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ATOMS
In the nucleus
protons +1 almost 2000x the mass of a
neutrons 0 slightly heavier than a proton
Around the nucleus, mostly empty space, containing
electrons
-- 1
Atomic Number = the number of protons = Z
Normally, this is the same for every atom of an element, therefore number of protons identifies the element.
In a “normal”, neutral atom, number of protons = number of electrons
Mass Number = the number of protons + number of neutrons = A
Number of neutrons in an atom of an element can vary. These are called Isotopes.
For lighter elements, up to about Z = 20, the most stable atoms are those with a neutron to proton ratio of 1:1
For heavier elements, the most stable atoms are those with a neutron to proton ratio of 1.5 : 1.
(see graph on Page 30)
Isotopes = atoms of the same element with the same number of protons but different number of neutrons.
= same atomic number ; different mass number
Almost all elements have isotopes. Some have many and some have only a few.
The isotopes of an element always occur in the same percentage.
Isotopes of an element have the same chemical properties (same number of electrons), but differ in their physical
properties such as mass (some are heavier).
Unstable isotopes are radioactive – they decay, (fall apart), and emit radiation (see Page 30. Three types)
Most of the time they will decay, emit radiation and eventually become a stable element.
Radioactive isotopes can be harmful but also have many uses, such as to diagnose and treat cancer, trace elements in
the soil and date very old artifacts.
Hydrogen is the only element for which the isotopes have been given names.
Protium – 1 proton, no neutrons. The most common isotope.
Deuterium – 1 proton, 1 neutron. Occurs in very small amounts. Also called Heavy Hydrogen.
Tritium – 1 proton, 2 neutrons. Radioactive. Found only in trace amounts.
Symbols = shorthand way to show the composition of an atom.
Nuclear Symbol :
Hyphen Notation :
Z
X
X–A
ex : 1. How many protons, neutrons and electrons in an atom of Br – 80
2. Write the nuclear symbol for silicon-28
3. Write the hyphen notation for an isotope with 15 electrons and 15 neutrons.
Practice : Page 29, Ex 2.1