Glossary

The structure of an atom:

atom - the smallest component of an element having the chemical properties of the element

atomic number – the number of protons found in the nucleus (that can be occupied by one or, at most, two electrons).

atomic orbitals - regions of space outside the nucleus that can be occupied by one or, more electrons. They are named s, p, d, and f. They have different shapes.

anion – negative ion which is formed by an electron gain.

cation – positive ion which is formed by electron loss.

electron – particle found outside the nucleus and has a negative charge.

element – a substance made of only one type of atom.

isotope – any of two or more forms of a chemical element, atoms with the same atomic number but with different mass numbers, having the same number of protons in the nucleus but having different numbers of neutrons in the nucleus.

ion - an electrically charged particle or group of particles formed by the loss or gain of one or more electrons.

mass number - the sum of the number of protons and neutrons in the nucleus of an atom.

neutron – particle found in the nucleus and has no charges and has the same mass as a proton.

nucleus – the central part of an atom that contains protons and nucleus.

proton – a positively charged subatomic particle in the nucleus.

relative atomic mass – the ration of the average mass of one atom of that element to 1/12th of the mass of one atom of carbon-12.

relative molecular mass – the ratio of the average mass of that molecule to 1/12th of mass of an atom of carbon-12.

atomic radius – half the shortest internuclear distance found in the structure of an element. Other values often used to represent the atomic radius are covalent radius, van der Waals radius and metallic radius.

charge density – the charge-to-size ratio of an ion. Ions with greater charge-to-size ratio have more polarizing power.

covalent radius – half the internuclear distance between two identical atoms in a single covalent bond. This is often used to represent the atomic radius of non-metallic elements.

diatomic – an element, such as chlorine, which exists as molecules consisting of two atoms.

effective nuclear charge – the resultant attraction from the nucleus experienced by the outermost electrons, after taking shielding effects into account.

metallic radius – half the shortest internuclear distance between two adjacent atoms in a metallic bond. This is often used to represent the atomic radius of metallic elements.

molecular compound – a substance containing molecules made up of covalently-bonded atoms. The molecules are held together by intermolecular forces. Also called simple covalent.

monatomic – An element, such as argon, which exists as single atoms.

Bohr’s model – a concept or model of the atom in which the negatively charged electrons move in circular or elliptical orbits around the positively charged nucleus, energy being emitted or absorbed when electrons change from one orbit to another.

in Thomson’s model ,the atom is composed of electrons surrounded by positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”.

the Rutherford model shows that an atom is mostly empty space, with electrons orbiting a fixed, positively charged nucleus in set, predictable paths.

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Electron shell of an atom:

Aufbau principle – electrons always fill the lowest energy orbitals first.

electron – particle found outside the nucleus and has a negative charge.

electronic configuration – the arrangement of electrons in an atom.

Quantum Numbers – The four numbers that define each particular electron of an atom. The Principle Quantum Number (n) describes the electrons’ energy and distance from the nucleus. The Angular Momentum Quantum Number (l) describes the shape of the orbital in which the electron resides.

the Magnetic Quantum Number

The Magnetic Quantum Number (m describes the orientation of the orbital in space. The Spin Quantum number describes whether the spin of the electron is positive or negative.

Hund’s rule – electrons never pair up in the same orbital until all orbitals of the same energy are singly occupied, and all unpaired electrons have parallel spin.

orbital – It is the space around the nucleus in which the electron is found with a probability of 90%.

Pauli exclusion principle – only two electrons may occupy the same orbital, and they must do so with opposite spin.

shielding – the ability of inner shells of electrons to reduce the effective nuclear charge on electrons in the outer shell.

subshells – regions within the principal quantum shells where electrons have more or less energy depending on their distance from the nucleus. They are given the letters s, p, d and f.

first ionization energy – the amount of energy required to remove the first electron from an atom. More accurately, it is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms.

mutual repulsion – repulsion between the two electrons paired within a single orbital.

Note

(Note that this is mutual repulsion within an electron pair, as in the context of explaining the low ionization energy of sulfur, not mutual repulsion between electron pairs, as in the context of working out the shape of a molecule).

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Mass spectrometry:

mass spectrum – a plot of the mass-to-charge ratio (m/z) of detected fragments against their relative abundance.

molecular ion – the ion formed when a molecule loses an electron during mass spectrometry but does not fragment. Represented by the symbol M+.

molecular ion peak – The peak on the mass spectrum of a compound at the highest mass-to-charge ratio, which corresponds to the relative atomic mass of the molecular ion.

acceleration – the stage of operation of the mass spectrometer when Ions accelerated to uniform speed by electric field.

deflection – the stage of operation of the mass spectrometer when Ions deflected by magnetic field; deflection depends on m/e ratio.

detection - the stage of operation of the mass spectrometer when electric current measured as ions land on plate.

mass spectrometer – an instrument for finding the relevant isotopic abundance of elements and to help identify unknown organic compounds.

Practice Exercises