Understanding atomic number and atomic mass is fundamental to grasping the basics of chemistry. This worksheet will guide you through the concepts, helping you confidently calculate atomic mass and identify isotopes. We'll break down each concept clearly, providing examples and practice problems to solidify your understanding.
What is Atomic Number?
The atomic number of an element represents the number of protons found in the nucleus of a single atom of that element. This number uniquely identifies each element on the periodic table. For example, hydrogen (H) has an atomic number of 1, meaning each hydrogen atom contains one proton. Similarly, oxygen (O) has an atomic number of 8, indicating eight protons in its nucleus. The atomic number is always a whole number and is represented by the letter Z.
Key takeaway: Atomic number = Number of protons
What is Atomic Mass?
The atomic mass (or atomic weight) is the average mass of all the isotopes of an element, weighted by their relative abundance in nature. Unlike the atomic number, atomic mass is usually not a whole number. It's expressed in atomic mass units (amu), where 1 amu is approximately the mass of a single proton or neutron.
The atomic mass considers the presence of isotopes, which are atoms of the same element with the same number of protons but a different number of neutrons. This difference in neutron number leads to variations in the atom's mass.
Key takeaway: Atomic mass is an average mass, considering the isotopes of an element and their abundance.
What are Isotopes?
Isotopes are atoms of the same element that have the same atomic number (number of protons) but different mass numbers (number of protons + neutrons). They have the same chemical properties because they possess the same number of electrons, influencing chemical reactivity. However, they can have slightly different physical properties due to their varying mass.
For example, carbon-12 (¹²C), carbon-13 (¹³C), and carbon-14 (¹⁴C) are isotopes of carbon. They all have 6 protons (atomic number 6), but they have 6, 7, and 8 neutrons, respectively, leading to different mass numbers.
Key takeaway: Isotopes have the same number of protons but different numbers of neutrons.
Calculating Atomic Mass: An Example
Let's calculate the atomic mass of chlorine (Cl). Chlorine has two main isotopes:
- ³⁵Cl (chlorine-35) with a mass of 34.97 amu and a natural abundance of 75.77%
- ³⁷Cl (chlorine-37) with a mass of 36.97 amu and a natural abundance of 24.23%
To calculate the average atomic mass:
(0.7577 x 34.97 amu) + (0.2423 x 36.97 amu) = 35.45 amu
Therefore, the atomic mass of chlorine is approximately 35.45 amu.
Worksheet Exercises:
Part 1: Identifying Atomic Number and Mass Number
| Element | Symbol | Number of Protons | Number of Neutrons | Atomic Number (Z) | Mass Number (A) |
|---|---|---|---|---|---|
| Hydrogen | H | 1 | 0 | ||
| Helium | He | 2 | 2 | ||
| Lithium | Li | 3 | 4 | ||
| Carbon | C | 6 | 6 | ||
| Oxygen | O | 8 | 8 |
Part 2: Isotope Identification
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Two isotopes of an element have mass numbers of 63 and 65. If the element has 29 protons, what are the isotopes' symbols?
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Explain why isotopes of the same element have the same chemical properties but might have slightly different physical properties.
Part 3: Calculating Atomic Mass
An element has two isotopes:
- Isotope 1: Mass = 10.01 amu, Abundance = 19.9%
- Isotope 2: Mass = 11.01 amu, Abundance = 80.1%
Calculate the average atomic mass of this element.
This worksheet provides a solid foundation for understanding atomic number, atomic mass, and isotopes. Remember to consult your textbook or other learning resources for further information and practice problems. Mastering these concepts is key to success in chemistry!
