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Atoms are the basic building blocks of matter and the smallest unit into which matter can be divided without the release of electrically charged particles. They are composed of three fundamental particles: protons, electrons, and neutrons. The protons and neutrons are packed into the atom's centre, known as the nucleus, while the much smaller electrons whizz around the outside. Atoms are extremely small, typically around 100 picometres across, and cannot be seen with conventional microscopes. They join together with other atoms to create matter, and it takes many atoms to create anything.

What are the basic particles that make up an atom?

Atoms are made up of extremely tiny particles called protons, neutrons, and electrons. Protons and neutrons are packed tightly together in the center of the atom, forming the nucleus, while electrons surround the nucleus in an electron cloud. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. The positive and negative charges of protons and electrons are of equal magnitude but opposite polarity, and their mutual attraction is what holds the atom together.

Protons and neutrons, collectively known as nucleons, are comparable in size, with dimensions on the order of 2.5 x 10^-15 m, and they contribute to the majority of an atom's mass. Neutrons are slightly heavier than protons, but their mass can be reduced by nuclear binding energy. In contrast, electrons are nearly four orders of magnitude less massive than protons and neutrons, with a mass of 9.11 x 10^-31 kg.

The number of protons in an atom is called its atomic number, and atoms with the same atomic number but differing numbers of neutrons are known as isotopes. The total number of protons and neutrons in an atom is referred to as the mass number. Atoms with the same number of protons, or atomic number, belong to the same chemical element.

How do atoms form molecules?

Atoms are the basic building blocks of the universe, combining in countless ways to form all matter. Atoms form chemical bonds with other atoms when there is an electrostatic attraction between them. This attraction is caused by the properties and characteristics of the atoms' outermost electrons, known as valence electrons. When two or more atoms chemically bond, they form a molecule.

Atoms of different elements have a unique number of electrons, protons, and neutrons. The protons and neutrons make up the central core of the atom, while the electrons circle the core in defined orbitals called energy levels. Electrons group themselves in pairs in their energy levels, with the number of electrons at each level growing as the number of the energy level increases. Very few atoms have the number of electrons they need to be stable, so they bond with other atoms to form molecules and get their full complement of electrons.

An atom with unpaired electrons in the outermost energy level will attract other atoms with unpaired electrons. When the valence electrons from two or more atoms form pairs, the atoms share their valence electrons and bond together, forming a molecule. For example, in a simplified model of a water molecule, two atoms of hydrogen share their valence electrons with an atom of oxygen. The three atoms bond together, forming a stable molecule. Each atom of hydrogen has two electrons, and the oxygen atom has eight.

The three primary types of chemical bonds are covalent, metallic, and ionic. Covalent bonds form between nonmetallic materials, with electrons being shared between atoms. Metallic bonds occur between metallic substances, with valence atoms moving freely and forming bonds easily. Ionic bonds form between a metal and a nonmetal substance, with electrons being transferred from the metal to the nonmetal.

What is the size of an atom?

The size of an atom is estimated to be about 10-10 meters or 10-8 centimeters. This is a very small size—under 1 nanometer, or one-billionth of a meter. To put this into perspective, trillions of atoms could fit inside the period at the end of this sentence.

The atomic radius is a measure of the size of an atom, usually defined as the distance from the center of the nucleus to the outermost electron. However, as electrons do not have definite orbits, the boundary of an atom is not a well-defined physical entity, and there are various non-equivalent definitions of atomic radius. These include the Van der Waals radius, ionic radius, metallic radius, and covalent radius.

The size of an isolated atom cannot be measured because the location of the electrons surrounding the nucleus cannot be determined. However, we can estimate the size of an atom by assuming that the radius of an atom is half the distance between adjacent atoms in a solid. This technique is best suited to elements that are metals, and the results are often referred to as metallic radii. The size of an atom can also be estimated by measuring the distance between adjacent atoms in a covalent compound, which is known as the covalent radius.

The atomic radius varies depending on the element, and there are several trends to be observed in the periodic table. Firstly, as we move down a column of the periodic table, the atomic radius becomes larger because the valence electrons are placed in larger orbitals. Secondly, as we move from left to right across a row of the periodic table, the atomic radius becomes smaller due to the increasing number of protons in the nucleus, which hold electrons more tightly.

How do atoms become ionized?

Atoms are electrically neutral, meaning they have the same number of negatively charged electrons and positively charged protons. However, atoms can become ionized, which means they gain or lose electrons and, in turn, acquire a net electrical charge. This process is called ionization, and the resulting electrically charged atom is called an ion.

Ionization can occur in several ways. One way is through collisions with subatomic particles, other atoms, molecules, electrons, positrons, protons, antiprotons, or other ions. Another way is through interactions with electromagnetic radiation, such as UV-radiation or X-rays, which can strip electrons from atoms through photoionization. In an astronomical context, for example, the intense UV-radiation from a white dwarf in a planetary nebula can ionize the surrounding hydrogen and helium gas, resulting in the formation of HII and HeIII, respectively.

Additionally, ionization can occur through radioactive decay, where an excited nucleus transfers its energy to an inner-shell electron, causing it to be ejected. This process is known as internal conversion. Another example is electron capture ionization, where a free electron collides with an atom and becomes trapped inside its electric potential barrier, resulting in a negatively charged ion.

Ionization can also be achieved by transferring energy to a bound electron through collisions with charged particles or photons. This process is associated with a specific threshold amount of energy required, known as the ionization potential.

The Townsend discharge is an example of a cascade reaction that leads to the creation of positive ions and free electrons due to ion impact. In this process, a positive ion drifts towards the cathode, while a free electron moves towards the anode in a device with a sufficiently high electric field. If the electric field is strong enough, the free electron gains enough energy to liberate another electron when it collides with another molecule. This initiates a chain reaction of electron generation, known as the avalanche.

The study of ionization is essential in various fields, including fundamental science, medical treatment, and air purification. It also plays a crucial role in understanding the chemical makeup of celestial bodies and the most abundant form of matter in the universe, which is believed to be the plasma state, consisting of positively charged ions and negatively charged electrons.

What are the different types of radioactive decay?

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. There are several types of radioactive decay, including:

• Alpha decay: This is a common mode of radioactive decay where a nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result is a new element with a lower atomic number. Alpha particles have a high ionizing power but low penetration power.
• Beta decay: This is another common mode of radioactive decay where a nucleus emits beta particles, which are high-energy electrons. Beta decay can increase or decrease the atomic number of the nucleus by one. Beta particles have less ionizing power but greater penetration power than alpha particles.
• Gamma decay: This type of decay results from a change in the energy level of the nucleus, leading to the emission of electromagnetic radiation. Gamma decay usually follows alpha or beta decay. Gamma rays have the least ionizing power but the greatest penetration power.

Other less common types of radioactive decay include:

• Spontaneous fission: This occurs in very heavy elements with an atomic mass number greater than 92. It results in the element splitting into two lighter nuclei and releasing neutrons.
• Neutron emission: This occurs in nuclei that are neutron-rich and proton-poor, where one or more neutrons are ejected from the nucleus. The atom becomes a different isotope of the same element.
• Positron emission: Positrons are the anti-matter equivalent of electrons and are emitted from unstable isotopes with too few neutrons. Positron emission results in the transformation of a proton into a neutron.
• Electron capture: This occurs when there are too many protons in the nucleus, and there isn't enough energy to emit a positron. An orbital electron is captured by a proton, creating a neutron and a neutrino. This changes the number of protons, transforming the nuclide into a new element.

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