The atom is a fundamental unit of matter, comprising a central nucleus of protons and neutrons, surrounded by negatively charged electrons bound by electromagnetic force. The Bohr model describes electrons orbiting the nucleus in defined paths, helping to introduce the concept of atomic structure, despite being considered an obsolete theory. Atomic orbitals represent the probabilistic locations of electrons and are defined by quantum numbers, with different categories such as s, p, d, and f orbitals indicating varying angular momentum.

1. The atom is a basic unit of matter that consists of a dense, central nucleus
surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of
positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1,
which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the
nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each
other, forming a molecule. An atom containing an equal number of protons and electrons is
electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is
classified according to the number of protons and neutrons in its nucleus: the number of protons
determines the chemical element, and the number of neutrons determines the isotope of the
element. Though the word atom originally denoted a particle that cannot be cut into smaller
particles, in modern scientific usage the atom is composed of various subatomic particles. The
constituent particles of an atom are the electron, the proton and the neutron. However, the
hydrogen-1 atom has no neutrons and a positive hydrogen ion has no electrons. In atomic
physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged
nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in
structure to the solar system, but with electrostatic forces providing attraction, rather than
gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model
(1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a
quantum physics-based modification of the Rutherford model, many sources combine the two,
referring to the Rutherford–Bohr model. The Bohr model is a primitive model of the hydrogen
atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using
the broader and much more accurate quantum mechanics, and thus may be considered to be an
obsolete scientific theory. However, because of its simplicity, and its correct results for selected
systems (see below for application), the Bohr model is still commonly taught to introduce
students to quantum mechanics, before moving on to the more accurate but more complex
valence shell atom. An atomic orbital is a mathematical function that describes the wave-like
behavior of either one electron or a pair of electrons in an atom. This function can be used to
calculate the probability of finding any electron of an atom in any specific region around the
atom's nucleus. These functions may serve as three-dimensional graphs of an electron’s likely
location. The term may thus refer directly to the physical region defined by the function where
the electron is likely to be. Specifically, atomic orbitals are the possible quantum states of an
individual electron in the collection of electrons around a single atom, as described by the orbital
function. The idea that electrons might revolve around a compact nucleus with definite angular
momentum was convincingly argued in 1913 by Niels Bohr, and the Japanese physicist Hantaro
Nagaoka published an orbit-based hypothesis for electronic behavior as early as 1904. However,
it was not until 1926 that the solution of the Schrödinger equation for electron-waves in atoms

2. provided the functions for the modern orbitals. Because of the difference from classical
mechanical orbits, the term "orbit" for electrons in atoms, has been replaced with the term
orbital—a term first coined by chemist Robert Mulliken in 1932. Atomic orbitals are typically
described as “hydrogen-like” (meaning one-electron) wave functions over space, categorized by
n, l, and m quantum numbers, which correspond to the electrons' energy, angular momentum,
and an angular momentum direction, respectively. Each orbital is defined by a different set of
quantum numbers and contains a maximum of two electrons. The simple names s orbital, p
orbital, d orbital and f orbital refer to orbitals with angular momentum quantum number l = 0, 1,
2 and 3 respectively.
Solution
The atom is a basic unit of matter that consists of a dense, central nucleus
surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of
positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1,
which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the
nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each
other, forming a molecule. An atom containing an equal number of protons and electrons is
electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is
classified according to the number of protons and neutrons in its nucleus: the number of protons
determines the chemical element, and the number of neutrons determines the isotope of the
element. Though the word atom originally denoted a particle that cannot be cut into smaller
particles, in modern scientific usage the atom is composed of various subatomic particles. The
constituent particles of an atom are the electron, the proton and the neutron. However, the
hydrogen-1 atom has no neutrons and a positive hydrogen ion has no electrons. In atomic
physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged
nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in
structure to the solar system, but with electrostatic forces providing attraction, rather than
gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model
(1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a
quantum physics-based modification of the Rutherford model, many sources combine the two,
referring to the Rutherford–Bohr model. The Bohr model is a primitive model of the hydrogen
atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using
the broader and much more accurate quantum mechanics, and thus may be considered to be an
obsolete scientific theory. However, because of its simplicity, and its correct results for selected
systems (see below for application), the Bohr model is still commonly taught to introduce
students to quantum mechanics, before moving on to the more accurate but more complex

3. valence shell atom. An atomic orbital is a mathematical function that describes the wave-like
behavior of either one electron or a pair of electrons in an atom. This function can be used to
calculate the probability of finding any electron of an atom in any specific region around the
atom's nucleus. These functions may serve as three-dimensional graphs of an electron’s likely
location. The term may thus refer directly to the physical region defined by the function where
the electron is likely to be. Specifically, atomic orbitals are the possible quantum states of an
individual electron in the collection of electrons around a single atom, as described by the orbital
function. The idea that electrons might revolve around a compact nucleus with definite angular
momentum was convincingly argued in 1913 by Niels Bohr, and the Japanese physicist Hantaro
Nagaoka published an orbit-based hypothesis for electronic behavior as early as 1904. However,
it was not until 1926 that the solution of the Schrödinger equation for electron-waves in atoms
provided the functions for the modern orbitals. Because of the difference from classical
mechanical orbits, the term "orbit" for electrons in atoms, has been replaced with the term
orbital—a term first coined by chemist Robert Mulliken in 1932. Atomic orbitals are typically
described as “hydrogen-like” (meaning one-electron) wave functions over space, categorized by
n, l, and m quantum numbers, which correspond to the electrons' energy, angular momentum,
and an angular momentum direction, respectively. Each orbital is defined by a different set of
quantum numbers and contains a maximum of two electrons. The simple names s orbital, p
orbital, d orbital and f orbital refer to orbitals with angular momentum quantum number l = 0, 1,
2 and 3 respectively.