Chapter 4 - Electronic Structure

An energy level is a region of definite energy within the atom that electrons can occupy.

An energy sublevel is a group of atomic orbitals within an atom, all of which have the same energy.

An atomic orbital is a region in space within an atom where the probability of finding an electron is relatively high.

s orbitals:

(i) are spherical in shape

(ii) can hold 2 electrons.

p orbitals:

(i) are equal dumb-bell shaped and are oriented at right angles to each other.

(ii) higher in energy than an s orbital.

(iii) can hold 2 electrons each

A spectroscope is used to study light emitted from elements:

• A continuous spectrum consists of a continuous band of colors.
• An absorption spectrum consists of a series of dark lines against a colored background.
• An emission spectrum is a series of colored lines against a dark background (also known as a line spectrum).

Niels Bohr explained the hydrogen line spectrum of hydrogen as follows:

He came up with the theory of energy levels: that is that electrons in any orbit have a fixed amount of energy.

1. If an atom absorbs an amount of energy equal to the energy difference between the energy level that an electron is in and a higher energy level, the electron will move to the higher energy level.
2. Electrons are less stable in the higher energy level and do not remain there for long.
3. The electron then falls back down to a lower energy level and a definite amount of energy equal to the difference between the two levels is emitted.
4. This energy is emitted in the form of light.
5. The definite amount of energy appears as a line of a particular color in the line spectrum.

____________________ n=3 Paschen Series in line spectrum of Hydrogen)

_____________________n=2 Balmer Series in line spectrum of Hydrogen)

 \n _____________________n=1Lyman Series in line spectrum of Hydrogen)

 \n TheHeisenberg Uncertainty principle states that it is not possible to determine at the same time the exact position and velocity of an electron.

Atomic Radii

The atomic radius of an element is half the distance between the nuclei of two atoms of the element that are joined together by a single covalent bond.

 \n The following factors affect atomic radius:

1. Nuclear charge: the more protons in the nucleus the greater the attraction between the nucleus and the outer electrons.
2. The Screening Effect: electrons in the inner levels shield the outer electrons from the attraction of the nucleus.

Atomic radius increases going down a group due to:

1. the addition of extra levels
2. an increase in screening effect.

 \n Similarly thereactivity increases as you go down any group due to:

1. an increase in atomic radius
2. an increase in screening effect cancels out the increase in nuclear charge
3. outermost electrons are less tightly held.

 \n Atomic radius decreases going across a period due to:

1. increasing nuclear charge and
2. no increase in screening effect.

 \n

Ionisation Energy

The first ionisation energy of an element is the minimum energy required to remove the most loosely bound electron from an isolated atom of the element in its ground state.

 \n The second ionisation energy is the minimum energy required to remove the most loosely bound electron from an ion of the element.

 \n Ionisation energies increase going across a period due to:

1. Increasing nuclear charge
2. decreasing atomic radius.

Ionisation energies decrease going down a group due to:

1. increasing atomic radius
2. increasing screening effect.