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Atom: Containership - Equilibrium - Peter O. Sagay.

An element is a pure substance. Substances are the different kinds of matter. A pure substance is made up of the same material(it is the same througout). An atom is the smallest part of an element that has all the characteristics of the element. Particle physicists have discovered paticles of matter smaller than the atom and they have grouped all subatomic particles into two groups: Hadrons which consists of baryons and Messons both of which consists of quarks; and Leptons. However, the atom is the smallest building block on Earth.

Natured packaged the atom using the following seven universal concepts: containership, identity, force, motion, change, grouping/interaction and equilibrium.

Containership

The Atom

The atom consists of a core called the nucleus and a configuration of orbits that surround the nucleus (figure 7.13). These orbits are called shells. The shells contain subshells that contain orbitals. The nucleus contains the neutrally-charged particles called the neutrons and the positively-charged particle called the protons. The neutron and protons are considered to be hadrons. The existence of the nucleus was discovered in 1909 by Earnest Rutherford (1891 AD - 1937).

The shells are numbered from 1 to 7 (or K -Q) from the nucleus outwards and they represent the energy -levels (also called the principal quantum numbers) of the shell. The subshells are assigned the letters s, p, d, f. Shell 1 nearest the nucleus contains a subshell named 1s. Shell 2 contains a pair of subshells named 2s and 2p. Shell 3 contains 3 subshells named 3s, 3p and 3d. Shell 4 contains 4 subshells named 4s, 4p, 4d and 4f. Subsequent shells are similarly configured. However, as the shells move outwards from the nucleus, their respective subshells begin to overlap with one another such that a 4s subshell may be occupied before a 3d subshell.

The orbitals hold negatively charged particles called electrons (considered to be a lepton). The number of electrons in a neutral atom equals the number of protons. The electron was discovered in 1897 by Joseph John (J.J.) Thomson (1856AD - 1940). J.J. Thomson is also credited with the discovery of the proton.

The electron capacity of an orbital increases the further away it is from the nucleus. The s subshell contains just 1 s orbital which can hold 2 electrons; the p subshell contains 3 p orbitals (each orbital can hold 2 electrons) which can hold a total of 6 electrons; the d subshell contains 5 d orbitals (each orbital can hold 2 electrons) which can hold a total of 10 electrons and the f subshell contains 7 f orbitals (each orbital can hold 2 electrons) which can hold a total of 14 electrons.

In general, a shell with principal quantum number n has n subshells. The nth subshell has 2n-1 orbitals which can hold a total of 2(2n-1) electrons. The total number of electrons in the shell is the sum of all the electrons in all the orbitals of the shell's subshells. The total electrons in an orbital type can be expressed as: nxy where n is the number of the shell (energy-level or principal quantum number) containing the orbital type x, and y is the number of electrons contained in the orbital type x (s, p,d, f, are orbital types).

The distribution of an atom's electrons in their respective orbitals is called the electron configuration of the atom. The electron configuration of an atom is usually expressed in an Electron Configuration Table . The following is the Electron Configuration Table for gold:

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s1

Identity

Atomic Protons

An atom is identified by its atomic number which represents the number of protons in its nucleus (figure 7.14). This number is unique for every atom. The atomic mass of an atom is an additional component of its identity. It differentiates the isotopes of an element. The isotopes of an element have the same number of protons but different number of neutrons therefore they have different atomic masses. In general, the identity of an element is represented as follows:

The Atom

where A is its atomic mass, S is the element's symbol and Z the atomic number.

Force

The Atom

There are four active forces in an atom (figure 7.16): (a) The strong force: opposes the repulsion force between protons and keeps protons together. It makes the nucleus possible and without it there would be no atoms. Its sphere of influence, that is, its force field has a limited range. It is effective only when protons are very close together. (b) The electromagnetic force: functions either as a force of attraction or as a force of repulsion. If particles of the same charge are in its force field, it acts as a force of repulsion between them. It acts as a force of attraction if the particles have opposite charges. Electrons are kept in orbit around the nucleus by the electromagnetic force. The negatively charged electrons are attracted to the positively charged nucleus. (c) The weak force: is responsible for radioactive decay, a process whereby a neutron in the nucleus changes into a proton and an electron. The weak force is very present in the sun. It plays a fundamental role in the nuclear processes responsible for the power of the sun. (d) Gravity: has a weak presence inside the nucleus of an atom and its role in the nucleus is yet to be fully understood. However, it is a force of attraction exerted between all objects in nature. It is more evident in the attraction between large objects.

