Periodic Table: order, groups and periods
Periodic Table: elements are placed in order of increasing atomic (proton) number, left to right and top to bottom. The vertical columns are groups; the horizontal rows are periods. For a main-group element the group number equals the number of outer-shell electrons and the period number equals the number of occupied shells — so magnesium (Group 2, Period 3) has 2 outer electrons over 3 shells, configuration .
Electronic configuration: the 2, 8, 8 rule
Electronic configuration: for the first 20 elements electrons fill shells 2, then 8, then 8, written as electrons-per-shell — sodium (11) is , calcium (20) is . Read position straight off it: the number of digits (shells) gives the period, and the last digit (outer-shell electrons) gives the group number for a main-group element. The digits always add up to the atomic number.
Metals, non-metals and Group 0
Metals lie towards the left and bottom (they conduct electricity and form basic oxides); non-metals towards the top right (generally poor conductors, forming acidic oxides). Elements in the same group have the same number of outer-shell electrons, so they show similar chemical properties. The noble gases (Group 0) are unreactive because they have a full outer shell (helium 2, the rest 8), with no tendency to lose, gain or share electrons.
Drawn from real examiner reports.
Group similarity needs the OUTER shell
Asked why a group reacts similarly, vague answers — "the same electrons" or "they are in the same group" — do not score. The mark-scheme point is the same number of electrons in the outer shell. The same trap hits Group 1 reactivity comparisons: "potassium is more reactive" with no reason loses the mark. Always name the outer shell, not electrons in general.
Jun24 1CR Q4ci — "same number of electrons" without "outer shell" did not score
Group ≠ period (column vs row)
Groups are the vertical columns; periods are the horizontal rows — the two words are often swapped. For a main-group element the group number = outer-shell electrons (the last digit) and the period number = number of shells. So is Group 7, Period 3 (chlorine), NOT Group 3. Transition metals and Group 0 are exceptions you are not asked to deduce here.
Same element ≠ same outer electrons
Match the answer to the level of the question. Asked why two particles are the same element, the credited points are the same number of protons and electrons — "same number of electrons in the outer shell" does not answer it, and neutron or isotope detail is irrelevant. An element is a substance made of only one type of atom (same proton number).
Ordered by atomic number, not mass number
Elements are arranged in order of increasing atomic number (proton number), NOT mass number (protons + neutrons). Confusing the two is a common slip when reading the table or placing an element. Whenever you are asked how the modern Periodic Table is ordered, the answer is proton number every time — mass number is not the ordering rule.
Group number ≠ ion charge
The group number is not always the ion charge. A metal in Groups 1-3 loses its outer electrons to form a positive ion equal to the group number (Group 2 gives ). A non-metal in Groups 5-7 gains electrons to form a negative ion of charge (group number 8), so Group 7 gives , not . Group 0 has a full outer shell and forms no ions at all.
State a group trend the right way round
Asked for a reactivity trend down a group, state the direction correctly. Reversing it — saying Group 1 reactivity decreases down the group when it increases, or that Group 7 reactivity increases when it decreases — shows a shaky grasp and loses the mark. Decide which end of the group is more reactive first, then write the trend in that direction.
Jun24 1CR Q4cii — reactivity trend stated the wrong way round lost the mark
Write the configuration first
Write the electronic configuration first (fill 2, 8, 8), then read it off: the digits (shells) give the period and the last digit gives the group. Metals lose their outer electrons for a positive ion equal to the group; non-metals gain for a charge of (group 8).
Use the exact mark-scheme wording
Precise terms earn the marks: say "outer shell", never just "electrons" or "same group"; say "full outer shell", not "stable" or "happy"; call the columns "groups" and the rows "periods". Vague wording is the commonest reason a correct idea scores nothing on this topic.
Answer at the level the command asks
Read whether the item is about the whole atom or the outer shell. "Same element" wants the same protons and electrons; "same group / similar properties" wants outer-shell electrons. Do not answer a whole-atom question with an outer-shell fact.
Metals vs non-metals: metals lie towards the left and bottom (they conduct electricity and form basic oxides); non-metals lie towards the top right (generally poor conductors, forming acidic oxides).
Reading position from the configuration of a main-group element:
Define a group in the Periodic Table.
State the order in which the elements are arranged in the modern Periodic Table, and name the vertical columns and horizontal rows. (3 marks)
