Relative Atomic Masses
Because the mass of an atom is very small, it is convenient to define a special unit that avoids large negative exponents. This unit, called the atomic mass unit and designated by the symbol u (some authors use the abbreviation amu), is defined as exactly 1/12 the mass of a ¹²C atom. Thus the mass of a ¹²C atom is exactly 12 u. The masses and abundances of some other nuclides are listed in Table 1-1.

Naturally occurring silicon is 92.23% ²⁸Si, 4.67% ²⁹Si, and 3.10% ³⁰Si. For chemical purposes, it is sufficient to know the average mass of a silicon atom in this isotopic mixture. These average masses are designated by Ar (E), where E is the symbol for the particular element. For example, the average mass of silicon atoms is

The term atomic mass will be understood to mean average atomic mass; nuclidic mass refers to one particular isotope of an element. Atomic masses are used in nearly all chemical calculations.

Table 1-1. Some Nuclidic Masses in Atomic Mass Units

مواضيع ذات صلة

الشكل الوضعي للسكريات الأحادية configuration of monosaccharides

نظام استخلاص الجزئيات البسيطة

اختيار الطور الجديد وعدد مرات الاستخلاص

ميكانيكية الاستخلاص

طبيعة الصنف المستخلص

الاستخلاص بالمذيب

امتصاص الشعاع الكهرومغناطيسي وطيف الامتصاص

الطرق الكيميائية الطيفية في التحليل.

طبيعة الكيمياء التحليلية

التحليل بالحقن الجرياني المتقطع Batch flow injection analysis

الحقن الجرياني متعدد التخفيف Multi-commutation in FIA

التحليل بالحقن المتعاقب Sequential Injection Analysis