Every Atom Is Trying to Reach the Same Comfortable Number

Because an outermost shell of eight is stable, and every atom takes the shortest route to it.

Sodium is $2,8,1$. It has one electron too many, so it loses that one and is left with $2,8$.

Chlorine is $2,8,7$. It is one electron short, so it gains one and becomes $2,8,8$.

Both arrive at a complete outer shell, by opposite moves — and conveniently, the electron sodium sheds is exactly the one chlorine needs. That is the whole reason common salt exists. This page covers the third part of the ICSE Class 8 Chemistry chapter on atomic structure: valency, ions, and isotopes.

Count the valence electrons, then decide which is fewer — losing them or gaining enough to reach eight.

The rules:

- 1 to 3 valence electrons — the atom loses them. Valency $=$ number of valence electrons.
- 5 to 7 valence electrons — the atom gains electrons. Valency $=8-$ number of valence electrons.
- 4 valence electrons — the atom shares. Valency $=4$.
- 8 valence electrons (or $2$ for helium) — the shell is already complete, so valency $=0$. The atom is inert.

Worked through the elements:

- Sodium, $2,8,1$ — $1$ valence electron, loses it, valency 1
- Magnesium, $2,8,2$ — $2$, loses them, valency 2
- Aluminium, $2,8,3$ — $3$, loses them, valency 3
- Carbon, $2,4$ — $4$, shares, valency 4
- Nitrogen, $2,5$ — $5$, gains $8-5=3$, valency 3
- Oxygen, $2,6$ — $6$, gains $8-6=2$, valency 2
- Chlorine, $2,8,7$ — $7$, gains $8-7=1$, valency 1
- Neon, $2,8$ — $8$, complete, valency 0

Why the rule splits at the middle. Reaching eight is the goal, and an atom takes whichever path needs fewer electrons moved. Sodium could in principle gain seven; losing one is far easier. Chlorine could lose seven; gaining one is far easier. Only at four are the two routes equally expensive, which is why carbon does neither and shares instead.

The point that makes valency useful. Valency is the combining capacity of an element — how many bonds it can form. Since sodium has valency $1$ and oxygen valency $2$, one oxygen needs two sodiums, giving ${\text{Na}}_2\text{O}$. Every formula you will write in the next chapter comes from these numbers.

And note what valency is not. It has no sign. Sodium's valency is $1$, not $+1$ — the sign belongs to the ion, which is the next section.

An atom that loses electrons becomes a positive cation. An atom that gains electrons becomes a negative anion.

An ion is simply a charged atom or group of atoms. The nucleus never changes — only the electron count does — so the number of protons stays fixed and the electron count moves away from it.

Cations — formed by metals, which have few valence electrons and lose them:

- Sodium: $\text{Na}\to {\text{Na}}^{+}+e^{-}$, so $2,8,1$ becomes $2,8$
- Magnesium: $\text{Mg}\to {\text{Mg}}^{2+}+2e^{-}$, so $2,8,2$ becomes $2,8$
- Aluminium: $\text{Al}\to {\text{Al}}^{3+}+3e^{-}$, so $2,8,3$ becomes $2,8$
- Potassium: $\text{K}\to {\text{K}}^{+}+e^{-}$, so $2,8,8,1$ becomes $2,8,8$
- Calcium: $\text{Ca}\to {\text{Ca}}^{2+}+2e^{-}$

Anions — formed by non-metals, which need only a few electrons and gain them:

- Chlorine: $\text{Cl}+e^{-}\to {\text{Cl}}^{-}$, so $2,8,7$ becomes $2,8,8$
- Oxygen: $\text{O}+2e^{-}\to {\text{O}}^{2-}$, so $2,6$ becomes $2,8$
- Nitrogen: $\text{N}+3e^{-}\to {\text{N}}^{3-}$, so $2,5$ becomes $2,8$
- Sulphur: $\text{S}+2e^{-}\to {\text{S}}^{2-}$

The size change, which is worth knowing. A cation is smaller than its parent atom, because it has lost its outermost shell entirely and the remaining electrons are pulled in more tightly by an unchanged nuclear charge. An anion is larger than its parent atom, because extra electrons have been added and the electrons now repel each other more while the nuclear pull is unchanged.

The result that surprises students. ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ both have the configuration $2,8,8$ or $2,8$ respectively — sodium ion is $2,8$, chloride ion is $2,8,8$. Both have achieved a noble-gas arrangement, yet they remain completely different substances, because the nucleus decides the identity. ${\text{Na}}^{+}$ has $11$ protons and neon has $10$; identical electron arrangements, different elements.

Why the charge equals the valency. Sodium's valency is $1$ and its ion is ${\text{Na}}^{+}$; aluminium's valency is $3$ and its ion is ${\text{Al}}^{3+}$. The number is the same; the ion carries a sign and the valency does not.

Isotopes are atoms of the same element with the same atomic number but different mass numbers — which means the same number of protons and a different number of neutrons.

Since $Z$ is unchanged, they are the same element with identical chemical behaviour. Since $A$ differs, they have different masses.

Isotopes of hydrogen, all with $Z=1$:

- Protium, ${}_1^1\text{H}$ — $1$ proton, $1-1=0$ neutrons
- Deuterium, ${}_1^2\text{H}$ — $1$ proton, $2-1=1$ neutron
- Tritium, ${}_1^3\text{H}$ — $1$ proton, $3-1=2$ neutrons

Hydrogen is the only element whose isotopes have their own names.

Isotopes of carbon, all with $Z=6$:

- ${}_6^{12}\text{C}$ — $6$ neutrons
- ${}_6^{13}\text{C}$ — $7$ neutrons
- ${}_6^{14}\text{C}$ — $8$ neutrons

Isotopes of chlorine, both with $Z=17$:

- ${}_{17}^{35}\text{Cl}$ — $35-17=18$ neutrons
- ${}_{17}^{37}\text{Cl}$ — $37-17=20$ neutrons

How to spot isotopes in a question. Compare the atomic numbers. Same $Z$ with different $A$ means isotopes. Different $Z$ means different elements, however close the masses.

The reverse case also has a name: atoms of different elements with the same mass number are isobars — such as ${}_{18}^{40}\text{Ar}$ and ${}_{20}^{40}\text{Ca}$.

Why chlorine's atomic mass is 35.5. A sample of natural chlorine is a mixture of the two isotopes in a ratio of about $3:1$. The average mass is therefore

$$\frac{(3\times 35)+(1\times 37)}{4}=\frac{105+37}{4}=\frac{142}{4}=35.5$$

This is the answer to a question that puzzles every student: how can an atomic mass be a fraction when protons and neutrons come in whole numbers? No single chlorine atom has a mass of $35.5$. The figure is an average over the mixture, and the fraction comes from the proportions, not from half a neutron.

Why isotopes are chemically identical. Chemical behaviour depends on valence electrons, and all isotopes of an element have the same electron configuration. ${}^{35}\text{Cl}$ and ${}^{37}\text{Cl}$ both have $2,8,7$, both have valency $1$, and both form chloride ions. Only physical properties that depend on mass — such as density and rate of diffusion — differ between them.