Topic 4E: atomic nucleus

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Atom: the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids.

Atoms are made up of subatomic particles (protons, neutrons, and electrons) which are responsible for the mass and charge of atoms.

Mass of the proton= 1.67 × 10−27 kg

Mass of the neutron= 1.68 × 10−27 kg

Mass of the electron= 9.11 × 10−31 kg

In neutral atoms, the number of protons is equivalent to the number of electrons.

Nuclear force: the force between quarks to form nucleons that holds nuclei together.

Its attractive power is dependent on distance; at farther distances it becomes insignificant, and at very short distances it becomes repulsive.

Nuclear binding energy is released when a nucleus is created from free nucleons or other nuclei.

All nuclei require net energy to separate into individual protons and neutrons; contributes to mass deficit (mass of an atom's nucleus is always less than the sum of the individual masses of the constituents when separated).

Binding energy is energy used in nuclear power plants and nuclear weapons.

1 atomic mass unit (AMU) is equivalent to 1.67 * 10^-24 grams, which is Equivalent to 1 Dalton. This is the mass of protons and neutrons (both protons and neutrons have about the same mass which is defined as the aforementioned value).

Atomic weight=mass number= # of protons + # of neutrons

Mass defect: In theory, due to the conservation of mass, we expect the mass of a nucleus to be equivalent to the sum of the masses of protons and neutrons, however the rest mass is actually less then the sum. This is caused by nuclear binding energy.

Isotopes: nuclei with the same number of protons but different number of neutrons; this would be atoms of the same element that have different numbers of neutrons.

Still have very similar physical and chemical properties; However some are unstable and will undergo radioactive decay to become other elements.

Example: deuterium and tritium are isotopes of hydrogen, carbon also has multiple isotopes.

The average atomic mass of an atom takes into account all of the atoms naturally occurring isotopes.

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Radioactive decay: A nuclear phenomenon typically exhibited by radioactive isotopes; an unstable atomic nucleus emits particles or light waves.

Because these elements generally contain more neutrons than protons, the repulsive electric forces in the nucleus become greater than the attractive forces, making the nuclei unstable.

The radioactive element then in a attempt to become more stabilized, spontaneously emits particles (photons, other charged and uncharged particles) thereby transforming to a different nucleus and a different element.

The process is dependent on the amount and identity of the radioactive element.

Alpha decay: the emission of an alpha particle, which is a helium nucleus consisting of 2 protons and 2 neutrons.

The radioactive nucleus decreases in mass number by 4 and decreases in atomic number by 2.

Beta decay: the emission of a beta particle, which can be negative or positive.

A β− particle is a electron; when this is emitted, the radioactive nucleus remains unchanged in mass number, and increases in atomic number by 1.

A β+ particle is a positron (positively charged electron); When this is emitted, the radioactive nucleus remains unchanged in mass number, and decreases in atomic number by 1

Gamma decay: the emission of highly energetic photons. The radioactive nucleus remains unchanged in both mass and atomic numbers.

The three types of radiation have different levels of penetrating power; this is the energy with which the radiation particles are ejected from the atom.

The higher the energy, the more the particles of light produced by decay will penetrate a substance.

Alpha particles can be stopped by paper, beta particles by aluminum foil, while gamma rays can only be stopped by thick lead and concrete.

radioactive decay rate is exponential and is dependent upon half-life, the activity and number of particles.

Half life: the time required for 1/2 of the radioactive atoms to remain unchanged; Can range from fractions of seconds to billions of years.

Using half-life information, scientists can date materials.

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mass spectrometry: The separation of compounds based on the mass-to-charge ratio (the mass of the atom divided by its charge).

The sample is vaporized into a gaseous state.

Gaseous samples are ionized (atoms converted into ions).

Ionized sample is passed through an electric or magnetic field wear it is separated out based on its mass (Particles mass can be calculated based on how long it takes to travel a certain distance or its angles of travel).

The charged particles are detected at the end by Faraday collectors, and the relative current generated by these ions will indicate their relative abundance.

mass spectrometers can determine the types of elements in a compound or measure the differences between the mass of different isotopes of the same element.​​​​​​​​​​​​​​

The behavior of the molecules is dictated by the charge-to-mass ratio of the ions.

A sample is bombarded with high-energy electrons, which transfer energy into the molecules.

These excited species relax by releasing an electron by forming a radical cation, aka molecular ion.