Constituents of the Atom

Unit 1, Lesson 1

Learning outcomes:

  • To be know the constituents of the atom with their masses and charges
  • To be able to calculate the specific charge of the constituents
  • To be able to explain what isotopes and ions are
Constituent Charge (C) Mass (kg)
Proton \(1.6*10^{-19}\) \(1.673*10^{-27}\)
Neutron \(0\) \(1.675*10^{-27}\)
Electron \(-1.6*10^{-19}\) \(9.1*10^{-31}\)

The Nuclear Model

Also seen in GCSE Physics 1 and 2

Rutherford's experiment shows us that the structure of an atom consists of positively charged protons and neutral neutrons in one place called the nucleus. The nucleus sits in the middle of the atom and is orbited by negatively charged electrons. At GCSE, constituent charges and masses were used relative to each other - the table above shows the actual charges and masses.

Almost all of the mass of the atom is in the nucleus which takes up practically no space when compared to the size of the atom. If the Solar System were shrunk so that the Sun was the size of a gold nucleus, the furthest electron would be twice the distance to Pluto. If the nucleus was a full stop it would be 25m to the first electron shell, 100m to the second, and 225m to the third.

Notation

Also seen in GCSE Physics 2

An atom of element \(X\) can be represented by \(^{A}_{Z}\textrm{X}\) where:

  • \(Z\) is the proton number. An uncharged atom has an equal number of protons in the nucleus as orbiting electrons. Also known as the atomic number.
  • \(A\) is the nucleon number. This is the total number of nucleons (protons and neutrons) in the nucleus. It can be defined as \(A=Z+N\) Also known as the atomic mass number.
  • \(N\) is the neutron number.

Isotopes

Also seen in GCSE Physics 1 and 2

Isotopes are different variants of an element. They have the same proton number but a different number of neutrons. Chemically, they behave the same as they have the same number of protons (and electrons).

Chlorine

Chlorine is represented in the periodic table as \(^{35.5}_{17}\textrm{Cl}\). How can it have 18.5 neutrons? It can't! There are two stable isotopes of chlorine:

  • \(^{35}_{17}\textrm{Cl}\), which accounts for ~75% of chlorine, and
  • \(^{37}_{17}\textrm{Cl}\), which accounts for ~25% of chlorine.

This averages to \(^{35.5}_{17}\textrm{Cl}\).

Specific Charge

Specific charge is also known as the charge-mass ratio. This is a measure of charge per unit mass and is calculated by dividing a particle's charge by its mass. It is measured in Coulombs per kilogram (of the given particle).

Constituent Charge (C) Mass (kg) Specific Charge (\(C kg^{-1}\))
Proton \(1.6*10^{-19}\) \(1.673*10^{-27}\) \(\frac{1.6*10^{-19}}{1.673*10^{-27}} = 9.58*10^{7}\)
Neutron \(0\) \(1.675*10^{-27}\) \(\frac{0}{1.675*10^{-27}} = 0\)
Electron \(-1.6*10^{-19}\) \(9.1*10^{-31}\) \(\frac{-1.6*10^{-19}}{9.1*10^{-31}} = -1.76*10^{11}\)

Ions

Also seen in GCSE Physics 2

An atom may gain or lose electrons. When this happens, the atom becomes negatively or positively charged, becoming an ion.

If the atom gains an electron there are more negative charges than positive, so the atom is a negative ion.

  • Gaining one electron would mean it has an overall charge of -1, which actually means \(-1.6*10^{-19}C\).
  • Gaining two electrons would mean it has an overall charge of -2, which actually means \(-3.2*10^{-19}C\).

If the atom loses an electron there are more positive charges than negative, so the atom is a positive ion.

  • Losing one electron would mean it has an overall charge of +1, which actually means \(+1.6*10^{-19}C\).
  • Losing two electrons would mean it has an overall charge of +2, which actually means \(-3.2*10^{-19}C\).