2.2The Particulate Nature of Matter
Elements, compounds and mixtures — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
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Lesson summary
2.2 Elements, compounds and mixtures — COMBINED SCIENCE (CHEMISTRY) notes: Define elements, compounds and mixtures and distinguish them by composition, bonding and separation.Element — = substance made of atoms that all have the same number of protons and that cannot be split int…
- Fixed ratio
- Properties
- Proportions
- Physical method
- Pure substance
- Number — there are about 118 elements in the Periodic Table
Section 1 of 14Elements
Element
- = substance made of atoms that all have the same number of protons and that cannot be split into anything simpler by chemical means.
Number
- Number there are about 118 elements in the Periodic Table.
Examples
- Examples iron, oxygen and sodium are elements.
Section 2 of 14Compounds
Compound
- = pure substance made of two or more elements chemically combined.
Fixed ratio
- Fixed ratio the elements are always combined in the same proportions, such as H2O for water.
Properties
- Properties a compound has properties different from its elements.
- Sodium chloride is unlike sodium or chlorine.
Separation
- Separation a compound can only be split into its elements by a chemical reaction.
Examples
- Examples copper(II) sulfate, calcium carbonate and carbon dioxide.
Section 3 of 14Mixtures
Mixture
- = two or more substances that are not chemically combined.
Proportions
- Proportions a mixture can have any proportions.
Properties
- Properties each part keeps its own properties.
- Iron filings in sand stay magnetic.
Separation
- Separation a mixture is separated by physical methods such as filtration, evaporation and distillation.
Examples
- Examples sand and water, oil and water, air.
Section 4 of 14Comparing the three types
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Made of | One type of atom | Two or more elements | Two or more substances |
| Chemically joined | — | Yes | No |
| Fixed proportions | — | Yes | No |
| Properties | Its own | Different from its elements | Each part keeps its own |
| Separation | Cannot be split | Chemical reaction | Physical methods |
| Examples | Iron, oxygen | Water, carbon dioxide | Air, salty water |
Section 5 of 14Common examples classified
Common examples classified
Air
- Air is a mixture of gases.
Water
- Water is a compound.
Oxygen
- Oxygen is an element.
Section 6 of 14More from the specification
1Noble gases
- The Group 0 elements, which are unreactive monatomic gases
2Atomic number (Proton number)
- The number of protons located in the nucleus of an atom, defining its chemical identity.
3Electronic configuration
- The specific arrangement of electrons in successive energy shells surrounding the atomic nucleus.
4Relative atomic mass (Ar)
- The weighted average mass of naturally occurring atoms of an element relative to 1/12th the mass of a carbon-12 atom.
5Isotopes
- Atoms of the same element containing identical numbers of protons and electrons but differing numbers of neutrons.
6Valence shell
- The outermost electron shell whose electron configuration determines chemical reactivity and bonding capacity.
7Symbol
- Shorthand for an element, e.g. Na.
8Electronic configuration
- The number of electrons in each shell, written with commas.
9Outer shell
- Highest-energy occupied shell.
10Electron
- Negative particle moving around the nucleus.
11Neutron
- Neutral particle in the nucleus.
12Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons.
13Isotope
- Atoms of an element with different numbers of neutrons.
14Nucleus
- Central part of an atom with protons and neutrons.
15Shell
- An energy level around the nucleus that holds a limited number of electrons.
16Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Section 7 of 14Particle diagrams
Particle diagrams
Element
- Element all particles are the same type of atom.
Compound
- Compound particles are groups of different atoms joined together, all groups identical.
Mixture
- Mixture different particles are present that are not joined, such as separate atoms and molecules.
Section 8 of 14Making a compound from elements
Iron and sulfur
- Iron and sulfur heating the mixture makes the compound iron sulfide.
- Before heating the mixture can be separated with a magnet.
- After heating the product is not magnetic because a new substance has formed.
Evidence
- Evidence a compound needs a chemical reaction to form, often with heat or light given out.
