Grade 8

Subatomic particles, isotopes, atomic number, mass number

Use proton, neutron and electron counts to identify atoms and ions. Isotopes share an atomic number but have different mass numbers.

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IntuitionIntuition: particles and change

Atoms are not indivisible: a tiny nucleus contains protons and neutrons, while electrons occupy the surrounding region. Isotopes of one element differ in neutron count.

Left to right: ¹²C, ¹³C, ¹⁴C. Each nucleus has 6 protons (red); only the number of neutrons (grey) differs. Drag to rotate.

Carbon atom (Z = 6): 6 protons in the nucleus, 6 electrons orbiting in shells.

SchoolSchool: models and rules

Definition: Key idea

Atomic number $Z$ is the number of protons. Mass number $A$ is the total number of protons and neutrons, so neutron number is $A-Z$ .

A neutral atom has equal proton and electron counts. A positive ion has lost electrons; a negative ion has gained electrons. Ion formation does not change the number of protons in the nucleus.

The three subatomic particles
Particle	Charge	Relative mass	Location
Proton	+1	≈ 1 u	nucleus
Neutron	0	≈ 1 u	nucleus
Electron	−1	≈ 1/1836 u	around the nucleus

A=Z+N,\qquad N=A-Z

{}^{A}_{Z}\mathrm{X}

Example: Worked example

How many neutrons are in sodium-23, with atomic number 11?

Solution

Neutron number is mass number minus atomic number: 23 − 11 = 12. A neutral atom also has 11 electrons.

Example: Second example

³⁵Cl and ³⁷Cl are two forms of chlorine. Same or different element?

Solution

Both are chlorine (Z = 17 protons each); they differ only in neutron count — isotopes of one element.

Write an isotope as $^{A}_{Z}\mathrm{X}$ . For carbon-14, $Z=6$ , so there are 6 protons, $14-6=8$ neutrons and, in the neutral atom, 6 electrons.

Almost all the mass concentrates in the nucleus, which occupies a tiny fraction of atomic volume. Most of an atom is electron-filled space — explaining why atoms are mostly empty.

Example: Calculation / application example

An oxide ion has the symbol ¹⁶₈O²⁻. Find its proton, neutron, and electron counts.

Solution

Protons Z = 8. Neutrons N = A − Z = 16 − 8 = 8. Electrons = Z − charge = 8 − (−2) = 10.