C1 Atomic Structure and the Periodic Table
C1 Core Knowledge Summary
1.1.1 Atoms, elements and compounds
Atom: Smallest part of an element; neutral particle with protons and neutrons in the nucleus, surrounded by electrons.
Reactants: Substances taking part in a chemical reaction (LHS of equation).
Products: Substances formed during a chemical reaction (RHS of equation).
Element: Substance made of one type of atom with the same number of protons.
Compound: Substance made of two or more different elements chemically joined.
Formulae: Combination of element symbols representing a substance.
Word equation: Representation of a chemical reaction using words.
Chemical reaction: Irreversible chemical change forming new products.
Chemical symbol: Letter(s) representing an element on the periodic table.
1.1.2 Mixtures
Mixture: Substance made of elements or compounds not chemically joined.
Filtration: Separating insoluble substances from soluble substances.
Filtrate: Liquid passing through the filter during filtration.
Residue: Insoluble material left behind during filtration.
Crystallisation: Separating a solid that has dissolved in a liquid in a solution.
Evaporation: Change of state from liquid to gas.
Simple distillation: Separating a mixture using evaporation, condensation, and collection.
Fractional distillation: Separating liquids by boiling off substances at different temperatures and condensing/collecting them.
Paper chromatography: Separating a mixture of soluble substances using paper and a liquid mobile phase (e.g., water).
Rf value: Ratio of the distance travelled by a solute divided by the distance travelled by the solvent.
Physical change: Reversible process with no new substances made, but with a change in appearance.
Soluble: A substance that can dissolve in a solvent
Insoluble: A substance that cannot dissolve in a solvent.
Solvent: Substance dissolving a solute to form a solution.
Solute: Substance dissolving in a solvent.
Solution: Mixture formed when one substance dissolves in another.
Solubility: Measure of how much solute dissolves in a given solvent to form a saturated solution.
1.1.3 Development of the model of the atom
John Dalton: Atoms are tiny solid spheres that cannot be broken down.
JJ Thomson: Plum pudding model (sphere of positive charge with electrons dotted around), discovered the electron.
Ernest Rutherford: Alpha scattering experiment identified the positive charged nucleus with mostly empty space; electrons orbit the nucleus.
Alpha scattering experiment: Firing alpha particles at gold foil; most went straight through, a few were deflected revealing a positive nucleus.
Niels Bohr: Orbiting electrons occupy fixed energy levels around the nucleus.
James Chadwick: Discovered the neutron.
1.1.4 Relative electrical charges of subatomic particles, 1.1.5 Size and mass of atoms, 1.1.6 Relative atomic mass, 1.1.7 Electronic structure
Atom: Smallest particle of an element.
Nucleus: Positively charged central part of an atom (protons and neutrons).
Proton: +1 charge, relative mass of 1.
Neutron: 0 charge, relative mass of 1.
Electron: -1 charge, relative mass of 1/2000.
Electron shells: Regions around the nucleus where electrons orbit (first shell: up to 2 electrons, other shells: up to 8 electrons).
Ion: Charged atom due to unequal numbers of protons and electrons (loss or gain of electrons).
Isotope: Forms of an element with the same number of protons but different numbers of neutrons.
Relative atomic mass: Average value taking account of the abundance of isotopes.
Diameter of an atom: 1 \times 10^{-10} m
Diameter of a nucleus: Less than 1/10,000th of that of the atom (1 \times 10^{-14} m).
Isotope: Atoms of the same element with different numbers of neutrons and different mass numbers.
1.1.6 Relative atomic mass
Relative atomic mass, Ar: Average mass of the atoms of an element compared with carbon-12, taking into account the percentage abundance of its isotopes.
1.2.1 The periodic table
Modern Periodic table: Arranged in order of increasing atomic number with similar properties in groups.
Atomic number: Number of protons in the nucleus.
Mass number: Number of protons and neutrons in the nucleus.
Chemical symbol: Letter(s) representing an element.
Element: Substance of only one type of atom; same atomic number.
Group: Vertical column with similar chemical properties and same number of outer shell electrons.
Group number: Indicates the number of outer shell electrons.
Period: Horizontal row; atomic structure increases by 1 from one element to the next.
Period number: Indicates the number of electron shells.
