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School Chemistry Notes: Important chemistry definitions explained with examples
Section 1.1 Introduction and Some keywords (see also pictures) Many of these terms overlap with each other, so there is some repetition - no harm in that? |
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1.1a ATOMS |
Why can't we trust atoms? Because they make up
everything! (i) Even as far back as ancient Greece ~500 BC philosophers had considered the concept of what would be formed on continuously dividing matter i.e. what was the smallest 'bit' left of any substance. In 1808 the English scientist-chemist Dalton proposed his 'atomic theory' - that all matter was made up of tiny individual units called atoms which could NOT be subdivided into simpler substances. What is more, he proposed the idea that there were different types of atoms which we now call 'elements' and combinations of them produce all the different substances which exist. The different types of atoms are called elements (examples below).
Therefore, more simply, 'atoms are the smallest bits ('building blocks') of an element that can exist'. This also means it is the smallest part of an element that can take part in a chemical reaction. (ii) BUT, remember atoms are built up of even more fundamental sub-atomic particles - the electron, proton and neutron. The centre of an atom, called the nucleus, consists of proton and neutron particles and the electrons move around the nucleus in 'orbital' energy levels. For more details see the Atomic Structure Notes. (iii) In chemical reactions, the atoms rearrange themselves in changing from reactants to new products, usually involving at least two elements and often many more (see chemical change on this page). The mass of atoms varies from about 1 x 10-23 to 1 x 10-21 g The diameter of atoms varies from about 1 x 10-10 to 5 x 10-10 m (0.1 to 0.5 nm) Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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1.1b ELEMENTs and symbols
H I Th Er Ho W Ar U? Element Symbol-name quizzes: easier-pictorial! Metals and non-metals |
(i) A basic
definition of an element
(ii) Extended ideas
Element symbol & name QUIZ - easier-pictorial or Element symbol & name QUIZ harder - no pictures! Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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1.1c MOLECULES and FORMULAE and their representations
and a FORMULA shows which atoms are in a compound and how many of each element are present in the molecule A formula can summarise the atoms present e.g. C2H6
or show how the atoms are connected by their chemical
bonds e.g.
and short note on Nanoparticles |
A MOLECULE is a larger particle formed by the
chemical
combination of two or more atoms.
The molecule may be made up of atoms of a single element e.g. hydrogen formula H2 (H-H, two atoms combined, all atoms the same). Other molecular examples are oxygen O2, nitrogen N2, one form of phosphorus is P4 and one form of sulfur consists of S8 molecules, and another form of carbon is a C60 molecule! OR A compound (a combination of two or more different elements - more examples below) e.g. carbon dioxide formula CO2 (O=C=O, three atoms combined) and in each case the atoms are held together by chemical bonds. (detailed GCSE bonding notes and examples) You can represent molecule in various styles of diagram. For example, you can colour and size code the atoms of different elements, so in the molecule pictured on the upper left, you can tell there are five types of atom (elements) and six atoms in total in the molecule. The second molecule (lower left) shows the molecular structure of ethanol ('alcohol') which consists of two carbon atoms, six hydrogen atoms and one oxygen atom. You will also come across shorthand versions of this diagrammatic style written like
Why they are combined in this particular number and order depends primarily on an atoms combining power (its valence) an advanced concept dealt with in Part 3 equations, formula and valence. There are also styles to give a much greater '3D' impression of the shape of a molecule and they attempt to show the '3D' spatial arrangement of the atoms in a molecule and how the bonds connect them together.
Watch out for some styles of abbreviated formula e.g. pentane can be written as CH3CH2CH2CH2CH3, but, using brackets it can be written as CH3(CH2)3CH3 This style is particularly useful for long molecules like decane CH3(CH2)8CH3 A shorthand way of expressing the 10 carbon atoms and 22 hydrogen atoms of decane! Nanoparticles are bigger than the simple molecules described above. They are typically composed of ~100 atoms and range from 1 nm to 100 nm in size. Particle size comparison: nuclear radius < atomic radius < simple 'small' molecule < nanoparticle < polymer molecules Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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1.1d CHEMICAL BOND |
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1.1e COMPOUNDS, and MOLECULES |
Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century chemistry examinations. |
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1.1f More on formulae and COMPOUNDS
methane CH4
ethane C2H6
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Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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A note on the names of chemicals |
Notes the
names of compounds In most cases the metallic element retains its elemental name in the compound. However, this is not the case with non-metallic elements. When the following non-metals combine with other different elements its name changes e.g.
There are examples where a non-metal retains its element name e.g.
