IB Chem Topic 2: Atomic Structure

Dalton’s Model of the Atom

“Billiard Ball” model (1808)

• Matter is made of indivisible, indestructible particles called atoms which are hard spheres

• Atoms are the same for one element, but different for other elements

• atoms can combine to form all known compounds

Thompson’s Model of the Atom

“Plum Pudding” model (1904)

• Matter is made of atoms, which contain electrons embedded in a spongy positive material

• electrons are a fundamental component of the atom and are the same regardless of the element they are in

• An element is characterised by the number of electrons

Rutherfords Model of the Atom

“Nuclear Atom” model (1911)

• atoms have very dense and small nuclei, which contain positive charges and most of the atom’s mass

• very small electrons are in motion around the nucleus and occupy most of the atom’s volume, they are held by electrostatic attraction

• most of the atom is empty space

Gold foil experiment

• Alpha particles were fired at gold foil and a very small number bounced back

• Most of them passed through the foil

• indicated that the atom is mostly empty space with a dense nucleus.

Bohr’s model of the atom

The Bohr Atomic Model (1913)

Used the H atom to determine that

• atoms are mostly empty space with a very dense nucleus that makes up most of the atom’s mass

• the nucleus contains protons and neutrons which together form the nucleon

• the nucleus is surrounded by electrons travelling along 3D pathways called orbits

• all electron orbits of equivalent energy belong to the same energy level

• electrons can possess only certain discrete energies called energy levels

• electron energy is said to be quantized (a specific quantity)

Proton relative mass and charge

1, 1+

Neutron relative mass and charge

1, 0

Electron relative mass and charge

0\.0005, -1

atomic number (Z)

• the magnitude of positive charge in the nucleus and defines which element an atom belongs to

• indicates the number of protons and the number of electrons

Mass number (A)

The number of protons and neutrons specific to an isotope of an element, always a positive integer

Physical properties of isotopes

may differ greatly due to the differences in mass of the nuclei

Chemical properties of isotopes

remain consistent among isotopes of an element because there are no differences in electron configuration and behaviour in chemical reactions

Relative abundance of isotopes

a measure of the percentage of an isotope that naturally occurs in a sample of an element

Relative isotopic mass

mass of an atom relative to the mass of a C-12 atom in amu (atomic mass units), is in decimal form, however, IB rounds it so it is the same as mass number

Average relative atomic mass

the weighted mean average of isotopic masses of stable isotopes based on their abundance in nature, appears on the periodic table as a decimal, in amu (atomic mass units) based on the mass of C-12

Mass spectrometer

a tool that can measure relative isotopic abundances and relative isotopic mass of a sample of an element

Mass spectrometer: Vaporization

is the sample isn’t already a gas, it is heated to a gaseous state

Mass spectrometer: Ionization

the sample is bombarded with high-energy electrons which forms ions with single positive charges

Mass spectrometer: Acceleration

the unipositive ions pass through slits in parallel plates under the influence of a magnetic field

Mass spectrometer: Deflection

• ions are passed over an external magnetic field which causes them to be deflected

• ions with smaller masses and ions with higher charges are deflected more than heavier ions and ions with smaller charges

Mass spectrometer: Detection

• Positive ions with a specific mass and charge are detected and a signal is sent to a recorder

• the strength of the signal is a measure of the number of ions with that mass/charge detected

Radioisotopes

isotopes of an element with too few or too many neutrons are unstable and change to form more stable nuclei by giving off radiation

Alpha radiation

nuclei with too many protons release particles identical to helium nuclei

Beta radiation

nuclei with too many neutrons eject electrons from nuclei (owing to neutron decay)

Gamma radiation

rays that are a high energy form of electromagnetic radiation

Co-60 in radiotherapy

• targets ionizing radiation at cancer cells, damaging their genetic material, and making it impossible for them to grow/divide

• Co-60 emits very penetrating gamma radiation when its protons and neutrons change their relative positions in the nucleus

I-131 as a medical tracer

• I-131 is used identically to other isotopes of iodine in the body, but it emits both beta and gamma radiation (which can be detected)

• it can be used in the form of sodium iodide to investigate activity of the thyroid gland and to diagnose/treat thyroid cancer

• I-125 can do the same thing with prostate cancer

photons

Particle with no mass that carries electromagnetic force and travels at the speed of light. It can behave like a wave or a particle and is responsible for all forms of electromagnetic radiation.

What is a continuous light spectrum?

A continuous light spectrum is a spectrum that contains all colors of light in a continuous range, without any gaps or missing colors. It is produced by a source that emits light at all wavelengths, such as the sun or a light bulb.

emission line spectrum

Light produced by excited atoms that have absorbed energy and then release it in the form of distinct wavelengths, creating a pattern of bright lines on a dark background.

The movement of an electron from one energy level to another within an atom is called ________.

electron transition

Ionization Energy

The amount of energy required to remove an electron from an atom or ion in its ground state. It is measured in electron volts (eV) or kilojoules per mole (kJ/mol) and increases across a period and decreases down a group in the periodic table.

Assumptions of Bohr’s atomic model

• electrons can travel indefinitely within an energy level without losing energy

• The greater the distance between the nucleus and the energy level, the greater the energy required for an electron to travel in that energy level

• an electron cannot exist between energy levels, but can move to a higher, unfilled orbit if it absorbs a specific quantity of energy, and to a lower, unfilled orbit if it loses a specific quantity of energy

Maximum energy level capacity expression

2n²

Continuous vs Emission line spectrums

• Continous spectrums display all wavelengths of visible light

• Emission line spectrums display only the wavelengths of light of the photons emitted by electron transitions to n=2 (Balmer series) for a specific element

Describe the hydrogen line emission spectrum

• 4 wavelengths of light which correspond to photons released by electron transitions to the second energy level (Balmer series)

• The energy of electron transitions gets closer together at higher energy levels at convergence until ionization occurs (n=infinity)