Motion

There are four types of motion in an atom: (a) The motion of electrons around the nucleus. The exact positions of the electron during this motion is nondeterministic. In other words, only the probability of an electron being at a position can be determined. The determination of this probability requires a general knowledge of quantum mechanics and the following concepts in particular: Heisenberg uncertainty principle (formulated by Werner Heisenberg); Erwin Schrodinger's wave equation (formulated by Erwin Schrodinger); and Pauli exclusion principle (formulated by Wolfgang Pauli). These concepts are too terse for this TECnote. (b) The rotation of an electron about its axis. This motion is called the spin of the electron. It is given the value 1/2 (the spin quantum number) for every electron. The spin is +1/2 if the electron rotates in the direction of the magnetic field that contains the it. The spin is -1/2 if the electron rotates in the direction opposite to the direction of the magnetic field that contains it. This orientation in the magnetic field is called the spin quantum number. (c) The motion of electrons from one energy-level to another energy-level. When an electron in a low energy level is sufficiently energized by external energy source, it moves to a higher energy level. The electron is said to have been excited. The excitation energy is eventually released (emission) and the electron returns to its prior energy-level. (d) The dislodgement of an electron from the atom. The electron moves out of the atom.

Change

(a) Change in the nucleus of an atom. This change is caused by radioactivity, a process in which atomic nuclei emit rays (particles) in other to become more stable. There are three types of rays that atomic nuclei emit: (i) Alpha rays (α): an alpha particle consists of 2 protons and 2 neutrons and is often represented as a He++ (a helium nucleus, the atom without its two electrons). The radioactive process that emits an alpha ray is called an alpha decay. Alpha decay changes the identity of an atom by reducing its atomic number (the number of its protons). The reduction in the number of neutrons also add to the identity change. Both atomic mass and atomic number of an atom change after an alpha decay. (ii) Beta rays (β): a beta particle is an electron formed inside the nucleus of an atom. An electron is formed inside the nucleus when a neutron decomposes into a proton and an electron. The radioactive process that emits a beta ray is called a beta decay. Beta decay changes the identity of an atom by increasing its atomic number without changing the atomic mass of the atom. (iii) Gamma rays (γ): a gamma ray is one of the electromagnetic waves in the electromagnetic spectrum (visible light, radio waves and infra-red waves are some members of the spectrum). A gamma ray has high energy, extremely high frequency and short wave length. It is also the most penetrating radiation emitted by radioactive elements. The radioactive process that emits a gamma ray is called a gamma decay. Gamma decay often accompanies Alpha and Beta decay. However, its emission does not change a nucleus into a different nucleus. The change that occurs due to gamma decay is a change in the energy-state of the nucleus. The high energy emitted by a gamma decay causes the nucleus to transit to a lower energy state.

A radioactive element consists of numerous radioactive nuclei. These nuclei decay gradually at a fixed rate over a period of time. The time it takes for half of the atoms in a given sample of an element to decay is called the half-life of the element. The variation in the half-lives of elements span a very large range. Some elements' half-lives span just a few seconds while some elements' half-lives span billion of years. For example, the element rhodium-106 has a half-life of 30 seconds while the element uranium-238 has a half-life of 4.5 billion years.

The spontaneous and continuous decay of a radioactive element form new elements, until a stable nonradioactive nucleus is formed. The series of steps that lead to the formation of the stable nonradioactive nucleus is called a decay series. The concept of decay series explains why some elements continue to be present in nature even though their half-lives indicate that they should have been extinct. For example, the element radium has a half-life of 1600 years, so it is not supposed to be present on Earth today. Its presence is due to the decay series of uranium-238 which has a half-life of 4.5 billion years.

