Atomic Structure and The Periodic Table

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Atom	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
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 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.
Simple Distillation	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.
Crystallisation/ Evaporation	To separate a soluble solid from a (nonflammable) liquid we use evaporation. If we want to create hydrated salt crystals then do not evaporate all the water from the mixture
Fractional Distillation	Type of mixture separated: Soluble liquids with different boiling points e.g. crude oil The fractionating column has a temperature gradient and is hotter at the bottom than at the top Explanation: When a 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.
Filtration	Type of mixture separated: insoluble solid suspended in a liquid (usually water) e.g. sand and water. Filter funnel Filter paper residue filtrate Filtration 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. 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. Neils Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations
The History of the Development of the Atom	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 The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.
Sub-Atomic Particles	Proton - 1 /+1 Neutron - 1 /0 Electron - Very small /-1
The Atom	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
Size of an Atom	Atoms are very small, having a radius of about 0.1 nm (1x 10-10 m). The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m).
Key Notes	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
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
Calculating Relative Atomic Mass	The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
Equation	R.A.M = Σ (isotopic mass x % abundance) --------------------- 100
Electronic Structure	Electrons occupy particular energy levels. Each electron in an atom is at a particular energy level. The electrons in an atom occupy the lowest available energy levels . The first energy level can hold a maximum of two electrons, the 2nd and 3rd can hold up to 8. (not really true for 3rd ). 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. Elements in the same period have the same number of energy levels.
The Modern Periodic Table	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.
The Early Periodic Table	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.
Key Notes on the Early Periodic Table	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.
Metals and Non-Metals	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- The Alkali Metals	The elements in Group 1 of the periodic table o all have 1 electron in their highest energy level o are metals with low density (the first three elements in the Group are less dense than water) o are stored under oil to prevent them from reacting with oxygen or water
Reactions of Group 1	o Group 1 metals react in a similar way as they all have 1 electron in their outer shell o react with non-metals to form ionic compounds in which the metal ion carries a charge of + 1. The compounds are white solids which dissolve in water to form colourless solutions o e.g. 2 Na + Cl2 -> 2NaCl
Reaction with Water	o Group 1 metals react with water releasing hydrogen and form hydroxides which dissolve in water to give alkaline solutions. (this is why they are known as alkali metals) o They react vigorously with water fizzing and moving around on the surface of the water o e.g. 2Na + 2H2O -> 2NaOH + H2
Increasing Reactivity Down the Group	In Group 1, the further down the group an element is the more reactive it becomes as the atoms get larger the outer electrons are further from the nucleus and thus less attracted to the nucleus and so can more easily lose electrons
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.
Trends in Group 7	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. o the less reactive the element as the atoms get bigger, electrons are further from nucleus and less strongly attracted so it can less easily gain electrons.
Displacement Reactions	A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
Transition Metals	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) and higher densities • are stronger and 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.

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