You might also come across the following 'parts' of names .. mono meaning one e.g. carbon monoxide (CO, one oxygen) di meaning two e.g. carbon dioxide (CO2, two oxygen atoms), sulfur dioxide (SO2, two oxygen atoms)
tri meaning three e.g. phosphorus trichloride (PCl3, three chlorine atoms), sulfur trioxide (SO3, 3 O atoms)
You can get 'mixed' numerical names e.g. Fe3O4 can be called triiron tetroxide !! There is another context in which mono..., di... and tri... etc. are used. It is used as a prefix to indicate the number of atoms in a molecule irrespective of the name. The noble gases exist as single atoms e.g. He, Ar, Ne etc. are sometimes referred to as monatomic molecules - but at this level best to think of them as elements consisting of single atoms. Diatomic molecules consist of two atoms e.g. H2, HCl, O2, N2, CO Triatomic molecules consist of three atoms e.g. O3, H2O, CO2
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1.1g CHEMICAL EQUATION |
A chemical change can be represented by a
chemical equation,
either in words or chemical symbols e.g. word equation: sodium hydroxide + hydrochloric acid ==> sodium chloride + water
picture equation:
symbol equation: NaOH + HCl ==> NaCl + H2O symbol equation with state symbols: NaOH(aq) + HCl(aq) ==> NaCl(aq) + H2O(l) The chemicals on the left-hand side of the equation are called the REACTANTS. The chemicals on the right-hand side of the equation are called the PRODUCTS. To summarise the essence of chemistry is:
For lots more examples and how to balance equations see HOW TO WRITE CHEMICAL EQUATIONS with lots of worked examples explained in detail. |
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1.1h (1) MIXTURE compared to a pure substance described below |
Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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1.1h (2) FORMULATION MIXTURES compared to a pure substance described below or a mixture described above |
Formulations are useful mixtures
made up of measured out precise amounts (precise ratio) of two or
more substances. They are special mixtures designed for some particular purpose. Each component in the mixture contributes to the properties of the formulation. Commercially, they are made up from recipes of tried and tested formulations for that particular purpose. If you look around the house, especially in the kitchen and bathroom, you will lists of ingredients on the side of the bottle or package of products - percentages or relative amounts of the components in the formulation. See also lots of detailed descriptions of examples of formulations |
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1.1i A PURE SUBSTANCE and a note on melting point and boiling point |
Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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1.1j IMPURE |
Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
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1.1k PURIFICATION |
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1.1l IONS |
An ion is an atom or a group atoms that carries an overall positive or
negative electric charge. In other words they are NOT a neutral particle like a molecule of water H2O or methane CH4. Examples of positive ions: sodium ion Na+, ammonium ion NH4+, aluminium ion Al3+ Examples of negative ions: chloride ion Cl-, sulfate ion SO42- The charge is notated as a superscript at the top right of the chemical symbol. Apart from a charge of single +/- a number must be used to indicate the overall charge e.g. 2+, 2-, 3+ 3- For ionic equations see a section HOW TO WRITE CHEMICAL EQUATIONS |
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Easy revision quizzes on some basic chemistry concepts 4 linked easy quizzes on "Separation of Mixtures" 7Hwf1 * 7Hwf2 * 7Hwf3 * 7Hwf4 Easy matching pair quiz based on "Solubility and Solutions" 7Hmp1 (important words and definitions) 4 Easy linked word-fill quizzes on "Atoms, Elements and Compounds" 8Ewf1 * 8Ewf2 * 8Ewf3 * 8Ewf4 5 Easy linked word-fill quizzes on "Compounds and Mixtures" 8Fwf1 * 8Fwf2 * 8Fwf3 * 8Fwf4 * 8Fwf5 Easy matching pair quiz on "Elements, Compounds and Mixtures" ecm1mp (matching particle model pictures) |
Section 1.2 Particle Picture examples of Elements, Compounds and Mixtures - useful visual images
The particle model for gases, liquids and solids is fully described and discussed on the States of Matter page
![]() These are changes which do NOT lead to new substances being formed. Only the physical state of the SAME material substance changes. The substance retains exactly the same chemical composition - the same atoms, ions or molecules. Examples ... Melting/fusing, solid to liquid, easily reversed by cooling e.g. ice and liquid water are still the same H2O molecules. Dissolving, e.g. solid mixes completely with a liquid to form a solution, easily reversed by evaporating the liquid e.g. dissolving salt in water, on evaporation the original salt is regained. So freezing, evaporating, boiling, condensing are all physical changes and may be involved in separating a mixture. Separating a physical mixture e.g. chromatography, e.g. a coloured dye solution is easily separated on paper using a solvent, they can all be re-dissolved and mixed to form the original dye. Distillation, filtering are also physical changes. See also '3 States of Matter' - gases, liquids and solids for more examples and particle theory models to explain state changes and the properties of gases, liquids and solids. Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. |
Section 1.4
CHEMICAL
CHANGES - CHEMICAL REACTIONS - reactants and products
A 'chemical change' involves the formation of at least one NEW substance AND there is always an energy change - which might, in some circumstances, be detectable with a thermometer. There are often colour changes and the products may look quite different from the reactants
Heating iron and sulphur is classic chemistry experiment to illustrate what is meant by CHEMICAL CHANGE and you can adapt the general conclusions described at the end of this section to any chemical reaction. A mixture of silvery grey iron filings and yellow sulphur powder is made. The iron can be plucked out with a magnet i.e. an easily achieved physical separation because the iron and sulphur are not chemically combined yet! They are still the same iron and sulphur e.g. even in the mixture iron will dissolve in acids to form a salt and hydrogen, sulfur will burn to form sulfur dioxide gas. However, on heating the mixture, it eventually glows red on its own and a dark grey solid called iron sulphide is formed. Both observations indicate a chemical change is happening i.e. a new substance is being formed. We no longer have iron or sulphur BUT a new compound with different physical properties (e.g. colour) and chemical properties (unlike iron which forms hydrogen with acids, iron sulphide forms toxic nasty smelling hydrogen sulphide!). word equation: iron + sulphur (sulfur) ==> iron sulphide (iron sulfide) or in a symbol chemical equation: Fe + S ==> FeS AND it is no longer possible to separate the iron from the sulphur using a magnet! Note that chemical changes can be expressed in a chemical equation. In chemical reactions, the atoms cannot be made or destroyed, but atoms rearrange themselves in changing from reactants to new products. There is no loss or gain of mass - see 'Law of Conservation of Mass' Further proof of a new substance formed: The original reactant iron, and the iron sulphide product, can be shown to be different substances by their reactions with dilute acid.
