Understanding the Element with Atomic Number 8

The question asks about an element with atomic number 8 and the number of electrons it gains when forming a compound with sodium. The atomic number of an element tells us the number of protons in its nucleus. In a neutral atom, the number of electrons is equal to the number of protons. Therefore, the element with atomic number 8 has 8 protons and 8 electrons.

Identifying the Element and its Properties

The element with atomic number 8 is Oxygen (O).

Let's look at its electron configuration:

• Total electrons: 8
• Electron configuration: The first electron shell (n=1) can hold up to 2 electrons. The second electron shell (n=2) can hold up to 8 electrons. So, the configuration is 2, 6.
• Valence electrons: The electrons in the outermost shell (n=2) are the valence electrons. Oxygen has 6 valence electrons.

Applying the Octet Rule for Stability

Atoms tend to gain, lose, or share electrons to achieve a stable electron configuration, typically resembling that of the nearest noble gas. This is known as the octet rule (most atoms aim for 8 valence electrons, except for very light elements like Hydrogen, Lithium, and Beryllium which aim for 2).

Oxygen has 6 valence electrons. To achieve a stable octet (8 valence electrons), it needs to gain electrons. The number of electrons it needs to gain is:

$\text{Required electrons} - \text{Current valence electrons} = 8 - 6 = 2$ electrons.

So, Oxygen needs to gain 2 electrons to become stable. When it gains 2 electrons, it forms an ion with a charge of 2- because it has gained 2 negative charges. This ion is the oxide ion, $O^{2-}$.

Forming a Compound with Sodium

Sodium (Na) has atomic number 11. Its electron configuration is 2, 8, 1. Sodium has 1 valence electron. To achieve a stable configuration, sodium tends to lose this 1 valence electron, forming a positive ion, $Na^+$.

When Oxygen forms a compound with a metal like sodium, Oxygen acts as the non-metal and accepts electrons from the metal. Since Oxygen needs to gain 2 electrons to form $O^{2-}$, and each sodium atom can donate 1 electron to form $Na^+$, two sodium atoms ($2 \times Na^+$) are needed to provide the two electrons required by one oxygen atom ($O^{2-}$). The resulting compound is sodium oxide, with the formula $Na_2O$.

The question specifically asks how many electrons the element with atomic number 8 (Oxygen) will gain. Based on the analysis, Oxygen needs and will gain 2 electrons to achieve a stable electron configuration when forming this compound.

Summary of Electron Transfer

Therefore, the element with atomic number 8 will gain two electrons.

Revision Table: Key Concepts

Additional Information: Bonding in Sodium Oxide

Sodium oxide ($Na_2O$) is an ionic compound. Ionic bonding occurs when there is a complete transfer of electrons from a metal to a non-metal. Sodium (metal) loses its valence electron to become $Na^+$, and Oxygen (non-metal) gains electrons to become $O^{2-}$. The electrostatic attraction between the positively charged sodium ions and the negatively charged oxide ion holds the compound together.

The formula $Na_2O$ arises because the total positive charge must balance the total negative charge for a neutral compound: $2 \times (+1) + 1 \times (-2) = +2 - 2 = 0$.

This example clearly illustrates how the element with atomic number 8 (Oxygen) gains electrons to fulfill its octet and form a stable ionic compound with sodium.