1.2.2 Development of the periodic table
Johann Dӧbereiner: Arranged elements with similar properties into triads (groups of three).
John Newlands: Arranged elements with similar properties into octaves (every eighth element reacted similarly).
Dmitri Mendeleev: Arranged elements by increasing relative atomic mass and properties, leaving gaps for undiscovered elements.
1.2.3 Metals and non-metals
Metals: Elements on the left side of the periodic table.
Non-metals: Elements on the right side of the periodic table.
Properties of metals: Shiny, strong, hard, high density, malleable, ductile, high melting and boiling points, good conductors.
Properties of non-metals: Dull, brittle, low density, low melting and boiling points, poor conductors.
1.2.4 Group 0
Noble gases: Non-metal elements located in group 0. Unreactive and stable, monoatomic gases, low boiling points, boiling point increases down the group.
1.2.5 Group 1
Alkali metals: Very reactive metal elements located in group 1. Soft, low density, low melting and boiling points, very reactive, increase in reactivity down the group, good conductors, shiny when freshly cut.
Reactivity of alkali metals: Increases down the group because it requires less energy to lose an outer shell electron.
1.2.6 Group 7
Halogens: Reactive non-metal elements located in group 7. Do not conduct electricity, brittle and crumbly when solid, poisonous and smelly, diatomic molecules, become darker down the group, boiling point increases down the group.
Reactivity of halogens: Decreases down the group because it requires less energy to gain an outer shell electron.
1.3.2 Typical properties of transition metals & 1.3.1 Comparison with Group 1 elements
Transition metals: Elements in the central block between Groups 2 and 3.
Properties of transition metals: Shiny, high melting and boiling points, solid at room temperature, malleable, ductile, good conductors.
Unique properties of transition metals: Form coloured ions in compounds, exist in variable oxidation states, and can act as catalysts.
Reactivity of halogens: Decreases down the group because it requires less energy to gain an outer shell electron.
Exam Technique Guidance
THINK before you write
T: Text: read it!
H: Highlight key words
I: Instructions: follow them
N: Now stop and think about your answer
K: Key words: write down the key words you will use
T: Read the text, look at the graph or results table or diagram before you even look at the rest of the question.
H: Highlight key words: it is useful to highlight a few key words in the question so that the focus of the question can be identified. It is also useful to indicate the command word in this way. Annotate graphs and tables to show key trends e.g. increase/decrease
I: Instructions-these are the command words
N: Stop and make sure you are answering the right question. Check the number of marks and make a plan for long answer questions (you can use bullet points) or check the table and graph carefully
K: Examiners will expect to see key terms
Command words
Name/Give: Short answer required.
Identify/State: One or two-word answer.
Label: Provide names on a diagram.
Calculate: Generate a numerical answer, with workings shown.
Estimate: Assign an approximate value.
Define: Give the meaning of
Describe: Recall or link together a series of facts or pieces of information in a logical order e.g. describe the effect of increasing light intensity on the rate of photosynthesis
Suggest: Apply knowledge to a new situation.
Explain: Say how or why something happens; link descriptions to reasons.
Compare: Give similarities and differences.
Evaluate: Make a judgment or justify your answer; weigh up the advantages and disadvantages, positives and negatives, or strengths and weaknesses.
Justify: Give reasons for your answer, providing evidence to back up your ideas
Practical and Maths Skills
Conclusions and data processing
Hint: ALWAYS read and annotate the information/data/graphs first and then read the question and then use the information/data/graphs to answer the question.
Describing data: make comparisons/look for patterns or trends-how does the dependent variable e.g. concentration vary with changes in the independent variable e.g. temperature. Use a ruler to read from graphs and quote units. State how the y axis changes with the x axis using key terms e.g. increases rapidly/fluctuates/plateaus
When comparing data, it is important that not just the data is quoted but is used in mathematical calculations e.g. simple ratios or percentage change using either the correct significant figures or decimal places (2) and units
Explaining data: look for the specification point and use scientific terminology to answer
Evaluating data: make a qualitative judgement taking into account the data e.g. does the data completely support the conclusion or only partially support it or not support it at all
Practical skills
Preparing slides
Use sharp blade so slide is thin enough to ensure high resolution and squash the slide so maximum light can penetrate the sample
Staining allows increased contrast allowing specific detail to be made visible
Presentation of results
Drawings (CLASS)
Single, clear lines drawn with a sharp pencil with no shading or colour
Labels and annotations-drawn with a ruler (no arrows).
