Atomic structure and the periodic table: Electron configuration, shells and periodic trends in atomic radii and valency

1. Where this topic sits in Criterion 4 — and the limits the course sets

Section A of the TASC Physical Sciences external examination is worth exactly 36 marks and is marked against Criterion 4: apply concepts and processes of atomic properties and nuclear reactions. Everything in Section A comes from that one criterion, so the atomic-structure material on this page is not background reading for the nuclear work later in the course — it is directly examinable in its own right, and it turns up in the opening items of almost every Section A booklet.

Two limits set by the PSC315118 course document decide how deep you need to go, and both work in your favour. First, electron shell diagrams are required for elements 1 to 20 only. Second, the course explicitly excludes s, p, d and f orbital theory. That means you will never be asked to write 1s²2s²2p⁶, and you should not do so in an answer: the required notation is the simple shell notation, written as comma-separated shell populations such as 2,8,1 for sodium. Students who have read ahead into a Chemistry textbook sometimes lose time and clarity by importing orbital notation into a paper that never asks for it.

The knowledge statements you are being assessed against are: that the structure of the periodic table is based on electron configuration; that atoms can be modelled as a nucleus surrounded by electrons in distinct energy levels held together by electrostatic forces of attraction; that similarities and trends in observable physical properties, including atomic radii and valencies, are evident in periods and groups; that the physical trends specifically named are those in periods 2 and 3 and groups 1, 2 and 17 (written in the course as I, II and VII); and that the charge on a stable ion is related to the number of electrons in the outer shell of the parent atom.

Your only permitted reference in the examination is the current TASC PSC315118 Physical Sciences Information Sheet plus a TASC-approved scientific calculator. The Information Sheet carries a periodic table, so atomic numbers and relative atomic masses are given to you. What is not given to you is the reasoning: why the radius shrinks across a period, why group 2 elements form 2+ ions, why argon forms none. That reasoning is what the marks are for.

2. The nuclear model: a tiny nucleus, electrons in energy levels, electrostatic attraction

The model this course uses is deliberately simple and you should be able to state it in two sentences. An atom consists of a nucleus containing positively charged protons and uncharged neutrons, surrounded by negatively charged electrons arranged in distinct energy levels (shells). The atom is held together by the electrostatic force of attraction between the positive nucleus and the negative electrons.

Three numerical facts follow, and examiners test them relentlessly in one-mark items. The atomic number (Z) is the number of protons and is what defines the element. In a neutral atom, the number of electrons equals the number of protons. The mass number (A) is the total number of protons plus neutrons, so the neutron count is A − Z. Write these carefully: a common one-mark loss is giving the number of neutrons as the mass number, or forgetting that an ion no longer has electrons equal to protons.

Notice the phrase electrostatic force of attraction. It is the course document's own wording and it should appear in your explanations. When a question asks why the outer electrons of chlorine are held more tightly than those of sodium, or why an atom does not simply fall apart, the mark is awarded for naming that force and linking its strength to charge and distance. Vague language such as 'the nucleus pulls the electrons in' is not wrong but rarely earns the explanation mark on its own.

Shells are described as distinct energy levels: an electron in the first shell is at a lower energy and closer to the nucleus than an electron in the second shell. This matters because it explains the whole of the rest of this topic. Electrons in inner shells sit between the nucleus and the outer electrons and reduce the attraction the outer electrons feel — the effect usually called shielding. Outer electrons, being furthest from the nucleus and shielded by the inner shells, are the least tightly held and therefore the ones involved in bonding and in ion formation. Every trend and every valency rule in this topic is an application of those two ideas: nuclear charge pulling in, distance and inner shells pushing the effect down.

The nucleus itself is extraordinarily small compared with the atom. Because almost all the mass sits in that tiny volume while the electrons define the size, the chemical behaviour of an atom is governed by its electron arrangement and the nuclear behaviour (which you meet later in Criterion 4) is governed by the composition of the nucleus. Keeping those two domains separate in your head is the single most useful organising idea in Section A.

