Arrangement of Electrons in the Atom - Chp 3

Continuous spectrum

light passes through a glass prism and is broken up into an array of colours

Line spectrum

• light emitted from the electrical current passing through an element is broken up into narrow coloured lines
• each element has a unique number and arrangement of electrons, resulting in different frequencies of light being emitted and a unique emission line spectrum

What did Bohr's theory state?

1. electrons orbit the nucleus on fixed paths called orbits
2. when in an orbit, electrons have a fixed/quantised amount of energy
3. when energy is supplied electrons can no longer stay in the same energy level. They jump one or more energy levels, which are closer together as you move away from the nucleus
4. the electron is now excited and unstable, and eventually it falls back to its original level giving off excess energy as light.
5. the light has a definite frequency and wavelength

E₂ - E₁ = hf

E₂ - energy of higher energy level
E₁ - energy of lower energy level
h - Planck's constant
f - frequency of light emitted

What is the quantisation of energy?

the idea that an electron in an atom can only have a fixed amount of energy, called a quantum of energy

Energy level

the fixed energy value that an electron in an atom may have

Ground state

of an atom, is one in which the electrons occupy the lowest available energy levels

What happens when an electron in it's ground state is given more energy?

• a specific amount of energy is absorbed (= the difference in energy between ground state and excited state)
• the electrons jump up from lower energy levels to higher energy levels
• the electrons are now in an excited state, unstable

Excited state

of an atom, is one in which the electrons occupy higher energy levels than those available in the ground state

What happens when the electrons fall back down to lower energy levels?

• since the electrons in excited are unstable they fall back down to lower energy levels after a short period of time
• the excess energy is released in the form of light of a definite frequency
• the frequency of the light depends on the difference in energy between the two energy levels

When electrons fall to the 1st level what light is given off?

• Lyman series
• invisible ultraviolet (UV) light

When electrons fall from higher levels to the 2nd level what light is given off?

• Balmer series
• visible coloured lines

When electrons fall from higher levels to the 3rd level what light is given off?

• Paschen series
• invisible infrared (IR) light

Sample Q: explain, terms of energy levels, why the spectra of elements consist of series of lines of definite frequency

Atomic absorption spectrum

it is the photo-negative of an emission line spectrum, it consists of a rainbow with a series of black lines, whereas the emission line spectrum was a black background with a series of coloured lines

How is atomic absorption spectrometry carried out?

• white light from a lamp is passed through a gaseous sample of an element
• the light that comes out is passed through a prism, and is found to have certain wavelengths missing
• these wavelengths are absorbed by the sample and correspond exactly to the wavelengths that would be detected in an emission line spectrum

How is atomic absorption spectrometry used?

• detecting the presence of elements in materials dissolved in water
• in the analysis of water for heavy metals such as lead, mercury and cadmium
• for measuring concentrations of lead in blood, as lead in blood is particularly harmful

Issues with Bohr's theory

• worked for hydrogen but needed to be modified for other elements with more than one electron
• when the emission line spectrum was closely examined, faint lines were discovered beside the main lines, which led to the discovery of energy sublevels

How many electrons do each of the sublevels take?

s- 2
c- 6
d- 10
f- 14

What is spd notation?

used to describe the electron arrangement in atoms, including the sublevels

Sublevel

a subdivision of a main energy level and consists of one or more orbitals of the same energy

Orbital

a region in space within which there is a high probability of finding an electron

What did Erwin Schrodinger do?

devised mathematical equations to predict the shape of an orbital

What are the main 2 shapes of an orbital + how many electrons can an orbital hold?

• s shaped (spherical)
• p shaped (dumbbell)
• can only ever hold 2 electrons

Heisenberg's Uncertainty Principle

states that it is impossible to measure at the same time both the velocity and the position of an electron

SPD notation example

What are the SPD notation exceptions?

• Copper (Cu)
• Chromium (Cr)

Ions + SPD notation

Describe the procedure for carrying out a flame test:

• using a presoaked wooden splint, hold a sample of the salt in the blue flame of the Bunsen burner

Flame Test: Lithium

Crimson

Flame Test: Potassium

Lilac

Flame Test: Barium

Green

Flame Test: Strontium

Red

Flame Test: Copper

Blue-green

Flame Test: Sodium

Yellow