Table
Single table with ruled lines and border
Label the columns: independent (changed) on left, dependent (measured) on right
Units should be in the column headings-not next to the number in the table
All raw data recorded to the same number of decimal places/clear observations.
Graphs (LUSH)
All plotted points should occupy at least 50% of the graph paper
Scale the axes correctly with ascending scale and equidistant intervals
Label the axis including full units (independent on X axis, dependent on Y)
Planning
Independent variable: what will you change and how will you change it
Dependent variable: what will you measure and how will you measure it
Controlled variables: what will you keep the same and how will you do this
Key variables (temperature/volume/concentration) must be the same for each repeat
Evaluating the validity of methods and conclusions
Hint: Whilst reading a procedure, constantly look for variables and limitations (design faults)
General limitations
Subjective results leading to variation between different investigators
Qualitative results not quantitative results
Not enough repeats so outliers cannot be identified and means calculated
Key variables (temperature/volume/concentration) must be the same for each repeat
A wide range (5 values) is needed to see patterns e.g. line of best fit
Intermediates are needed to be able to tell exactly when something happens
Imprecise apparatus leading to different volumes/masses/areas
L1. Atoms, Elements & Compounds
All substances are made up of tiny particles called atoms (from Latin 'pars' = part, 'particula' = little part).
A particle is a tiny piece of matter (atom, ion, molecule).
There are about 100 different types of neutral atoms found naturally on Earth (elements).
Elements can have different properties (e.g., copper, gold, silver are metals; oxygen, chlorine, argon are non-metals).
Non-metals often exist as molecules (two or more non-metal atoms chemically joined by covalent bonds).
Chemical symbols for elements are used for international understanding.
Most substances are compounds (different types of atoms chemically bonded together).
Compounds are formed from elements by chemical reactions.
Chemical reactions involve the formation of new substances and often involve a detectable energy change.
Compounds contain two or more elements chemically combined in fixed proportions and can be represented by formulae.
Compounds can only be separated into elements by chemical reactions.
L2. Mixtures & Filtration
Compounds have a fixed composition; mixtures do not.
Chemical bonds exist in compounds; no chemical bonds in mixtures.
Elements in compounds are separated chemically; mixtures can be separated physically.
Examples of mixtures: air (78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, 0.06% water vapor), alloys (steel), and toothpaste.
A solution forms when a solute dissolves in a solvent; the solute particles mix completely with the solvent particles.
Substances can be soluble in one solvent but insoluble in another.
The mass of solute + solvent = the mass of solution (law of conservation of mass).
Filtration separates insoluble solids from soluble substances using filter paper with microscopic holes.
Residue remains in the filter paper; filtrate passes through.
L3. Separating Mixtures – Crystallisation
Mixtures can be separated by filtration, crystallisation, simple distillation, fractional distillation, and chromatography because chemicals are not joined together.
Crystallisation: A separation process by which pure crystals usually of a salt are formed during evaporation of a solvent from a salt solution.
Gently heat a salt solution, so the solvent slowly evaporates leaving the solute behind.
Steps in crystallisation:
Make a solution (dissolve impure solid).
Filter (remove insoluble impurities).
Heat gently until crystals form (evaporation).
Dry the crystals in an oven or pat dry with filter paper.
L3 continued. Separating Mixtures – Simple Distillation
Simple distillation: Separates a solvent from a solution based on different boiling points (evaporation, condensation, collection).
Water (boiling point 100°C) evaporates from ink and water mixture, travels to the condenser, cools back into liquid water, and drips into the beaker.
Water in and out needs to be this way round to ensure the condenser full fills with water and cools efficiently
Ink remains in the round-bottomed flask.
L4. Separating Mixtures – Fractional Distillation
Fractional distillation: Separates two or more miscible liquids based on boiling points.
Used to separate ethanol from water for fuel, and crude oil into fractions.
During fractional distillation of an ethanol-water mixture, four things happen:
Ethanol vapour and water vapour can leave the liquid mixture and enter the fractionating column which is hot at the bottom and cool at the top.
The ethanol and water vapours condense on the inside surface of the column heating it up.
