From Atoms to Materials

The common symbol for mass number is A. Representing atoms We can use an atomic symbol* to represent an atom. An atomic symbol contains an atomic number and a mass number . Atomic number Atomic number is the number of protons in an atom. For example, a lithium atom contains 3 protons, so the atomic number of lithium is 3. The atomic number is unique* for each element. No two elements share the same atomic number. Mass number According to Table 13.1 on p. 10, the mass of an electron is extremely small. Therefore, the mass of an atom is almost equal to the mass of its nucleus, which is made up of protons and neutrons. Because of this, mass number is the total number of protons and neutrons in an atom. For example, the lithium atom in Table 13.2 on p. 15 has 3 protons and 4 neutrons. Its mass number is 3 + 4 = 7. number of protons + number of neutrons = mass number 3 number of protons 4 number of neutrons 7 mass number + = atomic number 原子序 mass number 質量數 atomic symbol 原子符號 unique 唯一 The common symbol for atomic number is Z. 7 3 mass number atomic number chemical symbol Li Atomic symbol The atomic symbol of the lithium atom is shown on the right. 18 © United Prime Educational Publishing (HK) Limited 2024 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publisher. Sample