I.E. = 519 kJ mol-1
- Na -1s22s22p63s11st I.E. = 494 kJ mol-1
Blocks in the Periodic Table
- The s block is group 1 and 2.
- The p block is group 3 and 8.
- The d block is between the s and p blocks.