c) Atomic Structure ​

1.9 Understand that atoms consist of a central nucleus, composed of protons and neutrons, surrounded by electrons, orbiting in shells ​

Atoms are the smallest particle of an element, consisting of a nucleus and electron shells. The nucleus is composed of protons and neutrons, and the electrons orbit around the nucleus in different shells. A model diagram for an atom is shown below:

Electron Shells ​

The electrons move around the nucleus in different energy levels. These energy levels are called electron shells:

• The first shell can hold up to 2 electrons.
• The second shell can hold up to 8 electrons.
• The third shell can hold up to 8 electrons.

1.10 Recall the relative mass and relative charge of a proton, neutron, and electron ​

1.11 Understand the terms atomic number, mass number, isotopes, and relative atomic mass (Ar) ​

• Atomic Number: The number of protons in an atom.
• Mass Number: The total number of protons and neutrons in an atom.
• Isotopes: Atoms of the same element with different numbers of neutrons.
• Relative Atomic Mass (Ar): The weighted average mass of the isotopes of an element, measured relative to 1/12th the mass of a carbon-12 atom.

Example: Chlorine has two isotopes:

• $^{35}_{17}\text{Cl}$ (75%)
• $^{37}_{17}\text{Cl}$ (25%)

To calculate the relative atomic mass:

$$ \text{Ar} = \frac{(75 \times 35) + (25 \times 37)}{100} = 26.25 + 9.25 = 35.5 $$

1.12 Calculate the relative atomic mass of an element from the relative abundances of its isotopes ​

Example: Boron has two isotopes:

• $^{11}_{5}\text{B}$ (80%)
• $^{10}_{5}\text{B}$ (20%)

To calculate the relative atomic mass:

$$ \text{Ar} = \frac{(80 \times 11) + (20 \times 10)}{100} = 8.8 + 2 = 10.8 $$

1.13 Understand that the Periodic Table is an arrangement of elements in order of atomic number ​

The Periodic Table is an arrangement of elements in order of their atomic numbers. It consists of 8 groups and 7 periods.

1.14 Deduce the electronic configurations of the first 20 elements from their positions in the Periodic Table ​

Rules for Electronic Configuration: ​

1. The 1st shell can hold up to 2 electrons.
2. The 2nd shell can hold up to 8 electrons.
3. The 3rd shell can hold up to 8 electrons.
4. The 4th shell can hold up to 18 electrons.

Examples:

• Sodium ($\text{Na}$): $2, 8, 1$
• Calcium ($\text{Ca}$): $2, 8, 8, 2$

1.15 Deduce the number of outer electrons in a main group element from its position in the Periodic Table ​

The group number determines the number of outer electrons in an element.

Examples:

• Group 1 elements have 1 electron in their outer shell.
• Group 7 elements have 7 electrons in their outer shell.
• Group 0 elements have 8 electrons in their outer shell (except helium, which has 2).