The Atom Notes

Cambridge IGCSE Physics: Revision notes

Key facts

  • An atom has a tiny, dense nucleus (protons and neutrons) with electrons in shells around it.
  • Most of an atom is empty space; the nucleus holds almost all the mass.
  • Atoms form ions by gaining or losing electrons only.
  • Rutherford's alpha scattering experiment gave evidence for the nuclear model.

Structure of the atom

An atom has a very small, dense nucleus of protons and neutrons, with electrons in shells around it.

The nucleus is tiny compared with the atom, so most of an atom is empty space, yet it holds almost all the mass. The positive protons attract the negative electrons, which keeps them in orbit.

Li3p 4n

Lithium atom

2,1

The nuclear model: a tiny, dense, positive nucleus with electrons in shells around it (lithium, 3 protons, 4 neutrons).

Proton

Charge:
Positive
Mass:
Large mass
Where:
Nucleus

Neutron

Charge:
No charge
Mass:
Large mass (about a proton)
Where:
Nucleus

Electron

Charge:
Negative
Mass:
About 2000 times lighter
Where:
Shells

Where is most of the mass of an atom?

Ions

Atoms form ions by losing or gaining electrons. The number of protons never changes.

A positive ion forms when an atom loses electrons, for example NaX+\ce{Na+}. A negative ion forms when it gains electrons, for example ClX−\ce{Cl-}. Only electrons move, never protons or neutrons.

Na11p 12n

Sodium atom

2,8,1

+Na11p 12n

Sodium ion

2,8

Cl17p 18n

Chlorine atom

2,8,7

−Cl17p 18n

Chloride ion

2,8,8

Sodium loses one electron to form Na+; chlorine gains one electron to form Cl-.

Positive ion

  • Loses electrons
  • NaX+\ce{Na+}, MgX2+\ce{Mg^2+}, AlX3+\ce{Al^3+}

Negative ion

  • Gains electrons
  • ClX−\ce{Cl-}, OX2−\ce{O^2-}, NX3−\ce{N^3-}

Worked example

A magnesium atom has 12 protons and 12 electrons (neutral). It forms MgX2+\ce{Mg^2+}. How many protons and electrons does the ion have?

An atom gains two electrons. Its charge is...

Alpha scattering

Alpha particles fired at gold foil showed that atoms are mostly empty space with a small, dense, positive nucleus.

Before this, the plum pudding model spread charge evenly through the atom. Positive alpha particles were repelled by the positive nucleus, and the closer one came, the stronger the repulsion. Link each observation to its conclusion.

Solid sphere

+++++++++−−−−−−−−

Plum pudding

+

Nuclear model

How the model of the atom changed: Dalton's solid sphere, the plum pudding model, then Rutherford's nuclear model.
gold foilmost pass straight throughsome deflected at large anglesvery few bounce back
Schematic of alpha scattering by gold foil (not to scale)

Observation

  • Most passed straight through
  • Some deflected at large angles, a few back

Conclusion

  • Atoms are mostly empty space
  • A small, dense, positive nucleus

Why did a few alpha particles bounce back?

Mass of the nucleus

Large deflections need a massive, dense target, and electrons have almost no mass, so nearly all the mass is in the nucleus.

A spread-out mass could not deflect alpha particles so strongly. The nucleus (about 10−1510^{-15} m across) is thousands of times smaller than the atom (about 10−1010^{-10} m) yet holds more than 99.9% of its mass. Electrons are about 2000 times lighter than protons.

+++++++++−−−−−−−−

Plum pudding (Thomson)

+

Nuclear model (Rutherford)

Plum pudding (spread-out mass) cannot deflect alpha particles strongly; the nuclear atom (dense nucleus) can.

Nucleus

  • About 10−1510^{-15} m across
  • Almost all the mass
  • Positive

Whole atom

  • About 10−1010^{-10} m across
  • Mostly empty space
  • Neutral overall

Compared with the atom, the diameter of the nucleus is...

Try an exam question

Describe two observations made in the alpha particle scattering experiment and state what each shows about the atom.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.