Module 2, Chapter 2


2.1 Atomic Structure and Isotopes

Nuclear atom


  • Relative charge = 1+
  • Relative Mass = 1


  • Relative charge = 0
  • Relative Mass = 1


  • Relative charge = 1-
  • Relative Mass = 1/1836

Overall charge on an atom is 0, the charges cancel out. 

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2.1 Atomic Structure and Isotopes

Atomic number - the number of protons. Every atom of each element contains the same number of protons.


Isotopes are atoms of the same element with different numbers of neutrons and different masses. But they contain the same number of protons.

Different isotopes of the same element react in the same way because they have the same number of electrons and it is the number of electrons which affects chemical reactions. The number of neutrons doesn’t affect chemical reactions.

Representing Isotopes

Mass Number (A) – A = number of protons + number of neutrons

Atomic number (Z) – Z = number of protons

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2.1 Atomic Structure and Isotopes

Atomic Structure of ions

Ions – charged atom where the number of electrons is different to the number of protons.

Ions and atoms of an element have the same number of protons but a different number of electrons

Cations – atoms with fewer electrons than protons. Overall positive charge.

Anions - atoms with more electrons than protons. Overall negative charge.

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2.2 Relative Mass


The mass of Carbon-12 is defined as 12 atomic mass units.

The standard mass for atomic mass is 1 u, the mass of 1/12th of an atom of carbon-12, 1 u is approximately the mass of a proton or a neutron.

Relative Isotopic Mass

The mass of an isotope relative to 1/12 th of the mass of an atom of carbon-12.

Relative Atomic Mass

The weighted mean mass of an atom relative to 1/12 th of the mass of an atom of Carbon-12

It takes into account the percentage abundance of each isotope and the relative isotopic mass of each isotope.

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2.2 Relative Mass

Mass Spectrometry

·         Sample placed in mass spectrometer

·         Sample is vaporised and then ionised to form positive ions

·         The ions are accelerated; heavier ions move more slowly and are more difficult to deflect than lighter ions, so the ions of each isotope are separated

·         The ions are deflected on a mass spectrum as a mass-to-charge ration m/z

·         Each ion which reaches the detector adds to the signal, so the greater the abundance, the larger the signal.

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2.3 Formulae and Equations

Ionic Charges

Metals – lose electrons and form cations (+ve ions)

Non-Metals – gain electrons and form anions (-ve ions)

Writing Formulae

Overall charge is zero so ionic charges must balance

Sum of positive charges = sum of negative charges

Balancing equations

Formula must not be changed when balancing equations.

Equations are balanced when there are the same number of atoms of each element on each side of the equation.

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