Order by atomic number; groups and periods
Elements are placed in order of increasing atomic number (proton number), left to right and top to bottom. Vertical columns are groups; horizontal rows are periods. For a main-group element the group number = outer-shell electrons and the period number = number of occupied shells. So magnesium (Group 2, Period 3) has 2 outer-shell electrons over 3 shells: configuration .
Electronic configuration: the 2, 8, 8 rule
For the first 20 elements electrons fill shells in the order 2, then 8, then 8. Write the configuration as electrons-per-shell separated by commas: sodium (11 electrons) is , calcium (20) is . Read the position from it — number of shells (digits) = period, last digit (outer-shell electrons) = group number for a main-group element. All the digits sum to the atomic number.
Same group reacts alike; Group 0 is inert
Elements in the same group have the same number of outer-shell electrons, and outer-shell electrons drive bonding — so they show similar chemical properties. The noble gases (Group 0) are very unreactive because they already have a full (stable) outer shell (helium has 2, the rest 8), so they have no tendency to lose, gain or share electrons. Metals sit to the left/bottom of the table, non-metals to the top right.
Drawn from real examiner reports.
Group similarity without "outer shell"
Vague answers — "the same electrons", "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 reactivity comparisons: "X is more reactive" with no reason, or talking about electrons in general instead of the outer-shell electron, loses marks. Always anchor the answer on the outer shell.
November 2024 Paper 2C Q4(c)(ii) — in a Group 1 reactivity comparison, candidates lost marks for mentioning electrons rather than the outer shell, or for not mentioning the outer shell at all.
Whole-atom question, outer-shell-only answer
Read exactly what is asked. For why two particles are the same element, adding neutrons or isotopes is irrelevant and "same number of outer-shell electrons" alone does not answer it — the credited points are the same number of protons and electrons (or the same electron configuration). Match the answer's level (whole-atom vs outer shell) to the question.
November 2024 Paper 2C Q1(b) — many gave irrelevant neutron/isotope detail or only "same number of electrons in the outer shell"; the answer needed the same number of protons and electrons / the same electron configuration.
Confusing groups with periods
Groups are the vertical columns; periods the horizontal rows — the two words are often swapped. For a main-group element the group number = outer-shell electrons (last digit) and the period number = number of shells (digits). So is Group 7, Period 3 (chlorine), not Group 3. Transition metals and Group 0 are the exceptions to "last digit = group".
Ordering by mass number, not atomic number
Elements are ordered by increasing atomic number (proton number), not by mass number (protons + neutrons). If a question asks the ordering principle, "arranged by mass" scores zero — the modern table orders by proton number, which is what groups elements of similar chemistry together. Atomic number is also what makes each element unique.
Group number vs ion charge
Group number and ion charge are related but not identical. A metal in Groups 1-3 forms a positive ion equal to its group number (Group 2 gives ); a non-metal in Groups 5-7 forms a negative ion of charge group number 8 (Group 7 gives ). Never give a Group 7 element a charge, and Group 0 forms no ions — its outer shell is full.
Same outer electrons is not the same element
Same outer-shell electrons means the same group, not the same element. Two atoms are the same element only if they have the same number of protons (same atomic number). Fluorine and chlorine both have 7 outer-shell electrons and sit in Group 7, yet they are different elements. Use the right property: outer-shell electrons for group, protons for element.
Deduce position from the configuration
Write the electronic configuration first (fill 2, 8, 8), then read it off: digits (shells) give the period, last digit gives the group. Metals (Groups 1-3) form positive ions of the group number; non-metals (Groups 5-7) form ions of charge group number 8.
Give the reasoning chain, not just the label
Explanation marks need a linked chain, not a bare statement. For "why same-group elements react alike": same group -> same outer-shell electrons -> these drive reactions -> similar chemical properties. A single link on its own loses marks — write each step.
State reactivity trends the right way round
State the direction correctly: down a group, Group 1 metals get more reactive and Group 7 non-metals get less reactive. And compare — "potassium is more reactive than sodium", not just "potassium is reactive". A reversed direction scores nothing.
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)
