Proton number, groups and periods
Elements are listed by increasing proton number (Z = protons in the nucleus). A group is a vertical column, numbered 1-7 plus Group 0 (noble gases); its elements share the same number of outer-shell electrons, so similar chemistry - Group 1 (Li, Na, K) react vigorously with water. A period is a horizontal row, numbered 1-7; its elements have the same occupied shells: Period 2 is Li to Ne (2 shells), Period 3 Na to Ar (3 shells).
Metals, non-metals and the stepped line
A stepped line divides the table. Metals lie left and below it; shiny solids (except mercury), conduct electricity, malleable, form positive ions and basic oxides. Non-metals lie to the right and above it; often gases or brittle solids, poor conductors (graphite excepted), form negative ions or covalent bonds. Hydrogen is the exception: in Group 1 but a non-metal. Elements on the line (silicon, germanium) are metalloids.
(Extended) Group = outer electrons; period = shells
For a main-group element, group number = outer-shell (valence) electrons and period number = occupied shells. So sodium (2.8.1) is Group 1, Period 3; oxygen (2.6) is Group 6, Period 2. Group 2 over Period 4 = 2 outer electrons across 4 shells - fill 2, 8, 8, 2 for calcium (2.8.8.2). Not for transition metals, whose outer counts vary. Trap: in 2.8.8.2 the 8 in shell 3 is not the group - only the outermost 2 set it.
Drawn from real examiner reports.
Group number vs period number
A group (column) counts outer-shell electrons; a period (row) counts occupied shells. Asked which has the most shells, candidates wrongly pick the highest group. Chlorine (2.8.7) is Group 7 but has only 3 shells, so Period 3 - not 7. Count shells in the configuration, never the group number. Rule: group = outer electrons, period = shells.
W22 P13 Q21; W22 P23 Q21; S23 P31 Q1(c)
Alkali metals give alkaline, not acidic
Group 1 metals react with water to form a metal hydroxide plus hydrogen; the hydroxide dissolves to an alkaline solution, pH > 7: 2Na + 2H2O -> 2NaOH + H2. Seeing fizzing, candidates wrongly call the solution acidic, or confuse the hydrogen with an acid. The name alkali metal is the clue. Universal indicator turns blue/purple, not red.
W22 P13 Q25; W22 P23 Q25
Iodine is a grey-black solid
At room temperature iodine is a grey-black solid, not a gas. Candidates borrow chlorine's yellow-green gas colour, or quote iodine's purple vapour. Down Group 7 the state changes gas (Cl2) to liquid (Br2) to solid (I2) and colours darken. Warmed, iodine sublimes to a purple vapour with no liquid stage - but the solid is grey-black.
S23 P32 Q1(d)
Hydrogen is a non-metal
Hydrogen is printed above Group 1, so candidates classify it as a metal like lithium and sodium. It is a non-metal: a colourless gas that does not conduct electricity and has no metallic lustre. It is the main exception to the rule that everything left of the stepped line is a metal. If asked, hydrogen is a non-metal, not a metal.
Group number is outer electrons only
The group number gives the outer-shell electrons of a main-group element - not the total electrons and not the protons. Elements in the same group have different proton numbers; what they share is the outer-electron count, which drives their chemistry. So a Group 6 element has 6 outer electrons, not 6 in total and not 6 shells.
Read configuration from table position
To read a configuration: period number = occupied shells; group number = outer electrons (main-group). Fill inner shells 2, 8, 8 inward-out, then add outer electrons. Phosphorus (Group 5, Period 3) is 2.8.5; check they sum to Z: 2 + 8 + 5 = 15.
Count shells, not the group
For 'most occupied shells', count the shells in each configuration or read the period number, not the group. Potassium (2.8.8.1) has 4 shells, Period 4. A high group number does not mean more shells; the lowest element in a column has the most.
Use the stepped line for metal/non-metal
To decide metal or non-metal, use the stepped line: metals to the left and below, non-metals to the right and above. Remember the exception - hydrogen is in Group 1 but a non-metal. Elements on the line (silicon, germanium) are metalloids.
Periodic Table — an arrangement of all known elements in order of increasing proton number (atomic number, symbol ). Elements with similar chemical properties appear in the same vertical column (group).
Group — a vertical column of elements in the Periodic Table. Elements in the same group have the same number of outer-shell electrons and therefore similar chemical properties. Groups are numbered 1, 2, 3, 4, 5, 6, 7 and 0 (noble gases).
Period — a horizontal row of elements in the Periodic Table. All elements in the same period have the same number of occupied electron shells. Periods are numbered 1–7 from the top.
Proton number (atomic number) — the number of protons in the nucleus of an atom. This is the fundamental organising property of the Periodic Table.
Define group in the context of the Periodic Table.
Sodium is in Group 1 of the Periodic Table.
(a) State whether sodium is a metal or a non-metal.
(b) Write a balanced symbol equation for the reaction of sodium with cold water. Include state symbols.
(c) State the pH of the solution formed and explain why the solution has this pH.
