Group 0

Group 0

  • Known as noble gases, Group 0 consist of helium, neon, argon, krypton, xenon, and radon.
  • These elements are unreactive due to their full outer shell of electrons.
  • They are monatomic gases - exist as single atoms.
  • The boiling points increase down the group due to increase in size of atoms.

Chlorine and Iodine

  • Both chlorine and iodine are part of Group 7 - the halogens.
  • Chlorine is a green gas at room temperature whereas iodine is a grey solid.
  • Both elements form salts with metals.
  • Chlorine is more reactive than iodine due to its ability to gain an electron more readily.

Alkali Metals

  • Alkali metals consist of lithium, sodium, potassium, rubidium, cesium, and francium, these metals occupy Group 1 of the periodic table.
  • All alkali metals have one electron in their outer shell.
  • They are highly reactive with water and oxygen, reacting more vigorously down the group.
  • Alkali metals have low melting points compared to most other metals, decreasing down the group.

Electronic Structures of the First 20 Elements

  • The structure details how electrons are arranged in energy levels or shells around the nucleus of an atom.
  • The first shell can hold up to 2 electrons, and the second and third can each hold up to 8 electrons.
  • To be stable, atoms aim to have a full outer shell, often sharing, donating or accepting electrons to achieve this.

Order of Elements in the Modern Periodic Table

  • Elements are ordered by increasing atomic number.
  • Rows and columns are known as periods and groups respectively.
  • Groups contain elements with similar chemical properties and reactions; they have the same number of outer shell electrons.
  • Periods represent another full set of electron shells.

Atomic Number, Mass Number, Isotope

  • Atomic number (Z) represents the number of protons in an atom.
  • Mass number (A) is the sum of the number of protons and neutrons.
  • Isotopes are atoms of the same element with different numbers of neutrons, hence different mass numbers but same atomic numbers.

Relative Masses and Charges of Protons, Neutrons and Electrons

  • Protons and neutrons both have a relative mass of 1.
  • Electrons have a much smaller relative mass, close to 0.
  • A proton carries a +1 charge, an electron carries a -1 charge, and a neutron is neutral (0 charge).


  • An atom is the smallest part of an element that retains the properties of that element.
  • Consists of a central nucleus containing protons and neutrons.
  • Electrons orbit the nucleus in energy levels or shells.
  • The number of protons equals the number of electrons in an atom, making it overall electrically neutral.