2.2 The symbol for the element silicon is:

Here are the solutions to the questions:

2.2 The symbol for the element silicon is:

$$\boxed{\text{Si}}$$

2.3 Study an atom of the element BORON below and use it to answer the questions:

2.3.1 Boron is element number 5 on the Periodic Table because its atomic number is 5. The atomic number is defined by the number of protons in the nucleus, and the diagram shows 5 protons.

2.3.2 Counting the neutral particles in the nucleus of the boron atom:

$$\text{Number of neutrons} = \boxed{6}$$

2.3.3 This atom is electrically neutral because the number of positively charged protons (5) in the nucleus is equal to the number of negatively charged electrons (5) orbiting the nucleus. The positive and negative charges cancel each other out.

2.4 Consider the diagrams of three different substances:

2.4.1 Identify each of the following:

*a) A diatomic substance:

$$\boxed{\text{Y}}$$

*b) A mixture:

$$\boxed{\text{Z}}$$

2.4.2 Assuming the substance Y represents oxygen, and each molecule consists of two atoms:
There are 4 molecules shown in diagram Y.
Each molecule has 2 atoms.
Total number of atoms = $4 \times 2 = 8$

$$\text{The number of Oxygen atoms in Y} = \boxed{8}$$