When the temperature inside the column reaches the boiling point of ethanol (78.4˚C), ethanol vapour cannot condense into liquid anymore, and turns back into a gas, then it passes through the condenser where it is cooled back into a liquid and collected in the conical flask.
Meanwhile, the water vapour can continue to condense in the column as it has a higher boiling point of 100˚C. Water droplets then fall back into the round-bottom flask. The two substances have now been separated.
L5. Separating Mixtures – Paper Chromatography
Paper chromatography: Separating technique separating a mixture of two or more substances.
Uses a liquid mobile phase (usually water) and paper as the stationary phase.
Separates solutes based on solubility (e.g., dyes in ink).
Steps for paper chromatography:
Draw a pencil line on chromatography paper and spot the sample using a capillary tube.
Lower the paper into solvent, ensuring the pencil line is above the level of the solvent, placing a lid on to ensure inside the beaker is saturated with solvent vapour.
The solvent travels up the paper by capillary action, taking some of the colored substances with it.
Calculate the R_f value for each spot and compare with a standard reference material.
R_f value: (distance travelled by solute) / (distance travelled by solvent).
R_f value is always less than 1 (ratio, no units).
Pure substances yield one spot; impure substances yield multiple spots.
L6. Required Practical – Paper Chromatography
Ink spots are placed along a start line and held in a small volume of water which acts as the solvent.
The start line is drawn 1 cm from the bottom of the paper.
Name the piece of apparatus used to make the ink spots on the paper.
Explain why the level of the solvent should be below the ink spots.
Suggest when the developing chromatogram should be removed from the solvent.
Once the solvent has risen up the chromatography paper the chromatography paper is dried before measurement.
Sam and Alex were given an ink mixture that was used in a forgery and asked to identify which separate inks had made up the mixture. They put spots of the separate ink along the start line, added a spot of ink mixture at the end of the paper and developed the chromatogram. They used water as the solvent to ‘elute’ the spots.
Making and recording results
From the start line Sam and Alex needed to measure (recorded these distances in the table):
The distance the solvent had travelled (the solvent front)
The distance each spot of ink had travelled
Calculating the R_f values
As the solvent front may not move from the same distance each time, it is important to calculate the ratio of distances travelled by solvent and ink. This is given in an index known as the R_f value.
Predict a value for Jo’s red spot using an Rf value calculated from Sam’s data.
Jo has one anomalous result (a result that does not fit the pattern).Identify the anomalous result,
Explain what Jo should do to check the result.
Suggest why Rf values change when the solvent is changed.
L7. Development of the Periodic Table
Before the discovery of protons, neutrons, and electrons, one of the first suggestions came from John Dalton in 1808. He arranged the elements in order of their atomic weights, which had been measured in various chemical reactions.
In 1828, Dӧbereiner had noticed that sometimes three elements had similar properties. He noticed properties with lithium, sodium and potassium; calcium, strontium and barium; chlorine, bromine and iodine. These were called Dӧbereiner triads, but only certain elements displayed this pattern.
In 1864, John Newlands put the elements in atomic weight order (even though some seemed to be in the wrong place) there was often a pattern of similar properties of every eighth elements. However, he assumed that all the elements had been found, and some elements did not fit his pattern.
Dmitri Mendeleev (1869) organized elements by mass number, grouped elements with similar chemical properties, swapped certain elements into different groups better their chemical properties, and left spaces for undiscovered elements, predicting their properties correctly.
Eventually, elements filled the gaps. Isotopes explained why orders were not always correct based on atomic weights.
Modern periodic table is organized by increasing atomic number.
L7 continued. Development of the Atomic Model
John Dalton (1803): Matter is made from atoms (tiny solid spheres that cannot be split);
J.J. Thomson (1897): Discovered the electron (negatively charged with mass 1/2000th of a proton). JJ Thomson’s model of the atom had to make sense of two observations: atoms contain electrons, and atoms are neutral overall Plum pudding model states that atoms are spheres of positive charge with electrons dotted around inside, no empty space , the mass spread throughout.
Ernest Rutherford (1909): Gold foil experiment led to the nuclear model (small, dense, positively charged nucleus with electrons orbiting outside).
Niels Bohr (1913): Electrons occupy fixed energy levels (shells) around the nucleus.
James Chadwick (1932): Discovered the neutron (neutral subatomic particle with no charge and the same mass as a proton).
