Atomic structure
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Define subatomic particles and give their relative properties.
• Proton: Charge +1, mass ≈ 1 amu, located in nucleus. • Neutron: Charge 0, mass ≈ 1 amu, located in nucleus. • Electron: Charge –1, mass ≈ 1/1836 amu, located in orbitals outside nucleus.
What is an atom?
The smallest unit of matter that retains the chemical properties of an element, consisting of protons, neutrons, and electrons.
What is the atomic number (Z)?
The number of protons in the nucleus of an atom. It defines the element and equals the number of electrons in a neutral atom.
What is the mass number (A)?
The total number of protons and neutrons in the nucleus of an atom.
Do isotopes have the same chemical properties? Why or why not?
Yes, isotopes of the same element have identical chemical properties because they have the same number of electrons and the same electron configuration.
What are isotopes?
Atoms of the same element (same atomic number, Z) but with different numbers of neutrons, giving them different mass numbers (A).
Do isotopes have the same physical properties? Why or why not?
No, physical properties (e.g. mass, density, diffusion rate, boiling/melting point) differ slightly because of differences in mass.
Define relative atomic mass (Ar).
The weighted mean of the masses of the isotopes of an element relative to 1/12 of the mass of a carbon-12 atom.
Define relative isotopic mass.
The mass of a particular isotope of an element relative to 1/12 of the mass of a carbon-12 atom.
Define relative molecular mass (Mr).
The sum of the relative atomic masses (Ar) of the atoms in a molecule, relative to 1/12 of the mass of a carbon-12 atom.
Define relative formula mass.
The weighted mean mass of a formula unit of a compound compared with 1/12 of the mass of a carbon-12 atom.
What is a mass spectrometer used for?
To determine the relative abundance and relative mass of isotopes, and to calculate the relative atomic mass (Ar) of an element.
Outline the steps of mass spectrometry.
• Ionisation – Atoms/molecules are bombarded with high-energy electrons to form positive ions. • Acceleration – Ions are accelerated by an electric field so they have the same kinetic energy. • Deflection – Ions are deflected by a magnetic field; the amount of deflection depends on their mass-to-charge ratio (m/z). • Detection – Ions reach a detector and generate a current proportional to their abundance.
What information does a mass spectrum provide?
• The m/z values of ions (x-axis). • The relative abundances of ions (y-axis). • Isotopic composition of elements. -The relative atomic mass (Ar) can be calculated from the weighted average of isotope peaks.
Define relative abundance.
The proportion of each isotope of an element, usually expressed as a percentage.
How do you calculate Ar from a mass spectrum?
Ar=∑(isotopicmass×%abundance) /100
Why is carbon-12 used as the reference standard?
Carbon-12 is stable, abundant, and provides a convenient standard for defining relative atomic masses.