Revision notes · Atomic structure and the periodic table
The periodic table
The periodic table4.1.2.1
Definition: The periodic table arranges all the elements by increasing atomic number, in rows (periods) and columns (groups).
Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.
- •A period is a horizontal row; elements in the same period have the same number of electron shells.
- •A group is a vertical column; elements in the same group have the same number of outer-shell electrons, and therefore similar chemical properties.
- •The group number (for groups 1–7) tells you the number of electrons in the outer shell.
⚠️ Common mistake: Thinking elements are ordered by relative atomic mass — the modern periodic table orders elements by atomic number (proton number).
🧠 Remember: Group = outer electrons. Period = number of shells.
Development of the periodic table4.1.2.2
Before the structure of the atom was known, scientists tried to organise the elements using the information they had — atomic mass.
- •Early tables ordered elements by atomic (relative) mass, which sometimes placed elements in groups with different chemical properties.
- •Dmitri Mendeleev left gaps in his table for undiscovered elements, and used them to accurately predict their properties.
- •Mendeleev sometimes switched the order predicted by atomic mass, to keep elements with similar properties in the same group.
- •Once protons, neutrons and electrons were discovered, the table was reordered by increasing atomic number, resolving the remaining anomalies.
⚠️ Common mistake: Thinking Mendeleev's table matches the modern one exactly — his was ordered by atomic mass with gaps, not atomic number; some elements only fit properly once atomic number was used.
🧠 Remember: Mendeleev left gaps and predicted — later confirmed once atomic number was known.
Metals and non-metals4.1.2.3
Definition: Metals are elements that lose electrons to form positive ions; non-metals are elements that gain electrons to form negative ions (or share electrons).
An element's position in the periodic table shows whether it is a metal or non-metal.
| Metals | Non-metals | |
|---|---|---|
| Position | Left and centre of the table | Right of the table |
| Ions formed | Positive ions (cations) | Negative ions (anions) |
| Physical properties | Shiny, malleable, good conductors, high melting/boiling points | Dull, brittle (if solid), poor conductors, lower melting/boiling points |
- •Hydrogen is an exception to the rule above — it is not a metal, but in some reactions it forms a positive ion (H⁺), so it can behave like one.
⚠️ Common mistake: Assuming all non-metals behave the same when reacting — some, like the noble gases, don't form ions at all because they already have a full outer shell.
🧠 Remember: Metals lose electrons (positive ions); non-metals gain electrons (negative ions).
Group 04.1.2.4
Definition: Group 0 elements (the noble gases) are unreactive because they already have a full outer electron shell.
The noble gases are found in the far right column of the periodic table.
- •Noble gases are all inert (unreactive) and exist as single, monatomic atoms — they don't form molecules.
- •They have very low boiling points, which increase going down the group as the atoms get bigger and more massive.
- •Noble gases have 8 electrons in their outer shell, except helium, which has a full outer shell with just 2 electrons.
⚠️ Common mistake: Thinking noble gases react easily because they're gases — it's their full outer shell that makes them almost completely unreactive.
🧠 Remember: Group 0 = full shell = zero reactivity.
Group 14.1.2.5
Definition: Group 1 elements (the alkali metals) are soft, reactive metals that each have one electron in their outer shell.
Alkali metals become more reactive going down the group.
- •Alkali metals react vigorously with water, producing a metal hydroxide and hydrogen gas.
- •Reactivity increases down the group, because the outer electron is further from the nucleus and more easily lost.
- •They have relatively low melting and boiling points, which decrease going down the group.
- •They form ions with a +1 charge by losing their one outer electron.
- •Alkali metals have unusually low density for metals — the first three (lithium, sodium, potassium) are less dense than water and float on it.
⚠️ Common mistake: Thinking reactivity decreases down Group 1 like it does for Group 7 — for Group 1, reactivity increases down the group.
🧠 Remember: Group 1: down = more reactive (outer electron easier to lose).
Group 74.1.2.6
Definition: Group 7 elements (the halogens) are non-metals that each have seven electrons in their outer shell.
Halogens become less reactive going down the group — the opposite trend to Group 1.
- •Reactivity decreases down the group, because the outer shell is further from the nucleus, making it harder to gain an extra electron.
- •Halogens exist as diatomic molecules (e.g. Cl₂), and form −1 ions by gaining one electron.
- •A more reactive halogen can displace a less reactive halogen from a solution of its salt (a displacement reaction).
- •Melting and boiling points increase down the group.
- •At room temperature, chlorine is a pale yellow-green gas, bromine is a red-brown liquid, and iodine is a dark grey solid.
⚠️ Common mistake: Muddling up the Group 1 and Group 7 reactivity trends — Group 1 gets more reactive going down; Group 7 gets less reactive going down.
🧠 Remember: Group 7: down = less reactive (harder to gain an electron).
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