Chapter 2.4: The Atomic Theory Today

Flashcards from Chapter 2.4 of Chemistry: The Molecular Nature of Matter and Change.

Atom

An electrically neutral, spherical entity composed of a positively charged central nucleus, surrounded by one or more negatively charged electrons

Electron

A rapidly-moving, negatively-charged particle held by the attraction of the nucleus

• 2000 times smaller in terms of mass than a proton or neutron

Nucleus

Protons and neutrons in the center of the atom

• Very small; diameter is 20,000 times smaller than the total diameter

• Contributes 99.97% of the mass, but only 1 quadrillionth of the volume, creating high density (10¹⁴ g/mL)

Proton

A particle within the nucleus with a positive charge

• Has the same magnitude of charge of an electron, but with the opposite sign

• This always equals the number of electrons

Neutron

A particle in the nucleus with no charge; these are neutral

Mass number (A)

The total number of protons and neutrons in the nucleus of an atom, can be used for simple isotope names

• Written as a left superscript, like ¹²C (carbon-12)

• Find neutrons by subtracting N from this

Atomic number (Z)

The number of protons in the nucleus of each of an element’s atoms

• Equals the number of electrons

• Stays consistent across isotopes

• Written as a left subscript: ₆C signifies 6 protons (and by extension 6 electrons)

Neutron count (N)

This changes across isotopes

• Found by subtracting the constant atomic number (Z) from the mass number (A)

Atomic symbol

A one or two letter abbreviation for the English, Latin, or Greek name of an element

Isotopes

Atoms of an element that have different numbers of neutrons and therefore different mass numbers

• These have nearly-identical chemical behavior, even with different masses, due to electrons

Atomic mass unit (amu)

1/12 of the mass of a carbon-12 atom; based on relative mass

• Also known as the dalton (Da)

• About 1.66054 × 10²⁴ grams

Mass spectrometry

A method for measuring the relative masses and abundances of atomic-scale particles

• Electrons collide with an atom’s electron to produce a particle with one positive charge, which are then attracted towards negatively-charged plates and eventually strike a detector to record positions and abundances

• Also provides the mass ratio of an isotope to the ¹²C standard, giving us the isotopic mass

Atomic mass

The weighted average of naturally occuring isotopes according to their abundances; these can sometimes change overtime with ranges provided