Atomic structureAtomic Structure What is Atom?
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). 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 by chemical bonds based on the same force, forming a molecule.
Basic Structure of an Atom
The picture below is an example of the arrangement of the particles in an atom. Most of the atom is just empty space. The rest of the atom consists of a positively charged nucleus of protons and neutrons that are surrounded by a cloud of negatively charged electrons. The nucleus is the center of the atom. An atom is an extremely small particle of matter that retains its identity during chemical reactions.
In an electrically neutral atom, the number of electrons equals the positive charge on the nucleus. The nucleus of the atom is composed of smaller particles called neutrons and protons. A proton has a positive charge equal in magnitude to the negative charge of an electron. This means that in an electrically neutral atom, the positive of charge the protons, combined with the negative charge of the electrons, would result in no charge because they would cancel each other out. A proton's mass, however, is a whopping 1836 times that of the electron. A neutron, however, has a mass almost identical to a proton's, but it has no electrical charge associated with it.
Atomic Number and Mass Number
The atomic number of an element is what distinguishes it from all other elements. An atom's atomic number is the number of protons there are in the nucleus. Hydrogen's atomic number is 1. Helium's atomic number is 2. Any atom that has an atomic number of 1 is a hydrogen atom no matter how many electrons or neutrons the atom has.
The mass number is the number of neutrons added to the number of protons. The mass number of the most common isotope can be obtained from the periodic table. If you take the decimal number on the periodic table and round it to the nearest whole number, you have the mass number. For example the atomic weight of Iron(Fe) is 55.847. When rounded it gives a mass number of 56.
The atomic number of Fe is 26. so most Fe atoms have 30 (56-26) neutrons. In addition, all neutral Fe atoms have 26 protons and 26 electrons. Atoms of the same element with a different number of neutrons are called isotopes. The most common isotope of an element is the one that is on the periodic table.
Rutherford Model
Rutherford determines true nature of atom and the phenomenon of radioactivity (decay of unstable atomic nuclei). Unstable nuclei emit alpha particles (He nuclei) and beta particles (electrons).
The alpha particle was heavy and positively charged, we now know that it is the helium nuclei (2 protons and 2 neutrons). The beta particle was light and negatively charged, the electron. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. His experiment looked like the following:
The Rutherford beamed alpha particles through gold foil and detected them as flashes of light or scintillations on a screen the gold foil was only 0.00004 centimeter thick, meaning on a few hundreds of atoms thick if the Thomson model of atoms was correct, then the alpha particles should pass through with relatively little deflection.
ATOMIC STRUCTURE
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). 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 by chemical bonds based on the same force, forming a molecule.
Basic Structure of an Atom
The picture below is an example of the arrangement of the particles in an atom. Most of the atom is just empty space. The rest of the atom consists of a positively charged nucleus of protons and neutrons that are surrounded by a cloud of negatively charged electrons. The nucleus is the center of the atom. An atom is an extremely small particle of matter that retains its identity during chemical reactions.
In an electrically neutral atom, the number of electrons equals the positive charge on the nucleus. The nucleus of the atom is composed of smaller particles called neutrons and protons. A proton has a positive charge equal in magnitude to the negative charge of an electron. This means that in an electrically neutral atom, the positive of charge the protons, combined with the negative charge of the electrons, would result in no charge because they would cancel each other out. A proton's mass, however, is a whopping 1836 times that of the electron. A neutron, however, has a mass almost identical to a proton's, but it has no electrical charge associated with it.
Atomic Number and Mass Number
The atomic number of an element is what distinguishes it from all other elements. An atom's atomic number is the number of protons there are in the nucleus. Hydrogen's atomic number is 1. Helium's atomic number is 2. Any atom that has an atomic number of 1 is a hydrogen atom no matter how many electrons or neutrons the atom has.
The mass number is the number of neutrons added to the number of protons. The mass number of the most common isotope can be obtained from the periodic table. If you take the decimal number on the periodic table and round it to the nearest whole number, you have the mass number. For example the atomic weight of Iron(Fe) is 55.847. When rounded it gives a mass number of 56.
The atomic number of Fe is 26. so most Fe atoms have 30 (56-26) neutrons. In addition, all neutral Fe atoms have 26 protons and 26 electrons. Atoms of the same element with a different number of neutrons are called isotopes. The most common isotope of an element is the one that is on the periodic table.
Rutherford Model
Rutherford determines true nature of atom and the phenomenon of radioactivity (decay of unstable atomic nuclei). Unstable nuclei emit alpha particles (He nuclei) and beta particles (electrons).
The alpha particle was heavy and positively charged, we now know that it is the helium nuclei (2 protons and 2 neutrons). The beta particle was light and negatively charged, the electron. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. His experiment looked like the following:
The Rutherford beamed alpha particles through gold foil and detected them as flashes of light or scintillations on a screen the gold foil was only 0.00004 centimeter thick, meaning on a few hundreds of atoms thick if the Thomson model of atoms was correct, then the alpha particles should pass through with relatively little deflection.
Observations:
- most alpha particles were observed to pass straight through the gold foil
- a few were scattered at large angles
- some even bounced back toward the source
- only a positively charged and relatively heavy target particle, such as the proposed nucleus, could account for such strong repulsion
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