UPCAT SCI - CHEM ATOMS

Atom

The basic unit of an element that can enter into a chemical combination.

A = Mass Number

Z = Atomic number

Q = Charge or difference in number of protons and neutrons

What is the notation of each symbol here?

John Dalton’s 4 Hypotheses

Atoms of same element identical in mass/size; atoms of different elements differ

Atoms cannot be created, divided, or destroyed

Compounds form when atoms of different elements combine in fixed whole-number ratios Chemical reactions rearrange atoms; atoms themselves are unchanged

Law of Conservation of Mass (Dalton)

States that atoms are neither created nor destroyed in a chemical reaction

Subatomic Particles

The three basic components of an atom: protons (positive), electrons (negative), and neutrons (neutral).

John Dalton’s atomic model

A positive solid ball

Electron

The first subatomic particle discovered

Plum Pudding Model (Thomson)

An early atomic model proposing that an atom is a uniform, positive sphere with negatively-charged electrons embedded like raisins.

Cathode Ray Experiment

The experiment by J.J. Thomson that led to the discovery of the electron and its negative charge-to-mass ratio.

Oil Drop Experiment

The experiment by Robert Millikan that calculated the exact charge and mass of a single electron.

Gold Foil Experiment

The experiment by Ernest Rutherford that proved atoms are mostly empty space with a tiny, dense, positive core called the nucleus.

Nucleus

The extremely dense, positively-charged core at the center of an atom containing protons and neutrons.

Neutron

An electrically-neutral subatomic particle discovered by James Chadwick to account for unexplained atomic mass.

Atomic Number (Z)

The number of protons in an atom's nucleus, which identifies the element and dictates its position on the periodic table.

Mass Number (A)

The total number of protons plus neutrons in an atom's nucleus (not explicitly written on the periodic table).

Atomic Mass

The mass of an atom, mostly attributed to its protons and neutrons since electron mass is negligible.

Planetary Model (Bohr)

An atomic model proposing that electrons revolve around a positive nucleus in fixed, predetermined circular orbits.

Quantum Mechanical Model

The modern atomic model (Schrödinger) stating that electrons are found in 3D probability spaces called orbitals, not fixed orbits.

Orbital

A three-dimensional space around the nucleus where there is a high probability of finding an electron.

Quantum Numbers

A set of four numbers that act as an "address" to completely describe the energy, shape, orientation, and spin of an electron.

Principal Quantum Number (n)

Indicates the main energy level (shell) and size of the orbital

Azimuthal Quantum Number (l)

Describes the specified subshell of an atom

Magnetic Quantum Number (ml)

Dictates the orientation of an orbital in three-dimensional space

Spin Quantum Number (ms)

Describes the direction of the electron's spin

Orbital Electron Capacities

Maximum number of electrons each orbital type can hold: s holds 2, p holds 6, d holds 10, f holds 14.

Aufbau Principle

States that electrons must fill lower-energy orbitals first before moving to higher-energy ones (governed by the n+l rule).

Hund's Rule of Multiplicity

States that every orbital in a subshell must be singly occupied with an electron of the same spin before any orbital is doubly occupied.

Pauli's Exclusion Principle

States that no two electrons in the same atom can have the exact same set of four quantum numbers

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Mnemonic for electron configuration

Electron Configuration

The specific arrangement or distribution of an atom's electrons across its available energy levels and subshells.