periodic table

• how did niels bohr adapt his nucleur model?

  • electrons orbit the nucleus
  • electrons are at specific distances from the nucleus
• mendeleev ordered his table in order of atmic mass (not strictly)- in some places he changed order based on atomic weights and group properties.
• Mendeleev left gaps to make sure elements with similar properties were in the same group, indicated existence of undiscovered elements (able to predict their properties).
• newlands ordered his table in order of atomic mass, he realised similar properties occured every eighth element (law of octaves) but it broke down after calcium.
• why is the periodic table periodic?

  similar properties occur at regular intervals, elements with similar properties are found in the same group/column.
• knowledge of isotopes confirms Mendeleev was correct to not order elements strictly using atomic weights as they have different weights of the same element.
• when electrons, protons and neutrons were discovered, elements were ordered in atomic number.
• alkali metals:

  • all react with chlorine to form a white precipitate
  • all react with oxygen to create an oxide
  • reactivity increases going down the groups
  • due to single electron in outer shell
• lithium reaction with oxygen: burns with a strong red flame and produces a white solid.
• lithium reaction with chlorine: white precipitate produced and settles on sides of container
• sodium reaction with oxygen: strong orange flame and produces white solid
• sodium reaction with water: fizzes rapidly, melts into a ball and quickly disappears
• sodium reaction with chlorine: burns with bright yellow flame, clouds of white precipitate produced
• potassium reaction with oxygen: large pieces produce lilac flame, smaller ones make a solid immediately.
• potassium reaction with water: ignites with sparks and lilac flame, floats, melts, disappears very quickly
• potassium reaction with chlorine: more vigorous than with sodium
• noble gases (group 0)

  • full outer shells
  • unreactive and do not easily form molecules because of full shell
  • boiling points of noble gases increase going down the group
• halogens (group 7):

  • 7 outer electrons so similar reaction to noble gases
  • exist in molecules of pairs
  • form ionic compounds with metals
  • form covalent compounds with nonmetals
  • melting and boiling point decrease as you go down the group
  • reactivity decreases as go down
• reactivity of halogens decrease as you go down the group as when the number of shells increase down the group, it is more difficult to attract electrons from other atoms (weaker electrostatic forces).
• in halogens, a more reactive halogen can displace a less reactive one in an aqueous solution of its salt.
• relative charges:

  • proton: +1
  • neutron: 0
  • electron: -1
• atoms have a radius of 0.1nm and the radius of a nucleus is less than 1/10,000 of the atom
• relative mass:

  • proton: 1
  • neutron: 1
  • electron: very small
• what is the mass number of an element?

  the sum of protons and neutrons
• what is the atomic number of an element?

  number of protons
• group 1 properties:

  • soft
  • low density
  • first 3 less dense than water
  • very reactive with similar properties
• transition element properties compared to group 1:

  • harder and stronger
  • higher melting points and higher densities
  • less reactive, so react less vigorously with oxygen or water
• transition metals form coloured compounds and are useful as catalysts
• fluorine: very reactive, poisonous yellow gas
• chlorine: fairly reactive, poisonous dense green gas
• bromine: dense, poisonous, red-brown volatile liquid
• iodine: dark grey crystalline solid or purple vapour
• lithium reaction with water: fizzes steadily, floats, gradually disappears.