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### Chapter 6

Atoms & Electronic Structure

Modern Atomic Structure

Atoms consist of subatomic particles:

Atomic Number

The number of protons in an atom of an element.

- Each element has a different atomic number or number of protons.
- Each element has no charge.
- Each element has the same number of electrons as protons to keep neutral.

Quantum Mechanics

- Bohr Model states that electrons circle around the nucleus of an atom using the hydrogen atom.
- Heisenberg's Uncertainty Principle – impossible to know simultaneously the exact momentum and location of an electron.
- Schrodinger’s Wave Equation – electrons exhibit both wave and particle behavior and the probability of the electron location can be determined.

Atomic Orbitals

Probable location of an electron.

Quantum numbers are used to describe the orbital

- n – principle quantum number- whole number 1,2,3 etc… - explains the energy level.
- l – second quantum number or angular momentum – positive whole number from 0 to n-1 – defines the shape of orbital
- ml - magnetic quantum number – number from l to –l – describes the orientation of orbital in space
Shell – collection of orbitals with the same principle quantum number (n).

Subshell – collection of orbitals with the same n and l. Designated by a number and a letter.

Atomic Orbitals

n=whole number

- Number of subshells = n
- Shape of orbitals from 0 to l = n-1
- Number of orbitals in each subshell ml = 2 l + 1

Orbitals Shapes

- s orbitals – spherical in nature and as n gets larger the orbital gets larger.
- p orbitals – 3 p orbitals in the x, y, z direction on the Cartesian coordinate system corresponding to
ml - as n increases the orbitals get larger.

- d orbitals – 5 orbitals
- f orbitals – 7 orbitals

Orbitals

Example: For n=2,

- How many subshells?
- What are the possible values of l ?
- What are the shapes of the orbitals?

Orbitals

Example: If l = 1, what are the possible values of ml and how many subshells?

Final Quantum Number

ms – spin magnetic quantum number – the spin of the electron in its orbitals. Possible value of + ½ or -½.

Since the electron creates a magnetic field, 2 electrons in the same orbital, spin in opposite directions.

Pauli exclusion principle – “no two electrons in an atom can have the same set of four quantum numbers n, l, ml and ms “

Electron Configuration

Distribution of electrons among orbitals.

Orbitals are filled in order of increasing energy with no more than 2 electrons per orbital.

Condensed Electron Configuration

Core electrons – electrons in the inner-shell of an atom.

Valence electrons – electrons in the outer- shell available for a reaction.

Example : Write the electron configuration for:

Example : Write the Condensed electron configuration for:

- N
- S
- Mn

Atomic Number

The number of protons in an atom of an element.

- Each element has a different atomic number or number of protons.
- Each element has no charge.
- Each element has the same number of electrons as protons to keep neutral.

Mass Number

Since elements can have the same atomic number but differ in number of neutrons and thus mass there needs to be a way to identify the difference.

mass number = # protons + # neutrons

Elements having the same atomic number but different mass number.

X=symbol of element

23=mass # (number of

protons and neutrons)

11=atomic #

Example

- Write the symbol for Magnesium atom with a mass number of 24. How many electrons, protons and neutrons does it have?
- Magnesium 25?
- Magnesium 26?

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