3. Writing electron configurations and drawing shell diagrams for elements 1 to 20

For elements 1 to 20 the shells fill in a fixed and easily memorised pattern. The first shell holds a maximum of 2 electrons, the second holds a maximum of 8, and for this range of the table the third shell takes 8 before the fourth shell begins. So the capacities you need are 2, 8, 8, 2.

Work from the atomic number. Fill shell 1, then shell 2, then shell 3, then shell 4, and stop when you have placed all the electrons. Worked through, this gives:

• hydrogen (Z = 1): 1
• carbon (Z = 6): 2,4
• neon (Z = 10): 2,8
• sodium (Z = 11): 2,8,1
• silicon (Z = 14): 2,8,4
• chlorine (Z = 17): 2,8,7
• argon (Z = 18): 2,8,8
• potassium (Z = 19): 2,8,8,1
• calcium (Z = 20): 2,8,8,2

Potassium and calcium are the two the course deliberately stops at, and they are the two students get wrong. After argon fills the third shell to 8, the nineteenth and twentieth electrons go into the fourth shell, giving 2,8,8,1 and 2,8,8,2 — not 2,8,9 and 2,8,10. Being able to state that confidently is worth an easy mark and it is also the reason potassium behaves like sodium rather than like chlorine.

An electron shell diagram is the picture version of the same information: a labelled nucleus at the centre (write the symbol, or the number of protons and neutrons if the question asks) and concentric circles carrying the correct number of electrons, usually drawn as crosses or dots. When you draw one in the examination: label the nucleus, keep the circles clearly separate, put the right count in each shell, and count your own crosses before moving on. If the question asks for an ion, draw the shells the ion actually has and write the charge outside a square bracket around the diagram — for example the sodium ion has 10 electrons arranged 2,8 with a 1+ charge shown.

One habit is worth building now. Whenever you write a configuration, immediately note the number in the outer shell, because that single number tells you the group, the valency and the stable ion charge. For sulfur, 2,8,6 means six outer electrons, which means group 16 (VI), valency 2 and a 2− ion. Getting from Z to all four of those facts in one line is exactly the fluency Section A rewards.

4. Reading the periodic table from an electron configuration

The course document states that the structure of the periodic table is based on electron configuration, and the examination expects you to be able to move in both directions between a configuration and a position in the table.

The period number is the number of occupied shells. Sodium (2,8,1) occupies three shells and is therefore in period 3. Calcium (2,8,8,2) occupies four shells and is in period 4. The group is determined by the number of electrons in the outer shell: for the main-group elements the course concentrates on, an element with one outer electron sits in group 1 (I), two outer electrons puts it in group 2 (II), seven puts it in group 17 (VII) and a full outer shell puts it in group 18 (VIII), the noble gases.

This is why elements in the same group behave alike. Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) all have exactly one outer electron, so they all form 1+ ions and show a family resemblance in their physical and chemical properties. Elements in the same period, by contrast, all have the same number of occupied shells but a steadily increasing nuclear charge, which is why properties change progressively across a period rather than repeating.

The table also divides into metals and non-metals. Metals are found on the left and centre, non-metals on the right, and the elements with few outer electrons (groups 1 and 2) lose them readily while those with nearly full outer shells (group 17) gain electrons readily. Group 18 elements have full outer shells, form no stable ions under normal conditions and are chemically very unreactive — which is precisely why a full outer shell is used as the reference point for explaining ion formation.

Two notation points are worth fixing. First, the course uses both the modern numbering (1, 2, 17, 18) and the Roman-numeral numbering (I, II, VII, VIII); either is acceptable in your answer, but be consistent within a response. Second, when you are told a property and asked to deduce a position, work backwards through the same chain: an element that forms a 2− ion has six outer electrons, is in group 16 (VI), and if it is in period 3 its configuration must be 2,8,6, which identifies it as sulfur. Section A items frequently give you a made-up label such as 'element X' precisely to check that you can run the chain rather than recall a memorised fact.

5. Valency and the charge on a stable ion

Valency is the measure of the number of bonds an atom can form — in practice, the number of electrons an atom must gain, lose or share to reach a full outer shell. Stable ion charge is a related but distinct idea: it is the charge left on the atom once it has lost or gained the electrons needed to reach a full outer shell. Keeping the two apart is worth marks, because a question that asks for a valency and gets a charge (or the reverse) does not score.

