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.

• 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.

• Electric Charge in the Atom

  • Atoms contain negatively charged electrons and positively charged protons; the number of each determines the atom's net charge.
  • The number of protons in an atom defines the identity of the element (an atom with 1 proton is hydrogen, for example, and an atom with two protons is helium).
  • As such, protons are relatively stable; their number rarely changes, only in the instance of radioactive decay.
  • In the ground state, an atom will have an equal number of protons and electrons, and thus will have a net charge of 0.
  • Atoms in such a state are known as ions.

• Multielectron Atoms

  • Atoms with more than one electron are referred to as multielectron atoms.
  • Atoms with more than one electron, such as Helium (He) and Nitrogen (N), are referred to as multielectron atoms.
  • As an approximation, the effective nuclear charge on each electron can be estimated by: Zeff=Z−σZ_\text{eff} = Z - \sigma, where $Z$ is the number of protons in the nucleus and σ\sigma is the average number of electrons between the nucleus and the electron in question. σ\sigma can be found by using quantum chemistry and the Schrodinger equation or by using Slater's empirical formula.
  • For example, consider a sodium cation, a fluorine anion, and a neutral neon atom.
  • Each has 10 electrons, and the number of nonvalence electrons is two (10 total electrons minus eight valence electrons), but the effective nuclear charge varies because each has a different number of protons:

• Atomic Theory of Matter

  • Philosophical proposals regarding atoms have been suggested since the years of the ancient Greeks, but John Dalton was the first to propose a scientific theory of atoms.
  • For this reason, Dalton is considered the originator of modern atomic theory.
  • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
  • Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
  • Atoms can be broken down into smaller pieces, and atoms of a given element can vary in mass and other properties (see isotopes and ions).