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C1.5-C1.8 (Electronic Structures (Electrons are arranged around the…
C1.5-C1.8
Electronic Structures
Electrons are arranged around the nucleus in shells. Each shell represents a different energy level.
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The first and lowest energy level (shell) can hold up to two electrons. The second energy level can hold up to eight electrons.
After eight electrons fill the second and third shell, the fourth shell starts to fill.
The electronic structure shows the number of electrons in each energy level. You start recording the numbers of electrons at the lowest energy level. For example, sodium has 11 electrons, so sodium's electronic structure is 2,8,1.
Elements in the same group of the periodic table have the same number of electrons in their highest energy level. These electrons are called the outer electrons, because they are in the outermost shell. For example, all the group 1 elements have one electron in their highest energy level (outer shell).
Because all the elements in the same group have the same number of electrons in their outer shell, they all react chemically the same way.
The elements in group 0 of the periodic table are called noble gases. They are very unreactive elements because their electrons have a very stable arrangement. Noble gases have eight electrons in their outer shell, except for helium, which has two in its first shell.
History Of The Atom
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In the early 1800s, Dalton linked his ideas to strong experimental evidence. Dalton suggested atoms were tiny, hard spheres. These atoms could not be divided or split.
At the end of the 1800s, Thomson discovered a tiny negatively charged particle called an electron. Thomson proposed the 'plum pudding' model for the atom. The model suggested that negative electrons were embedded in a ball of positive charge. He imagined the electrons as the bits of plum in a plum pudding.
Ten years later, Geiger and Marsden were experimenting with alpha particles (dense positively charged particles). They fired alpha particles at a very thin piece of gold foil. A few alpha particles were repelled showing that there must be a tiny spot of positive charge in the centre of the atom. Rutherford then proposed the nuclear model.
In the nuclear model, electrons orbit around a nucleus (centre of the atom). The nucleus contains positively charged protons.
Bohr then revised the nuclear model. He suggested that the electrons were orbiting the nucleus in energy levels (or shells). The electrons were a set distance from the nucleus. Bohr's theoretical calculations matched the experimental observations.
In 1932, Chadwick provided the experimental evidence that showed the existence of uncharged particles, called neutrons in the nucleus.
Ions, Atoms, and Isotopes
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If an atom gains electrons, it becomes a negative ion (-). The ion has an overall negative charge because it has more electrons than protons. For example, an oxygen atom gains two electrons to form a negative ion. The formula is written as O2-.
If an atom loses electrons, it becomes a positive ion (+), because it has more protons than electrons. For example, a lithium atom loses an electron to form a positive lithium ion, Li+.
You cannot see atoms because each individual atom is incredibly small. An atom is about a tenth of a billionth of a meter across (0.0000000001m, written in standard form as 1 x 10 m).
Atoms of the same element always have the same number of protons, however they can have different numbers of neutrons.
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Isotopes of an element have different physical properties, but always have the same chemical properties.
Structure Of The Atom
The nucleus at the centre of the atom contains two types of sub-atomic particles, called protons and neutrons. Protons have a positive charge and neutrons have no charge.
Electrons are tiny negatively charged particles that move around the nucleus. An atom has no overall charge. That is because the number of protons is equal to the number of electrons and their relative charges are equal and opposite (proton +1 and electron -1).
All atoms of an element contain the same number of protons. This number is called the atomic number (or proton number) of the element. Elements in the periodic table are arranged in order of their atomic number. The atomic number is also the number of electrons in atn atom of the element.
The mass number is the total number of particles in the nucleus of an atom, so it is the number of protons plus the number of neutrons.