Nucleus and electron shells

Split any atom in your mind and two very different neighbourhoods appear. Right at the centre sits a tiny, unbelievably dense core called the nucleus, carrying a positive charge. Around it, filling a space that is enormous by comparison, drift the atom's electrons — the extra-nuclear part. This single division, nucleus versus extra-nuclear part, is the backbone of the "Atomic Structure" chapter in CISCE Class VIII Chemistry, and it explains three things in one stroke: why the nucleus is positive, why the nucleus holds almost all the mass even though it is minuscule, and why a complete atom carries no overall charge. This article stays with that two-region picture — detailed shell-filling rules and orbit diagrams belong to a later class and are deliberately left out here.

The core insight

An atom has two regions — a tiny, dense, positively charged nucleus built of protons and neutrons, and a much larger surrounding extra-nuclear region occupied only by electrons — whose opposite charges balance to leave a neutral atom with no net charge.

The core idea

The nucleus: small, dense, positive, heavy. The nucleus sits at the exact centre of the atom and contains the two heavier subatomic particles, protons and neutrons, together called nucleons. Protons are positively charged and neutrons are uncharged, so the nucleus as a whole carries a net positive charge equal in size to the number of protons it holds. Because protons and neutrons are each roughly two thousand times heavier than an electron, essentially the entire mass of the atom is packed into this tiny central region.

The extra-nuclear part: large, light, negative. Surrounding the nucleus is the extra-nuclear part — the comparatively vast space where the atom's electrons are found. Electrons are extremely light, so this large region contributes almost nothing to the atom's total mass, yet it is what gives the atom its overall size. Every electron carries a negative charge, so the extra-nuclear part as a whole carries a net negative charge equal in size to the number of electrons present.

\text{nucleons in nucleus} = \text{protons} + \text{neutrons}

where: protons contribute the nucleus's positive charge and neutrons contribute none; both contribute almost all of the atom's mass.

\text{protons (nucleus)} = \text{electrons (extra-nuclear part)} \quad \text{for a neutral atom}

where: this equality of proton count and electron count is exactly why a complete, undisturbed atom shows no net charge — the positive charge concentrated in the nucleus is exactly cancelled by the negative charge spread through the extra-nuclear space around it.

Figure

A single atom diagram showing a small dense positive nucleus at the centre labelled protons and neutrons surrounded by a much larger shaded region labelled extra nuclear part containing light negative electrons

Why it works

The picture holds together because charge, mass and space are shared out along strict lines, and each fact fixes the next. This is a purely descriptive model — no numeric formula is being derived — so the reasoning below simply traces the logic from particle to whole atom.

Step 1. Sort the three subatomic particles by location. Protons and neutrons never leave the nucleus; electrons never enter it. This strict separation is what defines the two regions in the first place — there is no overlap and no particle that belongs to both.

Step 2. Sort them by charge. Protons are positive, neutrons are neutral, electrons are negative. Because only protons sit in the nucleus, the nucleus's charge is entirely positive; because only electrons sit outside it, the extra-nuclear part's charge is entirely negative.

Step 3. Sort them by mass. Protons and neutrons are each far heavier than an electron, and both live only in the nucleus. So no matter how large the extra-nuclear space looks, it barely adds to the total mass — the nucleus alone accounts for almost all of it.

Step 4. Combine the charge argument with the fact that a complete atom is neutral. If the atom overall has no charge, the positive charge contributed by the nucleus's protons must exactly equal the negative charge contributed by the extra-nuclear electrons — which is only possible if the number of protons equals the number of electrons.

Worked example

Question: A carbon atom has 6 protons, 6 neutrons and 6 electrons. (a) State what the nucleus contains and its net charge. (b) State what the extra-nuclear part contains and its net charge. (c) What is the overall charge on the complete atom, and why?

Step 1. Identify the nucleus's contents. The nucleus holds the nucleons: 6 protons and 6 neutrons, giving $6 + 6 = 12$ nucleons in total. Since only the 6 protons carry charge (each neutron is neutral), the nucleus carries a net charge of $+6$ .

Step 2. Identify the extra-nuclear part's contents. It holds only the 6 electrons, each negatively charged, so the extra-nuclear part carries a net charge of $-6$ .

Step 3. Combine the two regions. The whole atom's charge is the nucleus's charge plus the extra-nuclear part's charge: $(+6) + (-6) = 0$ . The complete carbon atom is electrically neutral, because its 6 protons in the nucleus exactly balance its 6 electrons in the extra-nuclear part.

Competitive stretch

If you remove one electron from this neutral carbon atom (without touching the nucleus), the nucleus still carries $+6$ but the extra-nuclear part now carries only $-5$ , so the atom as a whole is left with a net charge of $+1$ . This is exactly how a positively charged ion forms — by disturbing the electron count in the extra-nuclear part while the nucleus, and hence the mass and the proton count, stays untouched.

Common traps

Placing electrons inside the nucleus alongside protons and neutrons — electrons belong only to the extra-nuclear part, never the nucleus.
Assuming a tiny nucleus must also be light — the opposite is true; the nucleus holds almost all the atom's mass because protons and neutrons are much heavier than electrons.
Calling the nucleus neutral or negative — it is always net positive (or zero only if it somehow had zero protons, which never happens for a real atom), since it contains only positive protons and uncharged neutrons.
Reaching for shell-filling rules such as 2, 8, 8 or drawing electron orbits at this stage — those detailed models are outside this syllabus; here only the nucleus versus extra-nuclear-part picture is required.

Ask the tutor: want a quick check on nucleus versus extra-nuclear-part charge and mass for a different element, or help seeing how this two-region picture connects to ions?

If the nucleus is so small, how can it hold almost all the mass?

Size and mass are not the same thing. The nucleus is small in volume because protons and neutrons are packed extremely tightly together at the centre, but each of those particles is individually very heavy — about two thousand times heavier than an electron. The extra-nuclear part is large in volume but holds only the light electrons, so it contributes very little mass even though it takes up most of the atom's space. Think of it like a small bag of iron ingots inside a huge, mostly empty balloon — the bag is tiny but far heavier than the balloon around it.

Why doesn't the positive nucleus just pull all the electrons into itself?

This article's model does not go into the forces that keep electrons at a distance — that mechanism belongs to the more detailed shell/energy-level models you meet in a later class. For now, simply accept the observed picture: electrons occupy the extra-nuclear space around the nucleus rather than being packed inside it with the protons and neutrons, and a full explanation of why they stay out there is not required at this stage.

Does an atom always have equal numbers of protons and electrons?

A complete, undisturbed atom does, because that equality is exactly what makes it electrically neutral — the positive charge of the protons in the nucleus cancels the negative charge of the electrons in the extra-nuclear part. If an atom gains or loses electrons (its proton count and nucleus stay the same), the two counts become unequal and the atom is left with an overall charge, positive or negative; such a charged particle is no longer called a neutral atom but an ion, a topic for later study.