The pattern for the main groups is:

• Group 1 (I): 1 outer electron, valency 1, loses one electron to form a 1+ ion.
• Group 2 (II): 2 outer electrons, valency 2, loses two electrons to form a 2+ ion.
• Group 13 (III): 3 outer electrons, valency 3, forms a 3+ ion.
• Group 14 (IV): 4 outer electrons, valency 4; these elements characteristically share rather than transfer electrons.
• Group 15 (V): 5 outer electrons, valency 3, can gain three electrons to form a 3− ion.
• Group 16 (VI): 6 outer electrons, valency 2, gains two electrons to form a 2− ion.
• Group 17 (VII): 7 outer electrons, valency 1, gains one electron to form a 1− ion.
• Group 18 (VIII): full outer shell, valency 0, forms no stable ion.

Notice that valency rises 1, 2, 3, 4 and then falls 3, 2, 1, 0 across a period. Students who memorise 'valency equals outer electrons' get phosphorus, sulfur and chlorine wrong. The reliable rule is: valency is the smaller of (number of outer electrons) and (8 − number of outer electrons), because an atom takes whichever route to a full shell involves fewer electrons.

The explanation you write matters as much as the number. A model answer for why magnesium forms Mg²⁺ reads: magnesium has the configuration 2,8,2, so it has two electrons in its outer shell; losing those two electrons leaves it with the full outer shell 2,8; the atom has lost two negatively charged electrons while the number of protons in the nucleus is unchanged, so the resulting ion carries a 2+ charge. Every element of that answer — configuration, what is lost or gained, the resulting full shell, and the charge explained by the proton/electron imbalance — is a place a marker can award a mark.

Finally, be clear that forming an ion does not change the element. A sodium ion still has 11 protons and is still sodium; it simply has 10 electrons instead of 11. Changing the proton number would change the element, and that only happens in the nuclear reactions you meet later in Criterion 4.

6. Atomic radius: the trend across a period and down a group

Atomic radius is a measure of the size of an atom — effectively the distance from the nucleus to the outer occupied shell. It is one of only two physical trends the course names explicitly (the other is valency), so it is the trend most likely to be examined.

Across a period, atomic radius decreases. Moving from sodium to chlorine across period 3, each successive element has one more proton in the nucleus, so the nuclear charge increases; but the extra electron is added to the same shell, so there is no additional shielding from inner shells. The stronger electrostatic attraction draws the outer shell closer to the nucleus and the atom gets smaller. Approximate values from a standard data table make the size of the effect clear: sodium is roughly 186 pm, magnesium about 160 pm, aluminium about 143 pm, silicon about 118 pm and chlorine about 99 pm. Period 2 shows the same pattern, with lithium around 152 pm falling to fluorine around 72 pm.

Down a group, atomic radius increases. Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) each add a whole new occupied shell. The outer electrons are therefore further from the nucleus and are shielded by an extra full inner shell, so the attraction they feel is weaker and the atom is larger — roughly 152 pm, 186 pm and 227 pm respectively. The same pattern runs down group 2 (beryllium to magnesium to calcium) and down group 17 (fluorine to chlorine to bromine to iodine).

The examination phrasing is usually explain or account for the trend rather than state it, so learn the two explanations as complete sentences. Across a period: increasing nuclear charge with the same number of occupied shells and no extra shielding means a stronger electrostatic attraction on the outer electrons, pulling them closer. Down a group: an additional occupied shell places the outer electrons further from the nucleus and adds shielding, weakening the attraction.

A frequent trap is to explain the period trend by saying 'more electrons are added so the atom gets bigger'. That is the intuition students bring in and it predicts exactly the wrong direction. The number of electrons does increase across a period, but they go into the same shell, and the simultaneous increase in nuclear charge wins. Say so explicitly — naming the fact that the added electrons enter the same shell is usually the difference between a partial and a full mark.

7. Groups 1, 2 and 17: the physical trends the course names

The course document restricts the trend work to periods 2 and 3 and groups 1, 2 and 17, so you can prepare thoroughly with a manageable amount of material.

