Chemistry_Grade_10-12 (1).pdf

In 1932 james chadwick discovered the neutron and

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atom stays electrically neutral, this particle would have to be neutral itself. In 1932 James Chadwick discovered the neutron and measured its mass. proton neutron electron Mass (kg) 1.6726 x 10 27 1.6749 x 10 27 9.11 x 10 31 Units of charge +1 0 -1 Charge (C) 1.6 x 10 19 0 -1.6 x 10 19 Table 3.2: Summary of the particles inside the atom Interesting Fact erest Fact Unlike the electron which is thought to be a point particle and unable to be broken up into smaller pieces, the proton and neutron can be divided. Protons and neutrons are built up of smaller particles called quarks . The proton and neutron are made up of 3 quarks each. 39
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3.4 CHAPTER 3. THE ATOM - GRADE 10 3.4 Atomic number and atomic mass number The chemical properties of an element are determined by the charge of its nucleus, i.e. by the number of protons . This number is called the atomic number and is denoted by the letter Z . Definition: Atomic number (Z) The number of protons in an atom The mass of an atom depends on how many nucleons its nucleus contains. The number of nucleons, i.e. the total number of protons plus neutrons, is called the atomic mass number and is denoted by the letter A . Definition: Atomic mass number (A) The number of protons and neutrons in the nucleus of an atom Standard notation shows the chemical symbol, the atomic mass number and the atomic number of an element as follows: A Z X number of nucleons number of protons chemical symbol For example, the iron nucleus which has 26 protons and 30 neutrons, is denoted as 56 26 Fe , where the total nuclear charge is Z = 26 and the mass number A = 56 . The number of neutrons is simply the difference N = A Z . 40
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CHAPTER 3. THE ATOM - GRADE 10 3.4 Important: Don’t confuse the notation we have used above, with the way this information appears on the Periodic Table. On the Periodic Table, the atomic number usually appears in the top lefthand corner of the block or immediately above the element’s symbol. The number below the element’s symbol is its relative atomic mass . This is not exactly the same as the atomic mass number. This will be explained in section 3.5. The example of iron is used again below. Fe 26 55.85 You will notice in the example of iron that the atomic mass number is more or less the same as its atomic mass. Generally, an atom that contains n protons and neutrons (i.e. Z = n ), will have a mass approximately equal to n u. The reason is that a C-12 atom has 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison. Exercise: The structure of the atom 1. Explain the meaning of each of the following terms: (a) nucleus (b) electron (c) atomic mass 2. Complete the following table: (Note: You will see that the atomic masses on the Periodic Table are not whole numbers . This will be explained later. For now, you can round off to the nearest whole number.) Element Atomic mass Atomic number Number of pro- tons Number of elec- trons Number of neu- trons Mg 24 12 O 8 17 Ni 28 40 20 Zn 0 C 12 6 3. Use standard notation to represent the following elements: (a) potassium (b) copper (c) chlorine 4. For the element 35 17 Cl, give the number of ...
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