Key concepts in Chemistry

• Covalent compounds:

  A bond formed between two atoms, where electrons are shared, is called a covalent bond. Covalent compounds usually form between non-metal elements and are held together by covalent bonds. All compounds that exist as molecules are covalent.
• Ionic compounds:

  A bond formed between two atoms where an electron is transferred from one atom to the other is called an ionic bond. Ionic compounds are made up of atoms joined together by ionic bonds. They usually (but not always) contain at least one metal element and one non-metal element.
• Ions:

  An ion is a charged particle formed when an atom, or a group of atoms, loses or gains electrons.
• Example word equations

  Potassium hydroxide reacts with sulfuric acid. Potassium sulfate and water are formed in the reaction. This means that:

  • the reactants are potassium hydroxide and sulfuric acid
  • the products are potassium sulfate and water
  • the word equation is: potassium hydroxide + sulfuric acid → potassium sulfate + water
• A balanced ionic equation shows the reacting ions in a chemical reaction. These equations can be used to model what happens in precipitation reactions.
• Precipitation reaction: two soluble reactants form an insoluble product and a soluble product

  AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

  The Na+ ions and NO3- ions remain separate in the sodium nitrate solution and do not form a precipitate. Ions that remain essentially unchanged during a reaction are called spectator ions. You only need to model how the solid silver chloride forms:

  Ag+(aq) + Cl-(aq) → AgCl(s)

  • the number of positive and negative charges is the same
  • the numbers of atoms of each element on the left and right are the same
• Hazard Symbols

  
• Dalton's model

  John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls. Dalton's model included these ideas:

  • atoms cannot be broken down into anything simpler
  • the atoms of a given element are identical to each other
  • the atoms of different elements are different from one another
  • during chemical reactions atoms rearrange to make different substances

  The Dalton model has changed over time because of the discovery of subatomic particles.
• Date Discovery

  1897 - Electrons

  1909 - 1911 - Atoms have a nucleus with electrons around it

  1913 - Electrons occupy shells (energy levels)

  1918 - Protons

  1932 - Neutrons
• As a result of these discoveries, we now know that:

  • atoms can be broken down into three smaller particles: protons, neutrons and electrons
  • atoms of an element have identical numbers of protons and electrons, but can have different numbers of neutrons
  • atoms of different elements have different numbers of protons and electrons
• Nucleus and Shells

  
• Sub-atomic particles

  
• Subatomic particle - Relative mass - Relative charge

  Proton 1 +1

  Neutron 1 0

  Electron 1/1835 -1
• Atomic number:

  The atomic number of an atom is the number of protons in its nucleus. Remember that:

  • all the atoms of a given element have the same atomic number
  • the atomic number of each element is unique - no two elements have the same atomic number

  An atom contains equal numbers of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms are neutral overall
• • number of protons = atomic number
  • number of electrons = atomic number
  • number of neutrons = mass number - atomic number

  A) mass numberr

  B) atomic number

  
• Isotopes:

  Atoms of an element that have the same number of protons, but different numbers of neutrons in their nuclei are called isotopes. Isotopes of an element have:

  • the same atomic number
  • different mass numbers
• relative atomic mass: (mass 1 x abundance) + ( mass 2 x abundance)

  --------------------------------------------

  total abundance
• Features of Mendeleev's tables

  Mendeleev arranged the elements in order of increasing relative atomic mass. When he did this he noted that the chemical properties of the elements and their compounds showed a periodic trend. He then arranged the elements by putting those with similar properties below each other into groups. To make his classification work Mendeleev made a few changes to his order:

  • he left gaps for yet-to-be-discovered elements
  • he switched the order of a few elements to keep the groups consistent

  
• Predictions using gaps

  Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical properties and physical properties of the elements next to a gap, he could also predict the properties of these undiscovered elements. For example, Mendeleev predicted the existence of 'eka-silicon', which would fit into a gap next to silicon. The element germanium was discovered later. Its properties were found to be similar to the predicted ones and confirmed Mendeleev's periodic table.
• Pair reversals

  Iodine has a lower relative atomic mass than tellurium. So iodine should be placed before tellurium in Mendeleev's tables. However, iodine has similar chemical properties to chlorine and bromine. To make iodine line up with chlorine and bromine in his table, Mendeleev swapped the positions of iodine and tellurium.
• An electronic configuration is the way in which electrons are arranged in an atom.
• Electronic configuration feature - Link to the periodic table

