Atomic Structure: Understanding Atoms and their Electronic Configuration, Study notes of Chemistry

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Atomic Structure

Foundations

Dr. R.

Masses of Diatomic Elements There are Eight Diatomic Elements

These Eight elements are Hydrogen, Nitrogen,

Oxygen, Fluorine, Chlorine, Bromine, Iodine, and

Astatine. They occur in nature as diatomic

molecules when all other elements occur in the

atomic state.

These Eight Elements have two masses: Atomic

Mass and Molecular Mass. Since there are two

atoms in a molecule,

Molecular Mass = Atomic Mass x 2

At

Masses of Diatomic Elements: Compare a Diatomic with a Monoatomic element Example: Hydrogen

• 1 atom of hydrogen weighs 1.008 amu
• 1 mole of hydrogen atoms weigh 1.008 g
• Hydrogen is a diatomic molecule.
• 1 Molecule of hydrogen contains two atoms of hydrogen
• Therefore, 1 molecule of hydrogen weighs 2 x 1.008 = 2.016 amu
• 1 mole of hydrogen molecule weighs 2.016 g Example: Neon
• 1 atom of neon weighs 20.179 amu
• 1 mole of neon atoms weigh 20.179 g
• Neon does not form molecule – it is a monoatomic element
• Therefore, we do not have molecular mass for Neon.
• 1 mole of Neon atoms weighs: 20.179 g

The Fundamentals of Atomic Structure

1. Atoms are the fundamental building units of matter.
2. Atom has two major parts: Nucleus and Shells. The nucleus is the small sized, dense body found at the center of the atom; shells are the circular orbits found around the nucleus.
3. There are three types of particles (generally called subatomic particles) inside an atom. They are: Electrons, Protons, and Neutrons. Electrons are negatively charged. Protons are Positively charged; Neutrons are Neutral. The mass of electron is negligible. The mass of a proton is the same as the mass of a neutron = 1 a.m.u. (atomic mass unit). 1 amu = 1 / 6. 023 x 10 ^ 23 g. Or 6. 023 x 10 ^ 23 amu = 1 g; 6. 023 x 10 ^ 23 is the Avogadro Number.
4. Avogadro Number represents the number of atoms in 1 mole of a monoatomic element.
5. Atoms are electrically neutral even though they have charged particles inside of them. This means that the positive and the negative charges inside an atom are equal and they cancel out each other. Therefore, the number of Protons should be equal to the number of electrons.
6. Nucleus contains both the Protons and Neutrons. Therefore, the positive charge of the atom is entirely concentrated only in the Nucleus. Similarly the mass of the atom is concentrated only in the nucleus.
7. The negatively charged particles namely electrons revolve around the nucleus in shells. This is similar to the planetary motion Electrons also spin on their own axis like the planets.
8. Every shell can have only a definite number of electrons. (See Right) Q Shell – maximum of 8 electrons only P Shell – maximum of 18 electrons O Shell – maximum of 32 electrons N Shell - maximum of 32 electrons M Shell – maximum of 18 electrons L Shell - maximum of 8 electrons K Shell – maximum of 2electrons Your task: Position Protons and Neutrons Your Task: Draw Nucleus, Label Shells, Fill electrons, label valence shell, penultimate shell, antepenultimate shell, kernel, and valence sheath

The Fundamentals of Atomic Structure

9. The shells are arranged concentrically with nucleus at the center.

10. Each shell has a fixed diameter and energy. The shell is also called

Energy Level.

11. The diameter as well as the energy of the shell increases as move

from inside to the outside. The diameter of an atom is: 0.1 to 0.

nanometers or 1 to 5 Angstrom units, Å.

(nm = 1 × 10 -^9 m ; Å = 1 × 10 -^10 m so 10 Å = 1 nm ).

12. There are infinite number (countless) of shells in an atom.

13. We count shells from interior to the exterior. Shells are named K, L,

M, N, O, P, Q, respectively from shell 1.

14. The last shell to have electrons is called the Valence Shell or the

Outermost Shell

15. The shell immediately inner to the Valence Shell is called the

Penultimate Shell

16. The shell immediately inner to the Penultimate shell is called the

Antepenultimate shell.

17. The electrons present in the valence shell are called Valence

Electrons.

18. All electrons other than the valence electrons are called inner

electrons are core electrons.

19. Valence shell together with all the empty shells outside of the

valence shell is called the Valence Sheath

20. Nucleus together with all the shells from K up to the Penultimate

shell is called the Kernel.

Energy Diameter

Daily Formative Assessment – 1: Complete the Concept Map

Recap: Valence Shell and Valence Valence shell (the last shell to have electrons) together with the outer empty shells makes what is called a Valence Sheath The nucleus together with all the inner shells makes the kernel of the atom. You can visualize the atom as orange fruit or a corn with the sheath. The skin is like the valence sheath and the corn or the fruit is the kernel or core of the atom.

• There are indefinite number (countless) of shells in an atom.
• We count shells from interior to the exterior. Shells are labeled K, L, M, N, O, P, Q. There are only 7 shells that have been found with electrons. We have not yet found elements that have electron in Shell 8 or above.
• The last shell to have electrons is called the Valence Shell or the Outermost Shell
• The shell immediately inner to the Valence Shell is called the Penultimate Shell
• The shell immediately inner to the Penultimate shell is called the Antepenultimate shell.
• The electrons present in the valence shell are called Valence Electrons.
• All other electrons are called inner electrons are Core electrons or Inner electrons.
• Effective Nuclear Charge is a measure of the influence of the nucleus on the valence shell, which is determined by the inner or core electrons. Effective Nuclear Charge is = Number of protons minus Number of Inner Electrons. Effective Nuclear Charge has the symbol: Zeff

VALENCE OR VALENCY: COMBINING CAPACITY OF ATOMS Valence shell is where chemical reactions take place. Inner shells and inner electrons are not involved in chemical reactions Elements gain or lose electrons so as to attain the octet configuration in the valence shell. How many electrons are lost or gained represents the combining capacity of atoms, called Valence or Valency. So valence or valency is a whole number and can never be in decimal, because full electrons are lost or gained. Valency can be easily determined from the position of an element on the periodic table relevant to the nearest Noble Gase.

VALENCE SHELL: OCTET RULE and EXCEPTIONS TO OCTET RULE

• Shells contain electrons. The maximum capacity of a shell for electrons is given by 2 n 2 where n represents which shell – first, second, third, fourth, etc.
• Thus, the maximum number of electrons in shells are as follows”
• Shell 1 or K shell: 2 x 1 x 1 = 2
• Shell 2 or L shell: 2 x 2 x 2 = 8
• Shell 3 or M shell: 2 x 3 x 3 = 18 , --- etc.
• However, the valence shell of an atom cannot hold more than 8 electrons (excepting the K shell, which can hold only Two Electrons)
• This is called the famous Octet Rule.
  • The Octet Rule is a very important rule and guides the force behind most of the chemical reactions.
  • Octet configuration of the valence shell represents Stability. In order to understand the stability aspect, let us have a look at the noble gases. All the noble gases have the octet configuration. They are therefore quite stable, inert, and unreactive.
  • All other elements undergo chemical reactions to attain the noble gas configuration of the valence octet.
  • For example sodium attains neon configuration by losing one electron and fluorine gains one electron to attain neon configuration. However, there are exceptions to the Octet Rule. Read the content in the Handout on Octet Rule.