Relative Atomic Mass and the Periodic TableAQA GCSE Chemistry: Revision notes
Section 1
How did Newlands and Mendeleev attempt to classify elements?
Newlands' Approach (1865)
- Newlands arranged elements in order of relative atomic mass
- He noticed that every eighth element had similar properties and called this the Law of Octaves
- His classification only worked well for the first few elements
- He did not leave gaps for undiscovered elements
- His table did not account for elements that didn't fit the pattern
Mendeleev's Approach (1869)
- Mendeleev also arranged elements by relative atomic mass, but with crucial differences
- He left gaps in his table for elements he believed had not yet been discovered
- He reordered some elements where their chemical properties didn't match their atomic mass order (for example, he placed iodine before tellurium despite tellurium having a higher relative atomic mass)
- He grouped elements with similar chemical properties together in columns called groups
- His table predicted the properties of undiscovered elements with remarkable accuracy
Key Difference
Mendeleev prioritised chemical properties over strict atomic mass order, whereas Newlands rigidly followed atomic mass without exception.
Examiners want you to explain that Mendeleev's success came from prioritising chemical similarity over strict atomic mass order, and from leaving gaps for undiscovered elements. Always contrast this directly with Newlands' rigid approach.
Students often say Mendeleev was simply 'right' about atomic mass order, but actually he broke his own rule to improve accuracy—emphasise that he reordered elements when necessary.
Section 2
Why was Mendeleev's table more successful than Newlands'?
Reasons for Mendeleev's Success
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Left gaps for undiscovered elements — Mendeleev recognised that not all elements had been discovered. He left spaces in his table and predicted the properties of missing elements (gallium, scandium, and germanium were later discovered and matched his predictions almost exactly)
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Reordered elements based on chemical properties — He placed tellurium before iodine despite tellurium having a slightly higher relative atomic mass, because iodine's chemical properties were more similar to other halogens
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Grouped by chemical similarity — Elements in vertical columns (groups) showed clear patterns in their chemical behaviour, which Newlands' table did not emphasise
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Allowed for future discovery — His flexible approach meant new elements could be fitted into the table without destroying its structure
Why Newlands Failed
- His Law of Octaves only worked for light elements; it broke down for heavier elements
- He did not leave room for undiscovered elements
- He treated the pattern as a rigid mathematical rule rather than recognising underlying chemical principles
- Scientists rejected his work because it seemed too simplistic and didn't explain why the pattern existed
When explaining Mendeleev's success, give specific examples: mention gallium, scandium, or germanium and how their properties matched predictions. This shows deep understanding.
Think of Newlands' table as a rigid checklist: 'every eighth element must be similar' with no flexibility. Mendeleev's was like a puzzle that could accommodate missing pieces—it worked because it was adaptable.
Section 3
How is the modern periodic table arranged?
Arrangement by Atomic (Proton) Number
- The modern periodic table is arranged in order of increasing atomic (proton) number, not by relative atomic mass
- This was made possible by the discovery of atomic number (the number of protons in an atom)
- Arranging by atomic number revealed that chemical properties repeat in a predictable pattern
- This order is more fundamental than Mendeleev's atomic mass arrangement
Structure: Periods and Groups
- Periods are the horizontal rows in the periodic table (Period 1, Period 2, Period 3, etc.)
- Groups are the vertical columns in the periodic table (Group 1, Group 2, Group 13–18)
- There are 7 periods and 18 groups in the periodic table
- The period number indicates the number of electron shells (or energy levels) an atom has
- The group number indicates the number of outer (valence) electrons an atom has in its outermost shell
Positions of Metals and Non-Metals
| Region | Elements | Properties |
|---|---|---|
| Left side and centre | Metals | Conduct electricity, malleable, lustrous, form positive ions |
| Right side (upper) | Non-metals | Poor electrical conductors, brittle as solids, form negative ions |
| Boundary (staircase line) | Metalloids/Semi-metals | Intermediate properties |
- Most elements are metals, located on the left and centre of the periodic table
- Non-metals are found on the right side of the periodic table
- A staircase line (or zigzag line) separates metals from non-metals
Examiners test whether you can link period number to electron shells and group number to outer electrons. Always state: 'Period 3 means 3 electron shells; Group 5 means 5 outer electrons.'
Sodium (Na) is in Period 3, Group 1. This means: 3 electron shells, 1 outer electron, metallic properties. Chlorine (Cl) is in Period 3, Group 17. This means: 3 electron shells, 7 outer electrons, non-metallic properties.
Section 4
What does group number tell us about chemical properties?
Elements in the Same Group Have Similar Chemical Properties
- All elements in a group (vertical column) have the same number of outer electrons
- The number of outer electrons is the primary factor determining how an element reacts
- This is why elements in the same group show similar chemical behaviour
Examples of Group Properties
| Group | Name | Outer Electrons | Properties |
|---|---|---|---|
| Group 1 | Alkali metals | 1 | Highly reactive, form +1 ions, soft, shiny |
| Group 7 | Halogens | 7 | Reactive non-metals, form -1 ions, exist as diatomic molecules (F₂, Cl₂) |
| Group 0 (or 18) | Noble gases | 8* | Unreactive, inert, exist as single atoms |
*Noble gases have a full outer shell (2 electrons for helium, 8 for others), making them extremely stable.
Why Outer Electrons Matter
- Atoms react by gaining, losing, or sharing electrons with other atoms
- Since each group has the same number of outer electrons, atoms in that group behave similarly when reacting
- For example, lithium (Li), sodium (Na), and potassium (K) all have 1 outer electron, so they all react in similar ways and form similar compounds
Reactivity Trends Within Groups
- In Group 1, reactivity increases down the group (lithium < sodium < potassium)
- In Group 7, reactivity decreases down the group (fluorine > chlorine > bromine > iodine)
- These trends relate to the increasing number of electron shells and the distance of outer electrons from the nucleus
When asked why elements in a group have similar properties, always write: 'They have the same number of outer electrons, which determines how they react.' This is the key mark-scheme answer.
Chlorine and bromine are both in Group 7 with 7 outer electrons. Both form diatomic molecules (Cl₂, Br₂), both form -1 ions (Cl⁻, Br⁻), and both react with metals in similar ways. This similarity stems entirely from having the same number of outer electrons.
Must Know
- Newlands arranged elements by atomic mass and discovered the Law of Octaves, but his pattern broke down for heavier elements and he did not leave gaps for undiscovered elements
- Mendeleev also used atomic mass but prioritised chemical properties, left gaps for undiscovered elements, and reordered elements (like tellurium and iodine) when necessary—his predictions matched newly discovered elements' actual properties
- The modern periodic table is arranged by atomic (proton) number, not atomic mass
- Period number = number of electron shells; Group number = number of outer electrons
- Elements in the same group have the same number of outer electrons, which is why they have similar chemical properties
- Metals are on the left and centre of the periodic table; non-metals are on the right side; a staircase line separates them
That's the notes covered.
Carry on to the next subtopic.