Document page
chemistry igcse acapprinted 12 · PDF 12
145 pagesrevision 46934
printed 12 · PDF 12
Page image rendered from the stored file at 110 dpi; it sharpens as you zoom. Outlines are placed from each region’s stored position on the page (612 × 792 pt).
readable
Source of this text: embedded text layer. The better of the embedded text and OCR is kept.
12
• Between the proton number and atomic mass, the proton number is always the smaller number of the two,
and the atomic mass is always the larger number of the two. e.g. in silicon (Si), its proton number is 14 and
its atomic mass is 28.
• The periodic table is arranged in ascending order of the proton numbers of every element.
Ions
• Metal atoms can lose their valence shell electrons to form positive ions, known as cations, e.g. an Na atom
loses its valence shell electron to form an ion.
• Similarly, non-metal atoms can gain electrons to complete their valence shell, which form negative ions,
known as anions, e.g. an O atom gains 2 electrons to form an ion.
• The group numbers also signify the charge of their elements. Ions of elements in Groups I, II, and III, have
a charge of 1+, 2+, and 3+ respectively. Atoms of elements in Group IV can both gain and lose electrons,
while ions of elements in Groups V, VI, and VII have a charge of 3-, 2-, and 1- respectively. Ions of elements
in Group VIII have no charge.
Isotopes
• Isotopes are atoms of the same element but with different nucleon numbers. They have the same number
of protons and electrons, but they have a different number of neutrons, e.g. U-235 and U-239 .
• Isotopes also have the same chemical properties (as they have the same number of valence shell electrons),
but they have different physical properties due to their different atomic masses.
• Often, questions will ask you to determine the relative atomic mass of an element by giving you two or
more isotopes and their abundance.
Examples of Isotopes :
Carbon-12 (¹²C) and Carbon-14 (¹⁴C)
Carbon-12: Has 6 protons and 6 neutrons. It is the most common isotope of carbon.
Carbon-14: Has 6 protons and 8 neutrons. It is used in radiocarbon dating.
Hydrogen Isotopes
Protium (¹H): The most common form of hydrogen, with 1 proton and no neutrons.