Atomic Structure and The Periodic Table

• Atoms

  All substances are made of atoms. An atom is the smallest part of an element that can exist.

  Atoms of each element are represented by a chemical symbol, eg O represents an atom of oxygen, Na represents an atom of sodium.
• Compounds

  Some elements combine through chemical reactions to form compounds.

  Compounds are made from two or more different elements (types of atoms) combined together in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed, e.g CO2

  Compounds have different properties from the elements they are made from.

  Compounds can only be separated into elements by chemical reactions.
• Elements and the periodic Table

  An element is a substance with only one type of atom. Elements are listed in the periodic table. There are about 100 different elements.

  Elements can be classified as metal or non-metal depending on their properties.

  The columns in the periodic table are called groups and contain similar elements.

  The rows in the periodic table are called periods. Elements show a gradual change in properties across a period
• Mixtures

  A mixture consists of two or more elements or compounds not chemically combined together.

  The chemical properties of each substance in the mixture are unchanged.
• Separating Techniques

  Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography. These physical processes do not involve chemical reactions.
• Type of mixture separated:

  soluble solid dissolved in a liquid (usually water) e.g. salty water

  explanation:

  liquid boils off and condenses in the condenser. The thermometer will read the boiling point of the pure liquid.

  
• Type of mixture separated: A soluble solid and a liquid (E.g. salt and water)

  To separate a soluble solid from a (non- flammable) liquid we use evaporation. If we want to create hydrated salt crystals then do not evaporate all the water from the mixture

  The evaporating basin is wide and shallow, which gives the liquid a large surface area for quicker evaporation

  
• Explanation: When mixture of soluble liquids is heated all liquids are evaporated. The liquid with the lower boiling point, however, forms the greatest percentage of vapour. As the vapour moves up the fractionating column it becomes more rich with the component that has the lowest boiling point. This is due to the vapour mixture condensing and evaporating as it moves up the column.

  A thermometer measures the temperature of the fractions before they condense. The liquid with the lowest boiling point will be the first 'fraction' to collect.

  
• insoluble solid suspended in a liquid (usually water) e.g. sand and water.

  Explanation:

  The insoluble solid (called residue) gets caught in

  the filter paper, because the particles are too big to fit through the holes in the paper.

  The filtrate is the substance that comes through the filter paper.

  
• Plum-pudding model

  The discovery of the electron led to the plum-pudding model of the atom By JJ Thompson in 1904 after John Daltons discover of atom in 1803 The plum-pudding model suggested that the atom was a ball of positive charge with negative electrons embedded in it
• Nuclear model

  The results from the Rutherford and Marsden’s alpha scattering experiments led to the plum-pudding model being replaced by the nuclear model.

  This experiment showed that all the mass of the atom was in the centre. This was called the nucleus In 1911

  The electrons were thought to orbit the nucleus, like planets around the sun.

  In the experiment most of the alpha particles directed at thin gold foil passed through but a few bounced back, suggesting the positive charge was concentrated at the centre of each gold atom
• Neils Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.

  The electrons are on energy levels or shells

  The theoretical calculations of Bohr agreed with experimental observations nucleus

  Bohr Model

  Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles
• In 1932 James Chadwick discovered neutrons and made atomic model we still use today
• Atoms have a small central nucleus made up of protons and neutrons around which there are electrons.

  In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.

  All atoms of a particular element have the same number of protons.

  Atoms of different elements have different numbers of protons.

  Most of the mass of an atom is in the nucleus
• The total number of protons and neutrons in an atom is called its mass number
• Particle Relative Mass Relative C

  Proton 1 +1

  Neutron 1 0

  Electron Very small -1
• The number of protons in an atom is called its atomic number (proton number). Atoms are arranged in the modern periodic table in order of their atomic number (proton number).

  To work out the number of neutrons in an atom subtract the atomic number from the mass number
• The total number of protons and neutrons in an atom is called its mass number
• Isotopes

  Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element. Isotopes of an element have the same chemical properties because they have the same electronic structure
• The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.

  R.A.M =  (isotopic mass x % abundance) 100
• Elements in the same group in the periodic table have the same number of electrons in the highest energy levels (outer electrons) and this gives them similar properties. E.g. group 1 elements all have 1 electron in their outer shell.

  The elements in Group 0 of the periodic table are called the noble gases. They are unreactive because their atoms have stable arrangements of electrons.
• The elements in the periodic table are arranged in order of atomic (proton) number and so that elements with similar properties are in columns, known as groups.

  The table is called a periodic table because similar properties occur at regular intervals.

  Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties
• The modern periodic table can be seen as an arrangement of the elements in terms of their electronic structures.

  o atomic number gives number of protons or

  electrons

  o Elements in the same group have the same number of electrons in their highest occupied energy level (outer shell). (This explains similar reactivity)

  o Elements in the same period have the same number of shells

  o As you go down a group the number of shells increases.
• Early Periodic Tables

  Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.

  The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed.

  Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.
• Mendeleev – leaving gaps

  Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.

  Elements with properties predicted by Mendeleev were discovered and filled the gaps.
• Elements that react to form positive ions are metals. Elements that do not form positive ions are non-metals. (note- hydrogen is an exception to this rule)
• The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table. Non- metals are found towards the right and top of the periodic table.
• Noble Gases

  The elements in Group 0 of the periodic table are called the noble gases. They are unreactive and do not easily form molecules because their atoms have stable arrangements of electrons.

  The noble gases have eight electrons in their outer energy level, except for helium, which has only two electrons The boiling points of the noble gases increase with increasing relative atomic mass (going down the group)
• Group 1 of the periodic table

  The Alkali Metals
• Alkali Metals


  • Have 1 electron in their highest energy level
  • Are metals with low density (the first three elements in the Group are less dense than water)
  • Increasing reactivity down the group
• Reactions of Group 1 metals

  1. React with non-metals to form ionic compounds in which the metal ion carries a charge of + 1

  2. The compounds are white solids which dissolve in water to form colourless solutions
• Reaction with water

  1. React with water releasing hydrogen and form hydroxides which dissolve in water to give alkaline solutions

  2. React vigorously with water fizzing and moving around on the surface of the water
• Increasing reactivity down the group

  As the atoms get larger the outer electron is further from the nucleus, thus the outer electron is less attracted to the nucleus and so can more easily be lost
• Group 7: The Halogens

  The elements in group 7 of the periodic table :

  o all have 7 electrons in their highest energy level

  o have coloured vapours

  o consist of molecules which are made up of pairs of atoms (Cl2, Br2 ,I2 )

  o form ionic salts with metals in which the chloride, bromide or iodide ion (halide

  ion) carries a charge of -1

  o form molecular (simple covalent) compounds with other non-metallic elements.
• In Group 7, the further down the group an element is:

  o the higher its relative molecular mass

  o the higher its melting point and boiling point (due to increased intermolecular forces.) o the less reactive the element.
• Decreasing Reactivity

  Going down the group for halogens:

  • The atoms get bigger

  • and so outer shell electrons are further from nucleus

  • Outer electrons less strongly attracted to nucleus

  • can less easily gain electrons.
• In the periodic table between Groups 2 and 3 is a block of elements known as the transition elements. These elements are all metals.

  Compared with the elements in Group 1, transition elements: • have higher melting points (except for mercury)

  • have higher densities

  • are stronger

  • are harder

  • are much less reactive and so do not react as vigorously with water or oxygen. Many transition elements:

  • have ions with different charges form coloured compounds
  • are useful as catalysts.