The Atom (Cambridge (CIE) IGCSE Physics)

Atomic structure

• Atoms are the building blocks of all matter

• They consist of a small dense positively charged nucleus and negatively charged electrons in orbit around the nucleus

 Atomic structure

An atom: is a small positive nucleus, surrounded by orbiting negative electrons

• Atoms are incredibly small, with a radius of only 1 × 10^-10 m

  • This means that about one hundred million atoms could fit side by side across your thumbnail

• The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom

Rutherford's alpha scattering experiment

Extended tier only

• In Rutherford's alpha scattering experiment the scattering of alpha (α) particles by a sheet of thin metal supports the nuclear model of the atom, by providing evidence for:

  • a very small nucleus surrounded by mostly empty space

  • a nucleus containing most of the mass of the atom

  • a nucleus that is positively charged

• In 1909 a group of scientists were investigating the Plum Pudding model of atomic structure

  • Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the alpha scattering experiment

• They directed a beam of alpha particles (He²⁺ ions) at a thin gold foil

  • They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount

• As shown in the diagram, instead they discovered that :

  • Most of the alpha particles passed straight through the foil because the atom is mostly empty space (A)

  • Some of the alpha particles changed direction but continued through the foil because the positive nucleus repels the positive α-particles (B)

  • A few of the alpha particles bounced back off the gold foil because the nucleus is tiny

• This bouncing back could not be explained by the Plum Pudding model of atomic structure, so a new model had to be created

  • This was the first evidence of the nuclear atom as atomic structure

 Rutherford's alpha scattering experiment results

When α-particles are fired at thin gold foil, most of them go straight through but a very small number bounce straight back 

Atoms & ions

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons

  • An atom will lose or gain electrons to become more stable

• A stable atom is normally electrically neutral

  • This means it has the same number of protons (positive charge) and electrons (negative charge)

• Positive ions are therefore formed when atoms lose electrons
  

  • There will be more protons than electrons

• Negative ions are therefore formed when atoms gain electrons

  • There will be more electrons than protons

The atomic structure of ions

The difference between positive and negative ions