(b) Change in eletron occupancy. A neutral atom has as many electrons as there are protons. The atom changes when it donates or accept electrons. When it donates electron it becomes positively charged. When it accepts electrons it becomes negatively charged.

Grouping/Interaction

The periodic table presents the fundamental groupings of atoms. The periodic table was first established by Dmitri Mendeleeve and later refined by Henry Moseley. Mendeleeve grouped the 63 elements that had been discovered by the mid-1800s according to atomic mass. Moseley determined fifty years later that the atomic number presents a more accurate grouping. The periodic law which states that the physical and chemical properties of the elements are periodic functions of their atomic number forms the basis of the periodic table.

The periodic table without the expansion of the rare-earth elements (Lanthanides and Actinides) consists of 18 columns (groups) and 7 rows (period). Elements within the same group have similar but not identical properties. Elements within a period have neither similar nor identical properties. However, there is a pattern to the properties of the elements within a period. The leftmost element within a period is always an active solid. The rightmost element is always an inactive gas. The properties of elements within these two boundary positions of a period follows a transitional pattern. Elements of the periodic table can broadly be grouped into metals, non-metals (liquids and gases) and metalloids which have properties of both metals and nonmetals.

The Atom

Atoms interact primarily to bond together. Their valence electrons (electrons in their outermost shells) bring about the bonding. There are three basic types of atomic bonds:

(a)Ionic Bond (figure 7.16). Electrons are transfered from one atom to another. One atom donates electrons, the other accepts the donated electrons. The donor atom becomes positively charged (cation) because the atom has more protons than electrons after the transfer. The acceptor atom becomes negatively charged (anion) because the atom has more electrons than protons. For example, sodium (Na) has one valence electron and fluorine (Cl) has one valence electron. Sodium donates its valence electron to chlorine in a sodium-chlorine interaction in order to attain a more stable outermost shell and it becomes positively charged. Chlorine accepts the donated electron in order to attain a more stable outermost shell and it becomes negatively charged. The resulting electromagnetic force bonds the sodium and chlorine atoms.

The removal of electrons from atoms to form ions is called ionization. The energy required for ionization is called ionization energy. Acceptor atoms like fluorine have electron affinity. The regular reapeating arrangement of ions of ionic compounds is called a crystal lattice. Electron Dot Diagram

(b) Covalent Bond. Atoms share their valence electrons to attain a more stable outermost shell. The shared electrons spend most of their time between the atoms because of the simultaneous electromagnetic force between the nuclei and the shared electrons. A covalent bond is illustrated by an electron-dot diagram (figure 7.17). The electron-dot diagram represents the nucleus and all shells before the valence shell, by the symbol of the atom and represents the valence electrons are represented as dots surrounding the symbol. For example, figure 7.14 shows the electron-dot diagram for the covalent bonds between hydrogen atoms, chlorine atoms and hydrogen and chlorine atoms.

(c) Metallic Bond. This is a bond solely between metals. Metals are generally donors of electrons so in a metallic bond there is no atom with electron affinity to readily accept the donated electrons. Consequently, there is a resultant free-moving sea of electrons in a common electron cloud formed by the outer electrons of the atoms that are all simultaneously attracted by the positive nuclei of the atoms.

Equilibrium

Every atom desire equilibrium. equilibrium is attained when an atom's valence shell contains the maximum number of electrons it desires. The last group of the periodic table, the noble gases (also called the inert gases), presents the template for what should be the maximum number of valence electrons in the valence shell of a chemically stable atom. The noble gases are: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). The noble gases are considered chemically stable and do not react with other elements unless under special conditions. Helium has 2 valence electrons; neon, argon, krypton, xenon and radon each has 8 valence electrons. So, an atom with 2 or 8 valence electrons is chemically stable. Donor atoms like sodium tend to seek the 2 valence electrons equilibrium while acceptor atoms like fluorine tend to seek the 8 valence electrons equilibrium.