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Measurement: Standard Units

Measurement: Standard Units

Every measurement provides three kinds of information: the size or magnitude of the measurement (a number), a standard of comparison for the measurement (a unit), and an indication of the uncertainty of the measurement. While the number and unit are explicitly represented when a quantity is written, the uncertainty is an aspect of the errors in the measurement results.

Subatomic Particles

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller subatomic particles: protons, neutrons, and electrons. These three particles account for the mass and the charge of an atom.

Atomic Mass

Atomic Mass

Atoms — and the protons, neutrons, and electrons that compose them — are extremely small. For example, a carbon atom weighs less than 2 × 10−23 g. When describing the properties of tiny objects such as atoms, we use appropriately small units of measure, such as the atomic mass unit (amu). The amu was originally defined based on hydrogen, the lightest element, then later in terms of oxygen. Since 1961, it has been defined with regard to the most abundant isotope of carbon, atoms of which are...

Atomic Radii and Effective Nuclear Charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number and distribution of electrons in their valence shells.

Atomic Nuclei: Nuclear Magnetic Moment

Atomic Nuclei: Nuclear Magnetic Moment

All atomic nuclei are positively charged. When they have a nonzero spin, they behave like rotating charges. As a consequence of their charge and spin, these nuclei generate a magnetic field (B). This, in turn, gives rise to a magnetic moment (μ), which is randomly oriented in the absence of an external magnetic field. When an external magnetic field (B0) is applied, the magnetic moment vectors can align with the field or against it in 2 + 1 orientations. A hydrogen nucleus, which is just a...

Atomic Weight

Atomic Weight

Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists arbitrarily define this amount of mass as one atomic mass unit (amu) or one Dalton. Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. As a result, they do not contribute much to an element's overall atomic mass. This means that, when considering atomic mass, it is customary to ignore the mass of any electrons and calculate the...

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Related Experiment Video

Updated: Jul 6, 2026

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Published on: February 14, 2014

Atomic hydrogen on Mars: measurements at solar minimum.

J S Levine, D S McDougal, D E Anderson

Science (New York, N.Y.)

|June 2, 1978

Summary

New data from the Copernicus Observatory reveals significantly higher atomic hydrogen in Mars's upper atmosphere during solar minimum. This finding impacts our understanding of planetary atmospheric escape processes.

Area of Science:

Planetary Science
Atmospheric Science
Astrophysics

Background:

Understanding the composition and dynamics of planetary upper atmospheres is crucial for atmospheric escape studies.
Atomic hydrogen plays a key role in atmospheric escape mechanisms on Mars.

Purpose of the Study:

To determine atomic hydrogen concentrations in the Martian upper atmosphere during solar minimum.
To compare these concentrations with previous measurements taken during solar maximum.

Main Methods:

Utilized the Copernicus Orbiting Astronomical Observatory to measure Mars's Lyman-alpha emission.
Integrated Copernicus data with in situ temperature measurements from the Viking mission.

Main Results:

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Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

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Measurement of Aerosols Optical Thickness of the Atmosphere using the GLOBE Handheld Sun Photometer

Measurement of Aerosols Optical Thickness of the Atmosphere using the GLOBE Handheld Sun Photometer

Published on: May 29, 2019

Related Experiment Videos

Last Updated: Jul 6, 2026

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Published on: February 14, 2014

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Published on: March 29, 2016

Measurement of Aerosols Optical Thickness of the Atmosphere using the GLOBE Handheld Sun Photometer

Measurement of Aerosols Optical Thickness of the Atmosphere using the GLOBE Handheld Sun Photometer

Published on: May 29, 2019

Obtained the first measurements of atomic hydrogen concentrations in Mars's upper atmosphere at solar minimum.

Found atomic hydrogen number density at the Martian exobase is approximately 60 times greater than previously estimated from solar maximum data.

Copernicus results align with Hunten's diffusion-limited hydrogen escape hypothesis.

Conclusions:

Solar activity significantly influences atomic hydrogen abundance in the Martian upper atmosphere.
The findings support the theory of diffusion-limited escape for atomic hydrogen from Mars.