L8. Atomic Structure, The Modern Periodic Table, & Calculating Sub-Atomic Particles
Atomic structure: Structure of an atom (1 \times 10^{-10} m).
Protons and neutrons (nucleus, +1 charge and a relative mass of 1, charge of 0, and a relative mass of 1).
Electrons (shells around the nucleus, charge of -1 and a relative mass of 0.0005).
Atomic number: Number of protons in its nucleus; also the number of electrons.
Mass number: Total number of protons and neutrons.
Neutrons: mass number minus the atomic number.
Modern periodic table: Elements arranged in horizontal rows (periods) and vertical columns (groups).
Group: Vertical column with similar chemical properties and the same number of electrons in the outer shells.
Period: Horizontal row in which the atomic structure increases by 1 from one element to the next.
L9. Properties of Metals & Non-Metals, & Electronic Structure
Metals are on the left-hand side of the periodic table and non-metals are on the right-hand side with each having different characteristic physical properties.
Physical property is a feature of a substance that can be observed or measured, such as its melting point or color.
Generally, the physical properties of metals are:
shiny, high melting and boiling points, solid at room temperature, malleable (bend without shattering), ductile (can be pulled into wires), good conductors of heat and electricity.In comparison, non-metals are:
dull, have low melting and boiling points, solids or gases at room temperature, brittle (shatters when hammered), non-ductile (snap when pulled), poor conductors of heat and electricity (insulators).Electronic structure and the periodic table
The periodic table provides key pieces of information about each element. For example, the number of electrons in the outermost shell of the atom can be identified by the group number e.g. Lithium has 1 electron in its outer shell so it is in Group 1. Elements in the same group have similar chemical properties because they have the same number of outer shell electrons. Electronic structure is determined by the number of electrons, and the electronic structure of an element determines its chemical properties.The number of shells is indicated by the period number.
Calcium is in group 2 because it has 2 electrons in its outermost shell, and it is in period 4 because it has 4 electron shells occupied.
Likewise, electronic configuration provides the group number and period number
L10. Isotopes and Calculating Relative Atomic Mass
Isotopes: Atoms of the same element (same atomic number) with a different relative atomic mass number.
They have similar chemical properties because they have the same number of electrons in their outer shell.May have slightly different physical properties because they have different mass numbers.
Relative atomic mass: The mean mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom, defined as 12.0 exactly.
Its symbol is A_r.
L11. Formulae of elements and molecules
Each chemical symbol:
has one, two or three letters;
starts with a capital letter;
with any other letters in lowercase.
The formulae for metal elements are always written as they are found on the periodic table because metals exist as giant metallic lattices
Non-metal elements in Group 0 exist as individual atoms, monatomic, attracted to each other by weak intermolecular forces. Their formulae are the same as their chemical symbols. For example, Helium is He not He2.
The non-metal elements in Group 7 exist as diatomic molecules. A diatomic molecule that contains two atoms covalently bonded together. This means that the formulae for Group 7 elements have a subscript 2 in them. For example, the formula for chlorine is Cl_2.
Hydrogen, nitrogen and oxygen are not in Group 7 but they also exist as diatomic molecules.
Formulae of simple covalent molecules
The molecular formula for a simple covalent compound shows: the symbols for each element it contains and the number of atoms of each element in one of its molecules.
For example, the molecular formula for carbon dioxide is CO_2. It shows that each carbon dioxide molecule contains one carbon atom and two oxygen atoms.
L12. Formulae of ions and ionic compounds
Atoms are neutral because they have an equal number of protons and electrons
Hydrogen ions have a 1+ charge.
Metals in Groups 1, 2 & 3 produce positive ions in which the number of charges is the same as their group number.
Transition metals are in the block of elements between Groups 2 and 3. They produce positive ions that usually have 2+ charge, but there are exceptions.
Non-metals in Groups 5, 6 & 7 produce negative ions in which the number of charges is equal to eight minus the group number.
L13. Constructing word equations & balanced symbol equations
A word equation is a simple model for a chemical reaction. It shows the names of the reactants (starting substances) before an arrow, and the name of the products (new substances made) after an arrow. It should never include an equals sign.
An equation must always be balanced because when a chemical reaction takes place the atoms get rearranged. This is because atoms cannot be created or destroyed in a chemical reaction.