Problem 65 - Chemistry and Chemical Reactivity

Question

How many electrons are in a strontium atom (Sr)? Does an atom of Sr gain or lose electrons when forming an ion? How many electrons are gained or lost by the atom? When Sr forms an ion, the ion has the same number of electrons as which one of the noble gases?

Step-by-Step Solution

Answer

Strontium has 38 electrons and loses 2 to form an ion with 36 electrons, like krypton.

Step 1: Identify Atomic Number

Strontium (Sr) is an element in the periodic table with the atomic number 38. This means a neutral atom of strontium has 38 electrons.

Step 2: Determine Sr Ion Formation

Strontium tends to form ions to achieve a stable electron configuration, similar to a noble gas. It will lose electrons until it has the same electron configuration as the nearest noble gas.

Step 3: Establish Electron Loss for Ionization

The nearest noble gas to strontium is krypton (Kr), which has 36 electrons. Therefore, a strontium atom loses 2 electrons to form a Sr²⁺ ion.

Step 4: Compare with Noble Gas Configuration

With the loss of 2 electrons, the Sr²⁺ ion now has 36 electrons, matching the electron configuration of krypton.

Key Concepts

Atomic Number

The atomic number is an essential concept in chemistry, particularly in identifying elements on the periodic table. The atomic number represents the number of protons in the nucleus of an atom. Since atoms are electrically neutral, the atomic number also indicates the number of electrons. For example, strontium (Sr) is an element with an atomic number of 38. This number tells us that there are 38 protons in the nucleus of a strontium atom and, under neutral conditions, 38 electrons surrounding the nucleus. In summary:

• The atomic number = number of protons.
• In a neutral atom, it also equals the number of electrons.
• The atomic number helps us locate strontium on the periodic table and understand its basic properties.

Understanding atomic numbers is crucial for studying elements and their reactions, such as ion formation.

Electron Configuration

Electron configuration refers to the distribution of electrons in an atom's orbitals. Electrons orbit the nucleus in layers called energy levels or shells, each with specific subshells (s, p, d, f). In the case of strontium (Sr), with 38 electrons, its electron configuration is written as follows:1. First energy level: 2 electrons in the 1s subshell.2. Second energy level: 2 electrons in 2s and 6 in 2p.3. Third energy level: 2 electrons in 3s, 6 in 3p, and 10 in 3d.4. Fourth energy level: 2 electrons in 4s, 6 in 4p.5. Incomplete 5th level: 2 electrons in 5s (2 outermost valence electrons).Electron configuration of Sr: \[1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 5s^2\]This configuration highlights the two valence electrons in the 5s subshell. Valence electrons are crucial because they determine how the element will react chemically, such as when Sr forms an ion.

Noble Gas Configuration

Noble gases are elements in the far right column of the periodic table. They have fully filled outer electron shells, making them extremely stable and usually non-reactive. This full outer shell configuration is something many elements strive for when they react, including strontium.A noble gas configuration is a shortcut to writing electron configurations by using the nearest noble gas to simplify the notation for elements. For example, for strontium, we would start from the last known noble gas, which is krypton (Kr).When strontium (which has 38 electrons) becomes an ion, it loses its two valence electrons (from the 5s orbital), achieving a similar configuration to krypton:\[[Kr] = 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6\]This makes the resulting Sr²⁺ ion more stable, having the same electron configuration as krypton, the closest noble gas. This loss of two electrons gives the Sr ion a noble gas electron configuration, which is often the driving force behind ion formation.