Atomic structure

Sub atomic particles The 3 sub-atomic particles are the proton, neutron and electron. Protons are positively charged and have a relative mass of 1. Neutrons are neutral and have a relative mass of 1. Electrons are negatively charged and have a relative mass of 1/2000 the mass of a proton. 1 AMU = 1/12 the mass of an atom of Carbon-12.

Structure of the atom The Bohr model of the atom has the protons and neutrons in the nucleus of the atom. The electrons are located in shells (or energy levels) around the nucleus.

Atomic number and mass number The atomic number is the number of protons in the nucleus of an atom. Elements in the periodic table are arranged in order of increasing atomic number. The mass number is the number of protons and neutrons in the nucleus of an atom. The number of neutrons can be calculated by subtracting the atomic number from the mass number.

Atomic number and mass number Number of protons = 17 Number of neutrons = mass number – atomic number = 18 Number of electrons = 17 (in a neutral atom, the number of electrons is equal to the number of protons.

Isotopes Isotopes are atoms of the same element that have different numbers of neutrons and therefore a different mass number. Isotopes have the same chemical properties but different physical properties such as boiling and melting points. Some isotopes are radioactive – they emit radiation in the form of alpha, beta or gamma radiation. Uses of radioisotopes: Carbon-14 is used for radioactive dating. Cobat-60 is used in radiotherapy. Iodine-131 is used as a medical tracer.

Uses of radioisotopes Carbon 14 has a half life of 5730 years. The ratio of carbon -12 to carbon-14 falls by 50% every 5730 years. Therefore it can be used to find the age of organic material. Cobalt-60 is used in radiation therapy for the treatment of cancer. Cobalt -60 emits gamma radiation which ionizes the cancerous cells. Iodine 131 is used to diagnose and treat thyroid cancer. It emits both beta and gamma radiation. It has a short half life and is quickly eliminated from the body.

Relative atomic mass Definition of relative atomic mass (Ar) The weighted average mass of an atom compared to one twelfth the mass of one atom of Carbon – 12. It takes into account all the isotopes of the element and their percentage abundance in nature. It is a relative scale and therefore has no units. It can be used to compare the masses of different elements.

The mass spectrometer

The mass spectrometer Vaporisation – the sample is vaporised to produce a gas. Ionisation – high energy electrons from an electron gun knock electrons off the atoms to produce positive ions. Acceleration – the positive ions are accelerated in an electric field. Deflection – The positive ions are deflected depending on their mass to charge ratio. Heavier ions are deflected more than lighter ions. Detection – ions are detected through the production of an electric current.

Mass spectra Mass spectra show the percentage abundance and the mass to charge ratio of the isotopes of an element. From the mass spectrum, the Ar can be calculated: Ar = 23 × 80 +(28 × 20) 100 = 24

Electron arrangement Certain elements give off distinctive colours when heated in a flame.

The electromagnetic spectrum Frequency = number of cycles per second (Hz) Wavelength = distance between peaks (nm) Higher energy = higher frequency = shorter wavelength Lower energy = lower frequency = longer wavelength

Continuous, emission, and absorption spectra

Emission line spectra

Electron transitions e- Electrons can transition (move) between energy levels. Electrons absorb energy when they move up to higher energy levels. Electrons emit energy when they move down to lower energy levels. e- e- e- e- e- e- e-

Hydrogen absorption spectrum

Hydrogen emission (line) spectrum

Hydrogen spectrum Lines converge at higher energies. Transitions to n=3 – Infra Red radiation Transitions to n=2 - Visible light Transitions to n=1 – Ultra Violet radiation Lines converge at higher energies. At n=∞, the electron is no longer in the atom and the atoms has been ionized.

Ions A neutral atom has the same number of positive protons and negative electrons. If an atom loses electrons, a positive ion is formed. If an atom gains electrons, a negative ion is formed.

Electron arrangements The first energy level can hold 2 electrons. Subsequent energy levels can hold 8 electrons.