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Atomic Number and Mass Number

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Damian Khan

on 19 October 2012

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Transcript of Atomic Number and Mass Number

O 16 8 Atomic Number Z The Atomic Number (Z) is determined by the number of positively charged protons in the nucleus A Mass Number The Mass Number (A) is determined by the number of protons(Z) + the number of neutrons(N). A = Z + N 2- Charge The Charge on the atom is determined by the difference between the number of protons (Z) and the number of electrons. = no of protons - no of electrons Element Symbol Examples: C - O - N - Na - Pb - W- Tungsten Carbon Oxygen Nitrogen Sodium Lead Try some examples: How many neutrons are in one atom of the following? C 12 6 C 14 6 Cl 35 17 U 238 92 N = A - Z A = Z + N N = 12 - 6 N = 14 - 6 N = 35 - 17 = 8 neutrons = 18 neutrons = 6 neutrons N = 238 - 92 = 146 neutrons The Atomic Number also determines the number of electrons in a neutral atom. Each element has its own unique Z. Oxygen has Atomic Number 8. The mass of the atom is determined by the number of protons + neutrons i.e. the Mass Number The mass of a proton is almost identical to the mass of a neutron. More electrons than protons: negative (-ve) charge.
More protons than electrons: positive (+ve) charge. Unless it is important to distinguish between isotopes, it is common to use only the element symbol and the Charge. Isotopes Two atoms of the same element can have different numbers of neutrons These atoms would therefore have a different mass number. Atoms of the same element that have different mass numbers are called Isotopes. Carbon has 3 naturally occurring isotopes Carbon-12 : C 12 6 The nuclide symbol is This form of carbon has 6 protons and 6 neutrons. The 12 represents the mass number. Carbon-13 : C 13 6 The nuclide symbol is 6 protons and 7 neutrons. Much rarer than carbon-12 Carbon-12 and carbon-13 are the two stable isotopes of carbon. Carbon-14 C 14 6 The nuclide symbol is 6 protons and 8 neutrons. The only naturally occurring radio-isotope of carbon. (Radioactive) Used in carbon dating. N = A - Z Atomic Mass Units (amu or u) Defined using the mass of the carbon-12 atom. 1 carbon-12 atom has a mass of 12 amu 1 proton has a mass of approximately 1.007 amu 1 neutron has a mass of approximately 1.009 amu 1 amu is exactly 1/12 the mass of a carbon atom. Every element has a unique symbol. Many element symbols begin with the first letter of the element. MANY DON'T... Some symbols have two letters (second is lowercase). O 16 8 2- 8 -2 = - x No. of electrons = 10 electrons no. of protons charge = 8 x = + 2 - no. of electrons
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