  Number or numbers of circles Period number

  Number of electrons in outer shell Group number

  Total number of electrons in shells Atomic number
• Forming ions

  An ion is an atom or group of atoms with a positive or negative charge. Ions form when atoms lose or gain electrons to obtain a full outer shell:

  • metal atoms lose electrons to form positively charged ions
  • non-metal atoms gain electrons to form negatively charged ions
• Forming positive ions

  Metal atoms lose electrons from their outer shell when they form ions:

  • the ions formed are positive, with more protons than electrons
  • the ions formed have full outer shells

  For elements in groups 1, 2 and 3, the number of electrons lost is the same as the group number.

  
• Forming negative ions

  The outer shell of non-metal atoms gains electrons when they form ions:

  • the ions formed are negative, because they have more electrons than protons
  • the ions formed have full outer shells

  For groups 6 and 7, the number of electrons gained is equal to (8 - group number)

  
• Forming ionic bonds

  Positively charged ions are called cations, and negatively charged ions are called anions. These ions can form when a metal reacts with a non-metal, by transferring electrons. The oppositely charged ions are strongly attracted to each other, forming ionic bonds.
• Dot and cross diagrams

  A dot and cross diagram can model the transfer of electrons from metal atoms to non-metal atoms. The electrons from one atom are shown as dots, and the electrons from the other atom are shown as crosses. For example, when sodium reacts with chlorine, electrons transfer from sodium atoms to chlorine atoms.

  
• The ions in a solid ionic compound are not randomly arranged. Instead, they have a regular, repeating arrangement called an ionic lattice. The lattice is formed because the ions attract each other and form a regular pattern with oppositely charged ions next to each other. Ionic bonds are strongelectrostatic forcesbetween oppositely charged ions.

  
• Properties of ionic compounds:

  • high melting and boiling point- has strong electrostatic forces so a lot of energy is needed to overcome these ionic bonds so ionic compounds have high melting and boiling points.
  • conductivity of electricity- conduct electricity when molten or dissolved in water because ions are free to move from place to place. Ionic compounds cannot conduct electricity when solid, as their ions are held in fixed positions and cannot move.

  -insulators when solid
• Working out a formula:

  Example 1

  Sodium chloride contains Na+ and Cl- ions:

  • this is one positive charge and one negative charge
  • the charges are balanced
  • so the formula is NaCl

  Example 2

  Magnesium oxide contains Mg2+ and O2- ions:

  • this is two positive charges and two negative charges
  • the charges are balanced
  • so the formula is MgO

  Example 3

  Aluminium oxide contains Al3+ and O2- ions:

  • this is three positive charges and two negative charges
  • to balance, we need two Al3+ ions and three O2- ions
  • so the formula is Al2O3
• Polyatomic ions:

  Example

  Calcium hydroxide contains Ca2+ and OH- ions:

  • this is two positive charges and one negative charge
  • to balance it will need, one Ca2+ ions and two OH- ions
  • so the formula is Ca(OH)2
• The name of an ionic compound ends in:

  • -ide if it contains just two elements
  • -ate if it contains three or more elements, one of which is oxygen
• Forming a covalent bond

  A covalent bond is formed when a pair of electrons is shared between two atoms, usually non-metals.
• Covalent bonding

  
• A molecule consists of a group of two or more atoms joined together by covalent bonds
• Covalent bonding examples

  
• Properties of simple molecular structure:

  • low melting and boiling point- intermolecular forces are weak so very little energy is needed to overcome the intermolecular forces, so have low melting and boiling points.
  • no conduction of electricity- no overall charge, or charged particles that can separate, so cannot conduct electricity
• giant covalent substances, which contain many atoms joined by covalent bonds
• Giant covalent substances:

  • high melting and boiling point- giant covalent substances have strong covalent bonds, so a lot of energy is needed to break up these large structures during melting and boiling.
  • can't conduct electricity- have no overall charge so can't conduct electricity except carbon
  • insoluble in water- giant covalent substances cannot form strong attractions with water, so they are insoluble
• Diamond:

  • each carbon atom is joined to four other carbon atoms by covalent bonds
  • the carbon atoms form a regular tetrahedral network structure
  • there are no free electrons
  • hard so used for cutting tools or jewellery