Atomic structure

Split any substance again and again — water, iron, sugar — and you eventually reach a particle that cannot be cut further without losing the identity of the element: the atom. Yet the atom itself is not a solid ball. It is mostly empty space, built from three particles arranged in a very particular way, and how those particles are counted and arranged tells you almost everything about how an element behaves. This article builds the ICSE Class 9 picture of atomic structure from the ground up: what protons, neutrons and electrons are, how atomic number and mass number pin down an atom exactly, what Rutherford's gold-foil experiment proved about the nucleus, Bohr's shell picture of the electron, and the two rules — the $2n^2$ rule and the Octet Rule — that govern how electrons fill those shells and why atoms react the way they do.

The core insight

An atom is defined by its atomic number $Z$ (protons = electrons) and mass number $A = Z + n$ , with electrons filling shells K, L, M... up to a maximum of $2n^2$ per shell, and atoms reacting mainly to complete an outer shell of eight electrons (the Octet Rule).

The core idea

Three particles, one atom. Every atom has a tiny, dense nucleus at its centre holding protons (charge $+1$ ) and neutrons (no charge, mass close to a proton's). Around this nucleus, in a much larger volume, move the far lighter, negatively charged electrons. In a neutral atom the number of electrons exactly equals the number of protons, so the positive and negative charges balance and the atom carries no net charge overall.

Two numbers identify an atom. The atomic number $Z$ is the number of protons in the nucleus — it is what makes an element that element, and in a neutral atom it also equals the number of electrons. The mass number $A$ is the total count of protons and neutrons together (electrons are too light to matter for mass):

A = Z + n

where: $Z$ is the atomic number (protons), $n$ is the number of neutrons, and $A$ is the mass number. An atom is written in shorthand as $^{A}_{Z}X$ ; for sodium, $^{23}_{11}\text{Na}$ has 11 protons, 11 electrons, and $23 - 11 = 12$ neutrons.

Rutherford's gold-foil experiment fired a narrow beam of fast, positively charged alpha particles at an extremely thin sheet of gold foil, with a fluorescent screen around it to record where the particles landed. Three observations followed: most alpha particles passed straight through undeflected; a small fraction were deflected through noticeable angles; and a very small number bounced back almost the way they came. Rutherford reasoned that an atom must be mostly empty space (explaining the straight-through majority), with almost all its mass and positive charge concentrated in a tiny, dense central nucleus (explaining the rare, sharp deflections), and electrons occupying the large empty volume around it. Its merit was locating where mass and positive charge sit; its demerit was that it gave no reason why orbiting electrons would not simply lose energy and spiral into the nucleus — the gap Bohr's model was built to close.

Bohr's model answers that gap without needing any mathematics at this level: electrons move around the nucleus only in certain fixed circular paths called shells or energy levels, labelled K, L, M, N, corresponding to $n = 1, 2, 3, 4$ . Each shell has one fixed energy, and an electron neither gains nor loses energy while it stays within its own shell — which is exactly why atoms are ordinarily stable rather than collapsing.

Figure

Diagram of a sodium atom showing a central nucleus of eleven protons and twelve neutrons surrounded by three shells K L and M holding two eight and one electrons respectively

Filling of shells follows two linked rules. The $2n^2$ rule fixes the maximum electrons a shell $n$ can hold: K ( $n=1$ ) up to 2, L ( $n=2$ ) up to 8, M ( $n=3$ ) up to 18 — though a shell need not always be full, especially the outermost one. The Octet Rule states that atoms are most stable when their outermost (valence) shell holds 8 electrons (2, for the K shell alone, as in helium). Atoms react — transferring or sharing electrons with other atoms — mainly to reach this stable outer-shell arrangement, which is the reason the octet rule underlies chemical bonding, the next topic in this chapter.

Derivation AGetting from Z and A to an atom's full electron picture

The goal is to show, in one repeatable sequence of steps starting from nothing but an atomic number and a mass number, exactly how to find every particle count in a neutral atom and then distribute its electrons into shells using the $2n^2$ maximums — the same sequence an exam question expects you to run for any element, illustrated here for sodium, $^{23}_{11}\text{Na}$ .

Step 1. Read off the atomic number directly. For sodium, $Z = 11$ . Since the atom is neutral, this single number fixes both the number of protons (11) and the number of electrons (11) — protons and electrons must be equal for the overall charge to be zero.

Step 2. Use the mass-number relation $A = Z + n$ to isolate the neutrons: rearranged, $n = A - Z$ . For sodium, $A = 23$ , so $n = 23 - 11 = 12$ neutrons. This is the only place mass number is used — it never affects the electron count, only the neutron count.

