The periodic table

Arrangement of Elements in the Periodic Table

Elements in the Periodic Table are arranged in order of increasing atomic number [the number of protons in the nucleus of an atom]. This means hydrogen (atomic number 1) comes first and each element that follows has one more proton than the last.

The Table is organised into vertical columns called groups and horizontal rows called periods. A group contains elements with similar chemical properties stacked on top of each other. A period contains elements whose properties change progressively from left to right.

The group number tells you how many electrons are in the outermost shell for main group elements. The period number tells you how many occupied electron shells the atom has.

For example, sodium (Na) sits in Group 1, Period 3. This means sodium has 1 electron in its outermost shell and 3 occupied electron shells, giving it the electronic configuration 2,8,1.

MisconceptionStudents sometimes confuse groups with periods. Groups are vertical columns (same number of outer electrons), and periods are horizontal rows (same number of electron shells). Exam cue: Group = vertical = similar properties; Period = horizontal = same number of shells.

Deducing Electronic Configurations from the Periodic Table

The electronic configuration of the first 20 elements can be deduced directly from an element's position in the Periodic Table. The period number gives the number of occupied electron shells. The group number gives the number of electrons in the outermost shell (for Groups 1–7; Group 0 has a full outer shell of 2 or 8).

Electron shells fill in order: the first shell holds a maximum of 2 electrons, and the second and third shells each hold a maximum of 8 electrons. Once one shell is full, electrons begin filling the next shell.

Reading electronic configuration notation: a configuration such as 2,8,7 means 2 electrons in the first shell, 8 in the second, and 7 in the third. Each number separated by a comma represents one shell, read from innermost to outermost.

Examiner InsightExaminers frequently give an element's position (e.g. Period 3, Group 6) and ask for the electronic configuration. Fill shells in order: 2 first, then 8, then place the group number of electrons in the outermost shell. Exam cue: Period 3, Group 6 → 2,8,6 — practise this conversion until automatic.

Classifying Elements as Metals or Non-Metals

Elements can be classified as metals or non-metals using two experimental properties: electrical conductivity and the acid-base character of their oxides.

Metals are good conductors of electricity because they contain delocalised electrons that are free to move and carry charge. Non-metals are poor conductors of electricity (with the exception of graphite, a form of carbon). This difference in conductivity provides a direct experimental test.

The oxides of metals and non-metals also differ. Metal oxides are basic — they react with acids to form a salt and water. Some metal oxides dissolve in water to form alkaline solutions. Non-metal oxides are acidic — they dissolve in water to form acidic solutions or react with bases.

For example, magnesium oxide reacts with hydrochloric acid:

Carbon dioxide dissolves in water to form carbonic acid:

MisconceptionStudents sometimes state that all non-metals are insulators. Graphite (a form of carbon) conducts electricity because it has delocalised electrons within its layered structure. Exam cue: If asked for an exception, name graphite and explain it has delocalised electrons.

Position of Metals and Non-Metals in the Periodic Table

Metals and non-metals occupy distinct regions of the Periodic Table. Metals are found on the left-hand side and centre of the Table. Non-metals are found on the right-hand side. A stepped diagonal line (running roughly from boron to astatine) separates the two regions.

When given a Periodic Table in an exam, an element's position relative to this dividing line identifies it as a metal or non-metal without needing experimental data. Most elements are metals — they occupy the larger portion of the Table.

Electronic Configuration and Position in the Periodic Table

The electronic configuration of a main group element is directly related to its position in the Periodic Table. The number of occupied electron shells equals the period number. The number of electrons in the outermost shell equals the group number (for Groups 1–7). Group 0 elements have full outer shells (2 for helium, 8 for the others).

This relationship means position determines electronic configuration and electronic configuration determines position — they are two ways of expressing the same information. Because the outermost electrons are the ones involved in chemical reactions, elements in the same group react in similar ways.

Examiner InsightA common exam question gives an electronic configuration and asks you to state the group and period number, or vice versa. The outermost shell number = period; the outermost shell electrons = group. Exam cue: Always look at the last number in the configuration for the group, and count how many numbers there are for the period.

Similar Chemical Properties Within a Group

Elements in the same group of the Periodic Table have similar chemical properties because they have the same number of electrons in their outermost shell. The outermost electrons are the ones involved in chemical bonding and reactions, so elements with the same outer electron configuration react in similar ways.

For example, all Group 1 elements (lithium, sodium, potassium) have 1 electron in their outermost shell. Each reacts with water to produce a metal hydroxide and hydrogen gas. The reactions look similar because the same number of outer electrons is lost during each reaction.

Lithium with water:

Sodium with water:

Similarly, all Group 7 elements have 7 outer electrons, so they all gain 1 electron when they react, forming ions with a 1⁻ charge.

MisconceptionStudents sometimes write "elements in the same group have the same properties." They have similar, not identical, properties. Reactivity trends mean there are differences in vigour — for example, potassium reacts more vigorously with water than lithium does. Exam cue: Always write "similar chemical properties," never "the same."

Why Noble Gases Do Not Readily React

The noble gases (Group 0) — helium, neon, argon, krypton, xenon — do not readily react because they have full outer electron shells. Helium has 2 electrons in its only shell (which is the maximum for the first shell). Neon has the configuration 2,8 and argon has 2,8,8 — both have 8 electrons in their outermost shell.

A full outer shell is an extremely stable electronic configuration. Chemical reactions involve gaining, losing, or sharing electrons to achieve a full outer shell. Because noble gases already have this arrangement, they have no tendency to gain, lose, or share electrons. Therefore, they do not readily form bonds or take part in chemical reactions.

This stability explains why noble gases exist as single atoms (they are monatomic) rather than forming molecules or giant structures.

Examiner InsightWhen explaining noble gas unreactivity, examiners require you to state that they have a full outer shell (or "stable electronic configuration"). Simply writing "they have 8 electrons" is insufficient because helium has only 2 outer electrons yet is still unreactive. Exam cue: Write "full outer electron shell" — this covers both helium (2) and the others (8).