atomic number

Examples of atomic number in the following topics:

• Atomic Number and Mass Number

  • The atomic number is the number of protons in an element, while the mass number is the number of protons plus the number of neutrons.
  • The number of electrons can also be different in atoms of the same element, thus producing ions (charged atoms).
  • Isotopes of the same element will have the same atomic number but different mass numbers.
  • Given an atomic number (Z) and mass number (A), you can find the number of protons, neutrons, and electrons in a neutral atom.
  • Determine the relationship between the mass number of an atom, its atomic number, its atomic mass, and its number of subatomic particles

• Converting between Moles and Atoms

  • By understanding the relationship between moles and Avogadro's number, scientists can convert between number of moles and number of atoms.
  • The bridge between atoms and moles is Avogadro's number, 6.022×1023.
  • Given a known number of moles (x), one can find the number of atoms (y) in this molar quantity by multiplying it by Avogadro's number:
  • This can be written without a fraction in the denominator by multiplying the number of atoms by the reciprocal of Avogadro's number:
  • Convert between the number of moles and the number of atoms in a given substance using Avagadro's number

• Atomic Radius

  • The way atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity.
  • As the atomic number increases along a row of the periodic table, additional electrons are added to the same, outermost shell.
  • The increasing nuclear charge is partly counterbalanced by the increasing number of electrons, a phenomenon that is known as shielding; this explains why the size of atoms usually increases down each column.
  • Red numbers are ionic radii of cations, black numbers are for neutral species, and blue numbers are for anions.
  • A chart showing the atomic radius relative to the atomic number of the elements.

• Average Atomic Mass

  • The atomic number of an element defines the element's identity and signifies the number of protons in the nucleus of one atom.
  • Atoms of the same element can, however, have differing numbers of neutrons in their nucleus.
  • By adding together the number of protons and neutrons and multiplying by 1 amu, you can calculate the mass of the atom.
  • The atomic number of chlorine is 17 (it has 17 protons in its nucleus).
  • To calculate the average atomic mass, multiply the fraction by the mass number for each isotope, then add them together.

• Overview of Atomic Structure

  • Atoms have different properties based on the arrangement and number of their basic particles.
  • This can be determined using the atomic number and the mass number of the element (see the concept on atomic numbers and mass numbers).
  • Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge.
  • When considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom's mass based on the number of protons and neutrons alone.
  • Scientists define these charges as "+1" and "-1. " In an uncharged, neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus.

• Electronegativity and Oxidation Number

  • An atom's electronegativity is affected by both its atomic number and the size of the atom.
  • In ionic compounds, the ionic charge of an atom is its oxidation number.
  • The sum of the oxidation numbers of all the atoms in an ion or molecule is equal to its net charge.
  • The atom with higher electronegativity, typically a nonmetallic element, is assigned a negative oxidation number, while the other atom, which is often but not necessarily a metallic element, is given a positive oxidation number.
  • Apply the rules for assigning oxidation numbers to atoms in compounds

• Analysis of Molecular Formulas

  • The number of hydrogen atoms that can be bonded to a given number of carbon atoms is limited by the valence of carbon.
  • If nitrogen is present, each nitrogen atom (valence = 3) increases the maximum number of hydrogens by one.
  • For stable organic compounds the total number of odd-valenced atoms is even.
  • Thus, when even-valenced atoms such as carbon and oxygen are bonded together in any number and in any manner, the number of remaining unoccupied bonding sites must be even.
  • The number of hydrogen atoms in stable compounds of carbon, hydrogen & oxygen reflects the number of double bonds and rings in their structural formulas.

• Oxidations & Reductions

  • To determine whether a carbon atom has undergone a redox change during a reaction we simply note any changes in the number of bonds to hydrogen and the number of bonds to more electronegative atoms such as O, N, F, Cl, Br, I, & S that has occurred.
  • Bonds to other carbon atoms are ignored.
  • If the number of hydrogen atoms bonded to a carbon increases, and/or if the number of bonds to more electronegative atoms decreases, the carbon in question has been reduced (i.e. it is in a lower oxidation state).
  • If the number of hydrogen atoms bonded to a carbon decreases, and/or if the number of bonds to more electronegative atoms increases, the carbon in question has been oxidized (i.e. it is in a higher oxidation state).
  • If there has been no change in the number of such bonds, then the carbon in question has not changed its oxidation state.

• Avogadro's Number and the Mole

  • Avogadro's number is a proportion that relates molar mass on an atomic scale to physical mass on a human scale.
  • Avogadro's number is defined as the number of elementary particles (molecules, atoms, compounds, etc.) per mole of a substance.
  • With Avogadro's number, scientists can discuss and compare very large numbers, which is useful because substances in everyday quantities contain very large numbers of atoms and molecules.
  • This video introduces counting by mass, the mole, and how it relates to atomic mass units (AMU) and Avogadro's number.
  • Amedeo Avogadro is credited with the idea that the number of entities (usually atoms or molecules) in a substance is proportional to its physical mass.

• Applying the VSEPR Model

  • In the VSEPR model, the number of electron pairs around a central atom dictates a molecule's general shape.
  • The number of atoms bonded to a central atom combined with the number of pairs of its nonbonding valence electrons is called its steric number.
  • When calculating the coordination number around the central atom, count both the bonded atoms and the nonbonding pairs, because the nonbonding electrons are also in orbitals that occupy space and repel the other orbitals.
  • Use the molecule's formula, along with the number of bonded atoms and lone pair electrons, to determine its geometry.
  • The axial atoms are 90° away from the equatorial atoms.