Simple model of the atom, including the proton, neutron and electron.…

The idea

The simple nuclear model describes an atom as a small, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons. A proton has charge +1.60 × 10⁻¹⁹ C and mass 1.67 × 10⁻²⁷ kg; a neutron has zero charge and mass 1.67 × 10⁻²⁷ kg; an electron has charge −1.60 × 10⁻¹⁹ C and mass 9.11 × 10⁻³¹ kg. In relative units, proton and neutron each have relative mass 1 and relative charge +1 and 0 respectively, while the electron has relative mass about 1/1836 and relative charge −1. The atom is neutral because proton number equals electron number. Nearly all the mass is in the nucleus, but nearly all the volume is the electron region.

For example: A carbon-12 atom has six protons, six neutrons and six electrons, with total charge zero.

What earns the marks

• Describes the atom as a nucleus of protons and neutrons surrounded by electrons, with the nucleus small and dense and the electrons occupying most of the volume.
• States the charge of the proton as +1.60 × 10⁻¹⁹ C or relative charge +1, and the charge of the electron as −1.60 × 10⁻¹⁹ C or relative charge −1.
• States that the neutron is uncharged, with relative charge 0 and zero charge in coulombs.
• Quotes the masses in SI units: proton and neutron about 1.67 × 10⁻²⁷ kg each, electron about 9.11 × 10⁻³¹ kg.
• Uses relative mass values correctly, with proton and neutron each 1 and electron about 1/1836, and explains that the electron mass is negligible compared with the nucleon mass.
• Explains that an atom is neutral because the number of protons equals the number of electrons, and applies this to a named nuclide such as carbon-12.

Common mistakes and how to put them right

• Writing the electron charge as a positive value.

  Put it right: Include the negative sign, giving −1.60 × 10⁻¹⁹ C or relative charge −1.

• Confusing mass and charge values, for example giving the electron a relative mass of 1.

  Put it right: Remember that the electron relative mass is about 1/1836 while its relative charge is −1.

• Writing the electron mass as 9.11 × 10⁻³¹ kg but omitting the unit or using grams.

  Put it right: Keep the value in kilograms and label it clearly.

Exam tips

• Learn the three particles as a table of charge in coulombs, relative charge, mass in kilograms and relative mass so comparisons are quick.
• When explaining neutrality, refer explicitly to equal numbers of protons and electrons rather than saying the charges cancel without justification.
• Use standard symbols and units, and keep the negative sign on the electron charge throughout any calculation.