Atoms, Elements & Compounds

• Element


  • A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler
  • There are 118 elements found in the Periodic Table
• Compound


  • A pure substance made up of two or more elements chemically combined
  • There is an unlimited number of compounds
  • Compounds cannot be separated into their elements by physical means
• Mixture


  • A combination of two or more substances (elements and/or compounds) that are not chemically combined
  • Mixtures can be separated by physical methods such as filtration or evaporation
  • E.g. sand and water, oil and water, sulfur powder and iron filings 
• • All substances are made of tiny particles of matter called atoms which are the building blocks of all matter
  • Each atom is made of subatomic particles called protons, neutrons, and electrons
  • The protons and neutrons are located at the centre of the atom, which is called the nucleus
• • The electrons move very fast around the nucleus in orbital paths called shells
• • One relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom.
  • All elements are measured relative to the mass of a carbon-12 atom, so relative atomic mass has no units 
  • Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon
•  Table of subatomic particles

  A) +1

  B) 1

  C) 1

  D) 0

  E) 1/1840

  F) 1-

  
• Proton number


  • The atomic number (or proton number) is the number of protons in the nucleus of an atom
  • The symbol for atomic number is Z
  • It is also the number of electrons present in a neutral atom and determines the position of the element on the Periodic Table
• Mass number


  • The Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom
  • The symbol for nucleon number is A
  • The nucleon number minus the proton number gives you the number of neutrons of an atom
  • Note that protons and neutrons can collectively be called nucleons.
• Electronic configuration

  • We can represent the structure of the atom in two ways: using diagrams called electron shell diagrams or by writing out a special notation called the electronic configuration (or electronic structure or electron distribution)
• Electron shell diagrams

  • Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it
  • The further away from the nucleus, the more energy a shell has
  • Electrons fill the shell closest to the nucleus 
  • When a shell becomes full of electrons, additional electrons have to be added to the next shell
  • The first shell can hold 2 electrons
  • The second shell can hold 8 electrons 
• Electron shells

  A) 2

  B) 8

  C) 8

  
• • Period: The red numbers at the bottom show the number of notations 
  • The number of notations is 3
  • Therefore chlorine has 3 occupied shells 
  • Group: The last notation, in this case 7
  • This means that chlorine has 7 electrons in its outer shell 
  • Chlorine is therefore in Group 7

  
• The noble gases

  • In most atoms, the outermost shell is not full
  • These atoms react with other atoms in order to achieve a full outer shell of electrons making them more stable
  • In some cases, atoms lose electrons to entirely empty this shell so that the next shell below becomes a (full) outer shell
  • They then have the electronic structure of a noble gas (Group VIII)
  • All of the noble gases are unreactive as they have full outer shells and are thus very stable
• Isotopes


  • Isotopes are different atoms of the same element that contain the same number of protons but a different number of neutrons
• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
• Why isotopes share properties

  • Isotopes display the same chemical characteristics
  • This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry
  • The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only
  • The difference in mass affects the physical properties, such as density, boiling point and melting point
• Relative atomic mass


  • Atoms are so tiny that we cannot really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (Ar) is used
  • The relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom
  • All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units
  • For example, hydrogen has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon
• How do I calculate relative atomic mass?

  • The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element.
• Mass number vs Relative atomic mass

  • Relative atomic mass is a rounded number, and takes into account the existence of isotopes when calculating the mass.
  • Relative atomic mass is an average mass of all the isotopes of that element.
  • For simplicity, relative atomic masses are often shown to the nearest whole number (as the mass number)
• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
• The loss or gain of electrons (ionisation) takes place to obtain a full outer shell of electrons
• Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons
• Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons
• All metals lose electrons to other atoms to become positively charged ions
• All non-metals gain electrons from other atoms to become negatively charged ions
• Formation of a cation

  A) lose

  B) two full shells of electrons

  C) 7 more

  D) third

  
• Formation of an Anion

  A) gain 1 electron

  B) 1 more electron

  C) stable

  D) gained

  
• What is ionic bonding? 

  • Ionic compounds are formed when metal atoms react with non-metal atoms
  • Metal atoms lose their outer electrons which the non-metal atoms gain to form positive and negative ions
  • The positive and negative ions are held together by strong electrostatic forces of attraction between opposite charges
  • This force of attraction is known as an ionic bond and they hold ionic compounds together
• Dot-and-cross diagrams

  • Dot and cross diagrams are diagrams that show the arrangement of the outer-shell electrons in an ionic or covalent compound or element
  • The electrons are shown as dots and crosses
  • In a dot and cross diagram:
  • Only the outer electrons are shown
  • The charge of the ion is spread evenly which is shown by using brackets
  • The charge on each ion is written at the top right-hand corner

  
• Ionic compounds

  • Ionic compounds are formed when metal atoms and non-metal atoms react
  • The ionic compound has no overall charge
• Lattice structure

  • Ionic compounds have a giant lattice structure
  • Lattice structure refers to the arrangement of the atoms of a substance in 3D space
  • In lattice structures, the atoms are arranged in an ordered and repeating fashion
  • The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions

  
• Properties of ionic compounds

  • Ionic compounds are usually solid at room temperature 
  • They have high melting and boiling points
  • Ionic compounds are good conductors of electricity in the molten state or in solution 
  • They are poor conductors in the solid state
• Ionic compounds have high melting and boiling points

  • They have giant structures
  • There are strong electrostatic forces of attraction between oppositely charged ions in all directions
  • The forces need lots of energy to overcome them 
• Ionic compounds are poor conductors in the solid state

  • The ions are in fixed positions in the lattice
  • They are therefore unable to move and carry a charge 
• Ionic compounds are good conductors of electricity in the molten state or in solution 

  • When the ionic compound is melted or dissolved in water, the ions are able to move and carry a charge
• The greater the charge on the ions, the stronger the electrostatic forces and the higher the melting point will be

  • For example, magnesium oxide consists of Mg2+ and O2- so will have a higher melting point than sodium chloride which contains the ions, Na+ and Cl-
• Diagram to show the electrical conductivity of ionic compounds

  
• What is the structure of graphite?

  • Each carbon atom in graphite is bonded to three others forming layers of hexagons, leaving one free electron per carbon atom which becomes delocalised
  • The covalent bonds within the layers are very strong, but the layers are attracted to each other by weak intermolecular forces
• The structure and bonding in graphite

  A) covalent

  B) bonds