Science - Atomic structure & the periodic table (Chemistry)

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  • Atoms contain protons , neutrons and electrons
  • Every atom has a nucleus :
    • the nucleus is in the middle of the atom
    • it contains protons and neutrons
    • it has a positive charge because of the protons
    • Almost the whole mass of the atom is concentrated in the nucleus
  • The Electrons :
    • They move around the nucleus in electron shells
    • They're negatively charged and tiny , but cover a lot of space
    • The volume of their orbits determine the size of the atom .
    • Electrons have virtually no mass
  • Protons are heavy and positively charged .
    Neutrons are heavy and neutral .
    Electrons are tiny and negatively charged .
  • Protons : Relative mass of 1 : Charge +1
    Neutrons : Relative mass of 1 : Charge of 0
    Electrons : Relative mass is 0 or very small : Charge of -1
  • Atoms are neutral (no overall charge) because they have the same number of protons and electrons.
    The charge on the electrons is the same size as the charge on the protons .
    • The nuclear symbol of an atom tells you an atomic (proton) number and mass number
    • The atomic number shows number of protons
    • The mass number shows total number of protons and neutrons
    Example :
    23
    Na 23 is the mass number
    11 11 is the Atomic number

    There are 11 protons , 11 electrons (the number of both is always the same ) and 12 neutrons (23-11=12)
  • An element is a substance made up of atoms that all have the same number of protons in their nucleus .
    They consist of Atoms with the same atomic number .
    Atoms can have different amounts of protons , neutrons , and electrons . It's the number of protons in the nucleus that decides what type of atom it is .
    If a substance only contains atoms with the same number of protons it's called an element .
    All atoms of a particular element (e.g nitrogen) have the same number of protons and different elements have atoms with different numbers of protons .
  • Isotopes are different forms of the same element with different numbers of neutrons but same number of protons .
    Isotopes have the same atomic number and different mass numbers .
    Example :
    Carbon-12 : (6 protons)
    12 (6 Electrons)
    C (6 Neutrons)
    6
    Carbon-13 : (6 Protons)
    13 (6 Electrons)
    C (7 Neutrons)
    6
  • Many elements exist as a number of different isotopes so relative atomic mass is used instead of atomic mass when referring to the element as a whole . This is an average mass taking into account the different masses and abundances (amounts) of all isotopes that make up the element
  • Formula to work out relative mass of an element :
    relative atomic mass = sum of (isotope abundance x isotope mass no.)
    over sum of all abundances of all the isotopes .
  • Example to work out relative atomic mass :
    Copper has 2 isotopes . Cu-63 has an abundance of 69.2% and Cu-65 has an abundance of 30.8% . Calculate the relative atomic mass of copper to 1 decimal place .
    Relative atomic mass = (69.2 x 63)+(30.8 x 65) divided by 69.2 + 30 .8
    = 4359.6 + 2002 divided by 100
    = 6361.6 divided by 100 = 63.616 = 63.6
  • Atoms join together to make compounds
    1. Elements react
    2. Atoms combine with other atoms
    3. Atoms form compounds
  • Compounds
    Substances formed from two or more elements, the atoms of each are in fixed proportions throughout the compound and they're held together by chemical bonds
  • Making bonds
    1. Atoms give away electrons
    2. Atoms take electrons
    3. Atoms share electrons
  • Only the electrons are involved, the nuclei of the atoms aren't affected at all when a bond is made
  • Ionic bonding

    Metal atoms lose electrons to form positive ions, non-metal atoms gain electrons to form negative ions, opposite charges are strongly attracted
  • Covalent bonding

    Each atom shares an electron with another atom
  • Compounds covalently bonded
    • Hydrogen chloride gas
    • Carbon monoxide
    • Water
  • The properties of a compound are usually totally different from the properties of the original elements
  • Example of compound properties being different from original elements
    • Iron (lustrous magnetic metal) and sulfur (yellow powder) react to form iron sulfide (dull grey solid lump)
  • Formula
    Shows what atoms are in a compound
  • Elemental symbols
    Represent elements
  • Compound formulas
    Made up of elemental symbols in the same proportions that the elements can be found in the compound
  • Carbon Dioxide

    • CO₂
  • Carbon Dioxide

    • Formed from a chemical reaction between carbon and oxygen
    • Contains 1 carbon atom and 2 oxygen atoms
  • Sulfuric acid
    • H₂SO₄
  • Sulfuric acid

    • Each molecule contains 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms
  • Brackets in a formula
    The little number outside the brackets applies to everything inside the brackets
  • Calcium hydroxide

    • Ca(OH)₂
  • Calcium hydroxide

    • 1 calcium atom, 2 oxygen atoms and 2 hydrogen atoms
  • Example formulas
    • Carbon Dioxide - CO₂
    • Ammonia - NH₃
    • Water - H₂O
    • Sodium chloride - NaCl
    • Carbon monoxide - CO
    • Hydrochloric Acid - HCl
    • Calcium chloride - CaCl₂
    • Sodium carbonate - Na₂CO₃
    • Sulphuric acid - H₂SO₄
  • Chemical changes are shown using chemical equations :
    One way to show a chemical reaction is to write a word equation .
    Example :
    methane burns in oxygen giving carbon dioxide and water :
    (the molecules on the left are called reactants , because they react with each other)
    (the molecules on the right are called products because they've been produced with the reactants . )
    methane + oxygen = --> carbon dioxide + water
  • Symbol equations show the atoms on both sides .
    Chemical changes can be shown in a kind of shorthand using symbol equations . Symbol equations just show the symbols or formulas of the reactants and products
  • Symbol equations need to be balanced - there must be the same number of atoms on both sides .
  • Atomic structure theory

    Changed over time
  • John Dalton described atoms as solid spheres and said that different spheres made up the different elements

    Start of 19th century
  • JJ Thompson said from his experiment that atoms weren't spheres

    1897
  • JJ Thompson's measurements

    Showed that an atom must contain even smaller, negatively charged particles - electrons
  • The 'solid sphere' idea of atomic structure had to be changed