Classification and Composition of an Atom
– Smallest recognized division of a chemical element
– Consists of a compact nucleus of protons and neutrons
– Surrounded by an electromagnetically-bound swarm of electrons
– Different chemical elements are distinguished by the number of protons in their atoms
– Atoms with the same number of protons but different number of neutrons are called isotopes
– Atoms are extremely small, typically around 100 picometers across
– More than 99.94% of an atom’s mass is in the nucleus
– Protons have a positive electric charge, electrons have a negative charge, and neutrons have no charge
– If an atom has equal numbers of protons and electrons, it is electrically neutral
Forces within an Atom
– Electrons are attracted to the protons in the nucleus by the electromagnetic force
– Protons and neutrons in the nucleus are attracted to each other by the nuclear force
– Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, causing nuclear decay
– Atoms can attach to one or more other atoms by chemical bonds to form compounds
– Chemistry studies the changes that occur when atoms attach and detach from each other
History of Atomic Theory
– Ancient cultures had the idea that matter is made up of tiny indivisible particles
– John Dalton used the term ‘atom’ to refer to the fundamental units of matter in the early 19th century
– Dalton’s atoms were later discovered to be divisible, but the term stuck
– Dalton’s law of multiple proportions showed that elements combine in ratios of small whole numbers
– Dalton’s work laid the foundation for modern atomic theory
Discovery of Subatomic Particles
– J. J. Thomson discovered that cathode rays are particles, not electromagnetic waves
– Cathode rays can be deflected by electrical and magnetic fields
– The particles, later named electrons, were found to be 1,800 times lighter than hydrogen
– Electrons were identified as particles emitted by photoelectric and radioactive materials
– Electrons are responsible for carrying electric currents in metal wires
– The Geiger-Marsden experiment showed that a small portion of alpha particles were deflected by the concentrated positive charge of the nucleus
– Ernest Rutherford proposed that the positive charge of the atom is concentrated in a tiny nucleus at the center
– Rutherford and his colleagues bombarded thin foils of metal with alpha particles and observed deflections greater than 90°
– This led to the rejection of J. J. Thomson’s plum pudding model of the atom
– Frederick Soddy discovered that there are more than one type of atom at each position on the periodic table
– These atoms have the same properties but different atomic weights, known as isotopes
– J. J. Thomson developed a technique for isotope separation
– Stable isotopes were subsequently discovered
– The mass spectrometer allowed accurate measurement of atomic mass
– Francis William Aston showed that isotopes have different masses
– The atomic mass of isotopes can be determined by the ratio of mass to charge in a mass spectrometer
– The neutron, a subatomic particle with no charge, was discovered
Models of the Atom
– Niels Bohr proposed a model in which electrons orbit the nucleus in a finite set of orbits
– Electrons can only jump between these orbits in discrete changes of energy
– This model explained the stability of electron orbits and the discrete spectra of elements
– Werner Heisenberg formulated the first consistent mathematical formulation of quantum mechanics
– Louis de Broglie proposed that particles behave like waves
– Erwin Schrödinger developed the Schrödinger equation, describing electrons as three-dimensional waveforms
– The uncertainty principle, formulated by Heisenberg, states that precise values for both position and momentum cannot be obtained simultaneously
– The Schrödinger model explained observations of atomic behavior and replaced the planetary model
– Otto Hahn directed neutrons onto uranium atoms and discovered nuclear fission
– Lise Meitner and Otto Frisch verified Hahn’s results
– Hahn received the Nobel Prize in Chemistry for his work
– Particle accelerators and detectors in the 1950s allowed the study of high-energy impacts
– Neutrons and protons were found to be composed of smaller particles called quarks Source: https://en.wikipedia.org/wiki/Atom
The atom is the basic particle of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically-bound swarm of electrons. The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium, and any atom that contains 29 protons is copper. Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element.
| Atom | |
|---|---|
 An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case. The black bar is one angstrom (10−10 m or 100 pm). | |
| Classification | |
| Smallest recognized division of a chemical element | |
| Properties | |
| Mass range | 1.67×10−27 to 4.52×10−25 kg |
| Electric charge | zero (neutral), or ion charge |
| Diameter range | 62 pm (He) to 520 pm (Cs) (data page) |
| Components | Electrons and a compact nucleus of protons and neutrons |
Atoms are extremely small, typically around 100 picometers across. A human hair is about a million carbon atoms wide. This is smaller than the shortest wavelength of visible light, which means humans cannot see atoms with conventional microscopes. Atoms are so small that accurately predicting their behavior using classical physics is not possible due to quantum effects.
More than 99.94% of an atom's mass is in the nucleus. Protons have a positive electric charge and neutrons have no charge, so the nucleus is positively charged. The electrons are negatively charged, and this opposing charge is what binds them to the nucleus. If the numbers of protons and electrons are equal, as they normally are, then the atom is electrically neutral as a whole. If an atom has more electrons than protons, then it has an overall negative charge, and is called a negative ion (or anion). Conversely, if it has more protons than electrons, it has a positive charge, and is called a positive ion (or cation).
The electrons of an atom are attracted to the protons in an atomic nucleus by the electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by the nuclear force. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements. This is a form of nuclear decay.
Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. The ability of atoms to attach and detach from each other is responsible for most of the physical changes observed in nature. Chemistry is the science that studies these changes.
