Isotopes (Oxford AQA IGCSE Combined Science Double Award)

Isotopes

• Isotopes are atoms of the same element that contain a different number of neutrons

  • Isotopes also contain the same number of protons and electrons

• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number

  • For example, carbon-14 or C-14 has the chemical symbol

  • This means that it is the isotope of carbon with 6 protons

  • But, the 14 shows that it has 8 neutrons (14 - 6 = 8)

• Isotopes display the same chemical properties

  • This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry

Table to show the structures of isotopes of hydrogen

Isotope	Atomic Structure	Symbol
Hydrogen - 1
Hydrogen - 2
Hydrogen - 3

Calculating PEN Numbers

Protons

• The number of protons, p, is given by the atomic number

Electrons

• Since atoms are neutral, the number of electrons, e, is the same as the number of protons

  • The number of electrons in an element is also given by the atomic number

• Ions have a different number of electrons to the number of protons, depending on their charge

  • A positively charged ion has lost electrons and therefore has fewer electrons than protons

  • A negatively charged ion has gained electrons and therefore has more electrons than protons

Neutrons

• The mass number is the number of protons plus neutrons

• The number of neutrons , n, can be calculated by:

Number of neutrons = mass number - atomic number

• For example, the chemical symbol for an atom of beryllium is:

• Beryllium has an atomic number of 4

  • So, it has 4 protons

• Beryllium is an atom, it has not gained or lost any electrons

  • So, it has 4 electrons

• The mass number of beryllium is 9

  • So it has 9 - 4 = 5 neutrons

• So, the PEN numbers for beryllium are:

  • p = 4

  • e = 4

  • n = (9 - 4 =) 5

• With the right information, the number of neutrons equation can be rearranged to determine the mass number or atomic number, e.g.:

Mass number = Number of neutrons + atomic number

Atomic number = Mass number - number of neutrons