So signs that a chemical reaction has happened include: change in appearance e.g. change in colour or texture. temperature changes because an energy change has taken place, change in mass e.g.
and change in the chemical properties of the products compared to the original reactants. Therefore a chemical change is one in which a new substance is formed, by a process which is not easily reversed and usually accompanied by an energy (temperature) change. This is summarised as reactants ==> products as expressed in chemical equations in words or symbols. It doesn't matter how complex the reaction, the atoms you end up with are just the same as the ones you started with in terms of elements and number of atoms, BUT they have rearranged themselves into different substances with different properties e.g. copper carbonate + sulfuric acid ==> copper sulfate + water + carbon dioxide Check out the above reaction 12 atoms, 1Cu, 7O, 1C, 2H, 1S, and five completely different substances, all with different physical and chemical properties, definitely a big chemical change! Apart from experiments and preparations in the laboratory, plenty of chemical changes occur in the home. For a start, you are an extremely complex chemical structure with lots of reactions going on in your body all the time, but others in the home include ...
More advanced ideas [see GCSE notes on atomic structure and chemical bonding]: Atoms are held together in molecules or compounds by electrical forces of attraction between the positive nucleus and the outer negative electrons. Therefore, Atoms, ions or molecules react with each other to become electronically more stable. When chemical reactions occur chemical bonds are broken in the reactants and new bonds made in the formation of the products. Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations.
[SEARCH BOX] and [Main INDEX] See other web pages for:
QUESTIONS - QUIZZES - WORKSHEETS: GCSE Chemistry word-fill exercises Some "Chemistry Basics" online gap-fill worksheet exercises Word-fill quiz "Balancing chemical symbol equations" Word-fill quiz "(1) Important terms used in chemistry" Word-fill quiz "(2) Other terms and chemical symbols" Word-fill quiz "(3) More important terms used in chemistry" Word-fill quiz "Elements and compounds" Word-fill quiz "Particle models of elements, compounds and mixtures" Patterns in properties of elements, compounds & periodic table BIG crossword puzzle (Crossword puzzle answers) Common chemical reactions BIG crossword puzzle (Crossword puzzle answers) GCSE balancing and completing equation exercises: (1)
GCSE 'name and formula' of a compound quizzes (1)
Keywords and phrases: PART 1 This section describes, quotes with examples, definitions of elements, compounds and mixtures, other definitions of some important words common in chemistry, physical changes and chemical changes, particle pictures of elements, compounds and mixtures, what are atoms, molecules, particle pictures-diagrams-images of elements, compounds, mixtures, symbols, formula, pure substance, impure substance, purification, evidence and explanations of a chemical change or a physical change, what is a chemical reaction? Iron and sulfur reaction - index of keywords-terms-phrases explained below. These revision notes should prove useful for the new AQA, Edexcel and OCR GCSE (9-1) courses. aqa gcse 9-1 chemistry: Know that 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. There are about 100 different elements. Elements are shown in the periodic table. Compounds are formed from elements by chemical reactions. Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae using the symbols of the atoms from which they were formed. Compounds can only be separated into elements by chemical reactions. Chemical reactions can be represented by word equations or equations using symbols and formulae. You will be supplied with a periodic table for the exam and you should be able to: Revision notes on important definitions in chemistry e.g. atom, element, compound, molecule, formula, mixture, pure substance, impure, comparing physical change & chemical change when revising for AQA, Edexcel, OCR gateway science, OCR 21st century GCSE 9-1 chemistry examinations. information on chemistry definitions atom molecule formula element compound mixture impure pure substance purification definitions of physical change chemical change evidence KS4 Science chemistry definitions atom molecule formula element compound mixture impure pure substance purification definitions of physical change chemical change evidence GCSE chemistry guide notes on chemistry definitions atom molecule formula element compound mixture impure pure substance purification definitions of physical change chemical change evidence for schools colleges academies science course tutors images pictures diagrams of apparatus for 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Doc Brown's Chemistry |
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