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Electron Configuration
An atom is electrically neutral because it has an equal number of negatively charged electrons and positively charged protons. An atom of hydrogen has one proton and one electron, an atom of helium has two protons and two electrons, and so on. As the number of protons increases, the number of electrons increases also.
Scientists have long known that the electrons in an atom are the particles that interact with other atoms. What was hard to understand, however, was why only a few electrons in an atom were chemically reactive. Bohr’s model of the atom helped explain this.
While Bohr’s model was found to be wrong about electrons moving in fixed orbits, his model was correct in assigning a specific amount of energy to each electron in an atom. Today, scientists talk about an electron occupying a specific energy level. An energy level or electron shell is a state of energy that an electron has; it is not a location. As you move from one energy level to the next away from the nucleus, the energy of an electron in that level increases. Thus, the lowest energy level is the one closest to the nucleus. It is also important to know that the energy levels are quantized, or fixed. This means that if an electron loses or gains energy it must go all the way to another energy level; it cannot loiter around in between. The numbers in the diagram below show the maximum number of electrons that can fit in each energy level.
The number of electrons in an energy level is 2n2, where n is the number of the energy level, also known as shell number. That is, energy level 1 contains 2 electrons, level 2 contains 8 electrons (2 × 22), etc. Each energy level consists of one or more orbitals. An orbital, also known as sub-shell, indicates the probable spatial distribution of the electrons around the nucleus. In other words, an orbital gives the probability (around 90%) of finding the electron at a certain region of space. There are different types of orbitals, each having different energies, shapes, and sizes. They are the s, p, d, and f orbitals.
The first shell: n = 1, has n2= 1 orbital: one s orbital only. (1s)
The second shell: n = 2, has n2= 4 orbitals: one s and three p orbitals. (2s 2p)
The third shell: n = 3, has n2 = 9 orbitals: one s orbital, three p orbitals and five d orbitals. (3s 3p 3d)
The fourth shell: n = 4, has n2 = 16 orbitals: one s orbital, three p orbitals, five d orbitals and seven f orbitals. (4s 4p 4d 4f)
Electrons in atoms tend to have the lowest possible energy arrangement. Thus, they tend to occupy the lowest energy orbital first. Note that an s orbital of an energy level is always lower in energy than the s orbital of the next higher energy level. The same goes for the p, d, and f orbitals. Furthermore, p orbitals are always of a higher energy than s orbitals but lower than d orbitals in the same shell. Electrons fill the orbitals around the nucleus in the following order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p. The pattern of electrons filling these energy levels in an atom is called the electron configuration of that atom.
The electron configuration of any element can be determined from its atomic number, which equals the number of protons or electrons in an atom of that element. Carbon, with an atomic number of 6, has 6 protons and 6 electrons. The first 2 electrons go into the first energy level, closest to the nucleus as shown in Figure 9, and the next 4 electrons go into the next highest energy level, which has a maximum capacity of 8 electrons. The electron configuration of a carbon atom can thus be described as consisting of 2 electrons in the first energy level and 4 in the second energy level.
The valence electrons are the electrons in the valence shell, which is the outermost energy level of an atom. Having a full valence shell confers to the atom a stable electron configuration. The noble gases helium, neon, argon, krypton, xenon, and radon are particularly stable; they do not react easily because they all have full valence shells. Atoms of other elements that do not have full valence shells tend to either gain, lose, or share electrons to achieve a full valence shell, which is also known as the noble gas configuration.
For example, a magnesium atom has 2 electrons in the first energy level, 8 electrons in the second energy level, and 2 electrons in the outermost energy level. These 2 electrons are magnesium’s valence electrons. Consequently, to have a full valence shell, a magnesium atom either needs to gain 6 more electrons to fill this energy level, or to lose the 2 electrons it has. Magnesium tends to lose 2 electrons to achieve a stable configuration of 2 electrons in the first energy level, and 8 electrons in the second energy level which will become the outermost energy level. This is the same configuration as the noble gas neon.
Fill in the blank.
The
of an atom is the pattern of electrons that fill the energy levels of this atom.
Fill in the blank.
The
are the electrons that occupies the outermost energy level.
Fill in the blank.
A full valence shell is known as the
.
Comparing Subatomic Particles
As we have seen, the three particles making up an atom are the electron, the proton, and the neutron. These particles are classified as subatomic particles; they are smaller than an atom and make up an atom’s structure. When comparing these particles, scientists focus on their masses and their relative charges.
Mass data for subatomic particles shown in Table 1 are expressed in both kilograms and atomic mass units (symbol, amu). One amu is equivalent to 1.66054 × 10−27kg. Atomic mass units are often used to express the masses of individual atoms or subatomic particles because their masses are so small. You can see from the data in Table 1 that protons and neutrons are very similar in mass and much heavier than electrons —an electron is almost 1/2,000 the mass of a proton.
Relative charge indicates how the electrical charge of each particle compares to the charge of the other particles: neutrons have no charge, protons have a full positive charge, and electrons have a full negative charge. Because opposite charges attract each other, electrons in an atom are attracted to the positively charged nucleus. Moreover, electrons are in constant motion around the nucleus. Both the force of attraction between the electrons and the nucleus, and the energy of motion of electrons, help stabilize the atom’s structure.
The table below summarizes some properties of the three particles found in an atom.
Based on the masses of subatomic particles listed in the table above, you would say that
Based on the masses of subatomic particles listed in the table above, you would say that
