Structure and bonding

• Atoms are the smallest part of an element that can exist
• All substances are made up of atoms
• In an atom, the number of protons is equal to the number of electrons
• Atoms have no overall electrical charge
• The number of protons in an atom of an element is its atomic number
• Compounds are formed by elements in chemical reactions
• Chemical reactions always involve the formation of one or more new substances
• Compounds contain two or more elements chemically combined in fixed proportions
• Compounds can only be separated by chemical reactions
• Plum pudding model:

  • Suggested that the atom was a ball of positive charge with negative electrons embedded in it
  • Created by JJ Thomson
• Nuclear model:

  • The alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre and that this nucleus was charged
  • Created by Ernest Rutherford
• Niels Bohr:

  • Adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances
  • Theory agreed with experimental observations
• Ernest Rutherford discovered the proton
• Later experiments suggested that the positive nucleus could be subdivided into a whole number of smaller particles, each having the same amount of positive charge
• James Chadwick provided evidence to show the existence of neutrons within the nucleus
• Relative masses:

  • The sum of protons and neutrons is the atom’s mass number
  • Atoms of the same element can have different numbers of neutrons, these atoms are called isotopes of that element
• The elements in the periodic table are arranged in order of atomic number and so that elements with similar properties are in columns known as groups
• Elements in the same group have the same number of electrons in their outer shell
• John Newlands:

  • Looked at how a piano’s eighth note sounds very similar to its first note and he compared this to every eighth element in order of atomic weight
  • Called this his ‘Law of octaves’
• Johan Döberoiner created a pattern called the ‘Law of Triads’
• Dmitri Mendeleev arranged 60 elements in order of atomic weight
• Left gaps for undiscovered elements
• Called the ‘Periodic Table Of Elements’
• Relative atomic mass:

  • The average value that takes account of the abundance of the isotopes of the element
• Electronic configuration:

  • The inner shell only has 2 electrons
  • Any shell after that can have 8 electrons
• Isotopes:

  • When atoms of the same element have a different number of neutrons
  • Naturally occur
• Ions:

  • Atoms can gain or lose electrons for a full outer shell
  • An ion is an atom that has gained or lost electrons
• Bonding:

  • Compounds produced by alkali metals and non-metals are white solids that dissolve in water to form colorless solutions
  • Group 7 wants to gain one electron, whereas group 1 wants to lose one electron. Both react vigorously to have a full outer shell
• Shielding:

  • Outer electrons are farther away from the nucleus in an atom
  • Inner electrons ‘shield’ the outer electrons from the positive charge of the nucleus
• Groups and periods of the periodic table:

  • Periods: Atomic number increases down periods
  • Groups: Number of electron shells down a group
• Group 0 – Noble gases:

  • Non-reactive as have a full outer shell of electrons
  • Only react in extreme conditions
• Group 7 – The Halogens:

  • Reactivity increases as you go down the group; melting and boiling points increase
  • Want to gain an extra electron and fill up their outer shell
• Group 1 – Alkali metals:

  • Have 1 outer electron so are very reactive
  • Reactivity increases as you move down the group; melting and boiling points decrease
• Fractional distillation:

  • Separate a mixture of different liquids with different boiling points
• Key vocab:

  • Atoms: The smallest part of an element that still counts as an element
  • Elements: A substance made of any one atom
  • Compound: Two or more elements which are chemically bonded together and cannot be separated except by chemical reactions
  • Periodic table: The grouping of all the known elements in an organized way
• Word equations:

  • When three or more elements combine and one of them is oxygen, the product is ____ate
  • When two elements join, the product is usually ____ide
• State symbols:

  • Balanced when there’s the same number of each atom on the same side of the equation
  • Helps us see how much of each element is in the reaction