Group 1 (I), the alkali metals — lithium, sodium, potassium and below. All have one outer electron, all form 1+ ions, and all are soft, low-density metals that are good conductors of heat and electricity. Down the group the atomic radius increases and the melting point falls: lithium melts at about 181 °C, sodium at about 98 °C and potassium at about 63 °C. The single outer electron is progressively further from the nucleus and more shielded, so it is progressively easier to remove, which is the structural reason behind the group's characteristic reactivity increasing down the group.

Group 2 (II), the alkaline earth metals — beryllium, magnesium, calcium and below. Two outer electrons, valency 2, and 2+ ions throughout. Compared with the group 1 element in the same period, a group 2 atom has one more proton and the same number of shells, so it is smaller and holds its outer electrons more tightly: magnesium is smaller than sodium, calcium is smaller than potassium. Radius again increases down the group as shells are added.

Group 17 (VII), the halogens — fluorine, chlorine, bromine and iodine. Seven outer electrons, valency 1, and 1− ions throughout. Atomic radius increases down the group. The most visible physical trend is the change of state at room temperature: fluorine and chlorine are gases, bromine is a liquid and iodine is a solid, and melting and boiling points rise steadily down the group. Their appearances also change in a memorable sequence — chlorine a green-yellow gas, bromine a red-brown liquid, iodine a grey-black solid.

Across period 3, three linked trends should be at your fingertips: atomic radius decreases from sodium to chlorine; valency rises 1, 2, 3, 4 then falls 3, 2, 1 and reaches 0 at argon; and the elements change from metals on the left, through the metalloid region, to non-metals on the right. Period 2 repeats the same shape from lithium to fluorine. If an examination item hands you a data table of a physical property for four unnamed period-3 elements, the fastest route to the answer is usually to identify the trend direction first and only then match it to specific elements.

8. How Section A examines this — and what separates a top answer

Section A is one of five equally weighted 36-mark booklets, made up of between four and six compulsory questions broken into lettered items and roman-numeral sub-items arranged, where possible, in order of increasing difficulty. There are no multiple-choice questions anywhere in this paper. Every response is written in the space provided, and every mark is a mark the marker has to be able to find in your writing. Marks correspond to recommended minutes, so a 2-mark item deserves about two minutes and a 10-mark question about ten.

Atomic-structure material typically appears as the opening, most accessible items of Section A: Write the electron configuration of a named element; Draw an electron shell diagram; State the number of neutrons in a given nuclide; Identify the group and period from a configuration; Determine the charge on the stable ion an element forms; then, at the higher end, Explain a trend in atomic radius or Justify which of two elements has the larger radius. The command words this paper actually uses are Calculate, Show that, Write, Explain, Complete, Identify, Determine, Draw, Give, Define, Describe, Outline, Name, Justify, Suggest, Sketch, Balance, State, Estimate and Predict — plus straightforward 'What is...' and 'Which...' openers. Read them precisely: State wants the fact only, while Explain wants the mechanism, and writing a paragraph for a one-mark State item costs you time you will want back later in the booklet.

The A-standard element for Criterion 4 is the ability to interpret graphical and tabular nuclear and atomic data, generate additional data and make generalisations. In this topic, that is the item that gives you a table of atomic radii, melting points or valencies for unnamed elements and asks you to deduce group membership, predict the value for the next element in the group, or account for an anomaly. The move that separates the top answers is always the same: state the trend, name the two competing causes (nuclear charge versus number of occupied shells and shielding), say which one dominates, and then apply that reasoning to the specific data in front of you rather than reciting the general rule.

Three practical habits pay for themselves. First, quote the configuration you are reasoning from — writing '2,8,7' before an explanation about chlorine gives the marker the evidence your conclusion rests on. Second, use the course's own language: electrostatic force of attraction, energy level, outer shell, valency, stable ion. Third, remember that answers without working may not earn marks, so even in this largely qualitative topic, show the chain — atomic number, configuration, outer-shell count, conclusion. Section A is the booklet where careful students bank marks quickly and buy themselves time for the harder nuclear items later in the same 36.