Section 9 of 14Physical separation of mixtures
| Method | Separates | Example |
|---|---|---|
| Filtration | Insoluble solid from a liquid | Sand from water |
| Evaporation / crystallisation | Soluble solid from a solution | Salt from salt solution |
| Simple distillation | Solvent from a solution | Water from salty water |
| Fractional distillation | Miscible liquids | Ethanol and water |
| Magnet | Magnetic solid from non-magnetic | Iron filings from sand |
Section 10 of 14Getting pure salt from rock salt
1
Crush the rock salt.
2
Stir it into warm water so the salt dissolves.
3
Filter the mixture; sand stays on the filter paper as residue.
4
Heat the filtrate in an evaporating basin until the solution is saturated.
5
Leave it to cool so that salt crystals form, then filter and dry them.
Section 11 of 14Obtaining a soluble solid from an insoluble one
- Method
- dissolve the soluble solid, filter off the insoluble solid, then evaporate and crystallise the filtrate.
- Calcium sulfate is insoluble and magnesium chloride is soluble in water.
- Water dissolves the magnesium chloride and the calcium sulfate stays as the residue.
Section 12 of 14More from the specification
More from the specification
Subatomic Structure and Atomic Numbers
- An atom consists of a central nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons arranged in electronic shells;
- the atomic number equals the number of protons.
Definition and Properties of Isotopes
- Isotopes are atoms of the same chemical element possessing identical atomic numbers but different nucleon numbers due to differing counts of neutrons.
- (same number of protons and electrons)
Relative Atomic Mass Calculation
- The relative atomic mass (Ar) of an element listed in the periodic table is the weighted average mass of naturally occurring isotopes relative to 1/12th the mass of a carbon-12 atom: Ar = Σ / 100.
- (isotope mass × abundance)
Electronic Configurations and Valence Shells
- The arrangement of electrons across principal electron shells dictates an element's chemical properties, with elements in the same periodic table group possessing identical numbers of outer-shell valence electrons.
Periodic
- Describing properties that repeat at regular intervals along the table
Noble gases
- The Group 0 elements, which are unreactive monatomic gases
Shell
- An energy level around the nucleus that holds a limited number of electrons.
Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Symbol
- Shorthand for an element, e.g. Na.
Electronic configuration
- The number of electrons in each shell, written with commas.
Outer shell
- Highest-energy occupied shell.
Electron shell
- An energy level for electrons.
Electron
- Negative particle moving around the nucleus.
Proton
- Positive particle in the nucleus.
Neutron
- Neutral particle in the nucleus.
Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons.
Isotope
- Atoms of an element with different numbers of neutrons.
Nucleus
- Central part of an atom with protons and neutrons.
Section 13 of 14Key terms
| Term | Meaning |
|---|---|
| Element | Substance made of atoms with the same number of protons. |
| Compound | Pure substance of two or more elements chemically combined. |
| Mixture | Substances together but not chemically combined. |
| Atom | Smallest particle of an element. |
| Physical method | Separation method that does not make new substances. |
| Pure substance | Substance containing only one type of particle. |
| Term | Meaning |
|---|---|
| Residue | Solid left on the filter paper. |
| Filtrate | Liquid that passes through the filter paper. |
| Crystallisation | Formation of crystals from a saturated solution on cooling. |
| Saturated solution | Solution that cannot dissolve any more solute at that temperature. |
| Insoluble | Not able to dissolve in a solvent. |
| Soluble | Able to dissolve in a solvent. |
Section 14 of 14Exam technique
1
Saying a mixture and a compound are the same — a compound has chemically combined elements in a fixed ratio.
2
Saying a compound keeps the properties of its elements — its properties are different.
3
Saying air is a compound — it is a mixture.
4
Saying compounds are separated by filtering — a chemical reaction is needed.
1
Saying the iron and sulfur mixture and iron sulfide are the same — the compound is a new substance.
2
Boiling off all the water — crystals are formed by cooling a saturated solution.
3
Mixing up residue and filtrate.
4
Thinking a compound can be separated by filtering.
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