2. Elements, Isotopes & Relative Atomic Mass

Defining an Element: The number of protons in the nucleus determines which element an atom is. For example, any atom with 1 proton is hydrogen, while any atom with 6 protons is carbon.
Nuclear Symbols: In the periodic table, each element has a unique box. The number in the bottom left is the atomic number, representing the number of protons.
Chemical Symbols: Elements are represented by one or two letters (e.g., C for Carbon, Li for Lithium). Some are less obvious, such as Na for Sodium or Fe for Iron.

Definition: Isotopes are different forms of the same element that have the same number of protons but a different number of neutrons.
Mass Number: Because isotopes have different numbers of neutrons, they have different mass numbers. For example, Carbon-12 has 6 neutrons, while Carbon-13 has 7 neutrons.
Chemical Reactivity: Since isotopes only vary in their neutron count, they all react chemically in the same way.

Definition: Relative atomic mass is the average mass of all the isotopes that make up a particular element, weighted by their abundance (how common they are).
The Formula: Relative Atomic Mass = (Sum of isotope abundance × isotope mass) ÷ (Sum of abundances of all isotopes)

Copper has two stable isotopes: Copper-63 (69.2% abundance) and Copper-65 (30.8% abundance).

Step 1: Multiply each mass by its abundance: (63 × 69.2) = 4,359.6 (65 × 30.8) = 2,002
Step 2: Add the results together: 4,359.6 + 2,002 = 6,361.6
Step 3: Divide by the total abundance (which is 100%): 6,361.6 ÷ 100 = 63.616
Final Answer: The relative atomic mass of copper is 63.6 (to one decimal place).