For example, lithium atoms contain 3 protons. The atomic number of lithium is 3. Any atom that has 3 protons must be a lithium atom.

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Putting it in symbolsWe can show this information about an atom using its symbol and atomic number:lithium(The symbol for lithium is Li - note it starts with a capital letter although its name, lithium, starts with a small letter.)The atomic number is written to the left of the symbol.

lithiumn nucleus interactive graphicPicture 2.2 Lithium has 3 positve protons in its nucleus. However, a lithium atom is neutral because there are 3 negative electrons outside the nucleus.Neutral atomsA neutral lithium atom will also have 3 electrons. The negative electrons balance the charge of the positive protons in the nucleus. Although it is the number of protons that determines the element, the number of electrons will always be the same as the atomic number in a neutral atom.

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Neutrons and isotopesThe nucleus of every lithium atom has 3 ptotons. However, the number of neutrons can vary. This gives rise to different isotopes (see pages 8 and 9).

Question 6Look at the symbols below. In each case, decide on the missing information and type it into the space.a) nitrogeni. number of protonsii. atomic numberb)heliumnumber of electrons(in neutral atom)c)XThe symbol for the elementrepresented by Xd)oxygenThe amount of positive charge in the nucleusplease choose02468navigation bar