Step 3. Fill the shells from the inside outward, respecting the $2n^2$ maximum at each level, and stop once all 11 electrons are placed. The K shell ( $n=1$ ) fills first, taking its maximum of 2. That leaves $11 - 2 = 9$ electrons. The L shell ( $n=2$ ) fills next, taking its maximum of 8. That leaves $9 - 8 = 1$ electron, which goes into the M shell ( $n=3$ , maximum 18, but only 1 is available to place).

Step 4. Write the electron arrangement and read its chemical meaning. Sodium's arrangement is 2, 8, 1. The outer M shell carries only 1 electron, far short of a stable octet of 8. By the Octet Rule, sodium's easiest route to stability is to lose that single outer electron rather than gain seven more — which is exactly why sodium forms a $\text{Na}^{+}$ ion in reactions, a fact used directly when sodium bonds with chlorine.

Worked example

Chlorine has atomic number 17 and mass number 35 (from the earlier chlorine example: electron arrangement 2, 8, 7, an atom one electron short of an octet). Magnesium has atomic number 12 and, from the $A = Z + n$ relation with 12 neutrons, mass number 24 (electron arrangement 2, 8, 2, two electrons short of the stable 2, 8 arrangement below it). Using these two arrangements, predict the formula of the compound magnesium forms with chlorine.

Step 1. Read the outer-shell electron counts already found: chlorine's outer (M) shell has 7 electrons, one short of an octet, so by the Octet Rule each chlorine atom gains 1 electron to become $\text{Cl}^{-}$ .

Step 2. Magnesium's outer (M) shell has 2 electrons, and losing both leaves it with the stable 2, 8 arrangement of the shell below — so each magnesium atom loses 2 electrons to become $\text{Mg}^{2+}$ .

Step 3. The compound formed must be electrically neutral overall, so the total positive charge from magnesium ions must equal the total negative charge from chloride ions. One $\text{Mg}^{2+}$ (charge $+2$ ) needs two $\text{Cl}^{-}$ ions (charge $-1$ each, total $-2$ ) to balance.

Step 4. Combine one magnesium ion with two chloride ions. Answer: the formula is $\text{MgCl}_2$ (magnesium chloride), with the electron transfer being exactly the two electrons magnesium's octet-seeking M shell gives up, matched to the two separate chlorine atoms each needing just one electron to complete their own octet.

Common traps

Confusing mass number with the (often decimal) average atomic mass shown on a periodic table: mass number is a whole-number count of protons plus neutrons for one specific atom, not an averaged value.
Believing Rutherford's experiment proved the arrangement of electrons in shells — it only proved a small, dense, positively charged nucleus surrounded by mostly empty space; the shell idea came later, from Bohr.
Treating the $2n^2$ rule as a rule that a shell must always be completely filled — it only gives the maximum capacity; the outermost shell is very often only partly filled.
Mixing up which particle carries which charge: protons are positive, electrons are negative, neutrons are neutral, and only protons and neutrons contribute to mass number.

Ask the tutor: Want to work through the electron arrangement and reactivity of another element, such as oxygen or aluminium, or trace through Rutherford's experiment diagram step by step? Ask the AI tutor for more practice atoms.

Why does the outermost shell not always fill up to its full 2n squared capacity before a new shell starts?

The $2n^2$ rule gives the maximum a shell can ever hold once it is completely filled, but electrons start occupying the next shell once the current outermost shell reaches a stable configuration, typically eight electrons (the Octet Rule), well before hitting its own theoretical maximum. That is why sodium's arrangement is 2, 8, 1 rather than piling more electrons into the L shell — the L shell already has its stable octet of 8, so the eleventh electron must begin a new M shell.

If Rutherford's experiment did not tell us about electron shells, how do we know Bohr's shell picture is right?

Rutherford's alpha-scattering results only pinned down where the mass and positive charge sit — the tiny central nucleus — and said nothing about how electrons are arranged. Bohr proposed the shell model separately, based on how atoms absorb and emit light in fixed amounts (a topic beyond this syllabus), to explain why electrons do not simply spiral into the nucleus as classical physics would otherwise predict. At the ICSE Class 9 level you are only asked to describe Bohr's model in words and apply the $2n^2$ and Octet rules, not to justify it from first principles.

Why does chlorine gain one electron while sodium loses one, instead of both doing the same thing?

Which way an atom moves depends on how close its outer shell already is to a stable octet. Chlorine's outer shell has 7 electrons, so gaining just 1 more completes the octet with minimum effort. Sodium's outer shell has only 1 electron, so losing that single electron exposes the already-complete octet of its inner L shell, which is far easier than trying to gain 7 more electrons to fill its own outer shell. Atoms generally take whichever route — losing or gaining — needs the fewest electrons to reach a stable outer arrangement.

Atomic structure

The core idea

Worked example

Common traps

Before this

Where this leads