Question

The atomic number of an element is 35 . What is the total number of electrons present in all the \(\mathrm{p}\) orbitals of the ground state atom of that element? (a) 17 (b) 11 (c) 23 (d) 6

Short answer

(a) 17

Step-by-step solution

Understand the Atomic Number

The atomic number of an element, which is given as 35, tells us the number of protons in the nucleus of an atom. In a neutral atom, the number of electrons is equal to the number of protons. Thus, this element has 35 electrons.

Determine Electron Configuration

We need to determine the electron configuration for an element with 35 electrons. Starting from the lowest energy level, electrons fill the orbitals in the order given by the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.

Fill Electron Orbitals

Distribute the 35 electrons into orbitals: 1s² (2 electrons), 2s² (2 electrons), 2p⁶ (6 electrons), 3s² (2 electrons), 3p⁶ (6 electrons), 4s² (2 electrons), 3d¹⁰ (10 electrons), 4p⁵ (5 electrons). This uses all 35 electrons. The electron configuration is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵.

Count Electrons in p Orbitals

Now, count only the electrons in p orbitals. From the configuration: 2p⁶ accounts for 6 electrons; 3p⁶ accounts for another 6 electrons; 4p⁵ accounts for 5 electrons. This results in a total of 17 electrons in the p orbitals.

Key concepts

Atomic Number

The atomic number is a fundamental property of an element. It represents the number of protons found in the nucleus of an atom.
In a neutral atom, which has no net charge, the atomic number also equals the number of electrons. This is because the positive charges of the protons balance out with the negative charges of the electrons.
For example, if an element has an atomic number of 35, it means it has 35 protons. Correspondingly, in its neutral state, the element will also have 35 electrons.
Understanding the atomic number is crucial because it defines the identity of an element. Every element on the periodic table has a unique atomic number, starting from Hydrogen with an atomic number of 1 to elements with much higher atomic numbers. This property not only helps in identifying the element but also plays a vital role in determining its electron configuration and chemical behavior.

Electron Orbitals

Electron orbitals are regions around an atom's nucleus where electrons are most likely to be found. These orbitals are not just simple circles around the nucleus, but rather have complex shapes that are defined by quantum mechanics.
The main types of orbitals are designated as \(s\), \(p\), \(d\), and \(f\). Each type of orbital can hold a specific number of electrons:

• \(s\) orbitals can hold up to 2 electrons.
• \(p\) orbitals can hold up to 6 electrons across 3 sub-orbitals.
• \(d\) orbitals can hold up to 10 electrons across 5 sub-orbitals.
• \(f\) orbitals can hold up to 14 electrons across 7 sub-orbitals.

The capacity to hold electrons means that \(p\) orbitals play an especially significant role in the chemical properties of an element.
Electrons fill these orbitals in order of increasing energy levels, according to principles like the Aufbau principle, leading to the formation of an atom's electron configuration.

Aufbau Principle

The Aufbau principle is a key rule in determining the electron configuration of an element. It states that electrons are added to the lowest energy orbitals first before moving to higher energy orbitals.
The order of filling is not linear but follows a specific sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. This sequence helps in arranging electrons in the most stable way possible in an atom.
An easy way to remember the sequence is by using the diagonal rule or referring to an electron configuration chart.
Applying the Aufbau principle, when determining the electron configuration of an element with an atomic number of 35, you start filling from 1s up to 4p.
This leads to an electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵.
Therefore, knowing the Aufbau principle is essential in predicting how electrons arrange themselves in atoms and understanding the resulting structure and reactivity.