The atomic number of chlorine is 17.
25. give the electron configuration for a neutral atom of chlorine ( explain because i see the same answer everywhere, but i don't understand it)
26. explain how the understanding of the atomic emission spectrum led to the development of the atomic theory.
27. explain why the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added.
28. describe the trends in first ionization energy within groups and across periods in the periodic table. provide examples.

Explanation:

Question 25:

Give the electron configuration for a neutral atom of chlorine:

Chlorine is an element made up of 17 electrons in its orbitals.

A neutral atom of Chlorine has 17 electrons due to the atomic number(number of protons) being the same as that of electrons for atoms in their neutral state.

To write the electronic configuration of chlorine follow these principles:

              2 for s-subshell

              6 for p-subshell

              10 for d-subshell

              14 for f-subshell

   order of filling : 1s 2s 2p 3s 3p 4s 3d 4p e.t.c

Using these principle the electronic configuration of Cl is given as:

                          1s² 2s² 2p⁶ 3s²  3p⁵

   each super script is the number of electrons in the orbital.

Question 26:

Explain how the understanding of the atomic emission spectrum led to the development of the atomic theory:

Solution:

Neils Bohr model of the atom was based on the quantum mechanics. The concept is based on the premise that electrons only move the nucleus in only permissible orbits. For an electron to move from a lower energy level to a higher one, it must absorb energy.

Electrons will also have to emit energy in form of photons to move from excited to ground state. This is the basis of the atomic emission spectrum in which photons release gives rise to a line in the spectrum.

Bohr's model adequately explained the discrete lines in the atomic spectrum of hydrogen.

Question 27:

Explain why the 4s sublevel fills before the 3d sublevel begins to fill as electrons are added:

This is in compliance with Aufbau's principle. The principle states that sublevels with lower energies are filled up before those with higher energies.

The 4s is of lower energy compared to 3d sublevel according to the pattern shown below:

                1s 2s 2p 3s 3p 4s 3d 4p 5s 4d

           energy increases to the right

This is why the 4s will fill before the 3d

Question 28:

Describe the trends in first ionization energy within groups and across periods in the periodic table. Provide Examples

Ionization energy is a measure of the readiness of an atom to loose an electron. Different kinds of atoms bind their valence electrons with different amounts of energy. The first ionization energy is required to remove the most loosely bound electron of an atom in the gas phase. After this first ionization energy, it becomes more difficult to remove succeeding electrons due to the nuclear pull.

Down a group ionization energy decreases because of the increasing atomic radii and shielding effect of the nuclear charge.

Across a period, it increases progressively from left to right.

Example:

Ionization potential in down group I:

     Lithium      5.37ev

     Sodium      5.12ev

     Potassium    4.32ev

Across the third period:

    Sodium           5.12ev

    Magnesium     7.61ev

the answer is symmetrical : )