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Accretion Disk

A disk of gas surrounding a star. The star may be a white dwarf, a neutron star, or a stellar black hole in a close stellar binary system. In the binary, the material of the accretion disk comes from the nearby companion star.Accretion disks may also surround newborn stars or the massive black holes believed to lie in the centers of many galaxies.



Atoms

An atom is the smallest quantity of an element that still retains the chemical properties of the element.



Atomic Structure

Atoms have a small, dense nucleus of protons and neutrons held together by the strong force. The nucleus has a net positive charge. It is surrounded by a negatively charged cloud of electrons. The nucleus and its electrons are held together by the electromagnetic force.

The identity of an atom is defined by the number of protons in the nucleus. All iron atoms, for example, have 26 protons. For neutral atoms, the number of protons always equals the number of electrons.

Atoms of the same element can have different masses because they have different numbers of neutrons. Atoms of a single element that differ by neutron number are called isotopes. Isotopes have identical chemical properties, but vary in their physical properties. Hydrogen has two stable isotopes. The more common isotope has 1 proton, 0 neutrons and 1 electron. Heavy hydrogen, also called deuterium, has 1 proton, 1 neutron, and 1 electron.

The atomic number of an element is defined as the number of protons in the nucleus. The atomic mass is the number of (protons + neutrons) in the nucleus.



SUMMARY OF ATOMIC STRUCTURE

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Particle
Location
Charge
Mass
Notes
Proton
nucleus
+1
1.67 X 10-27 kg

( 1 AMU )
Determines the identity of the element. Changing the number of protons changes the identity of the element.
Neutron
nucleus
0
1 AMU
Determines the identity of the isotope of the element. Changing the number of neutrons changes the mass of the element, but not the identity. Also responsible for determining the stability of a nucleus.
Electron
orbiting
nucleus
-1
9.1 X 10-31kg

1/1836 AMU

so small that is often assigned a mass of
0 AMU
Determines the reactivity of an element. Electrons are gained,lost or shared when atoms bond together. Changing the number of electrons changes the charge of the element, and creates an ion.

ATOMIC NUMBER = the number of protons in the nucleus

ATOMIC MASS = the number of (protons + neutrons) in the nucleus

ATOMIC MASS UNIT = 1/12 the mass of an atom of carbon-12 (carbon with 6 protons and 6 neutrons)








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