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A question that is frequently asked is, “What is an atom?” Atoms are the smallest units of ordinary matter or chemical elements. Each solid, liquid, and gas contains a certain number of neutral atoms. Atoms can also be ionized and form plasma. These tiny particles are very small, usually a hundred picometers wide.

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## Structure

The structure of an atom is made up of two parts: the atomic nucleus (positively charged) and the electron cloud (negatively charged). The nucleus is the centre of the atom and is surrounded by positively charged protons and neutral neutrons. The protons and electrons attract one another and hold the atom together. Most atoms contain all three subatomic particles, although hydrogen, for example, only contains one proton and one electron.

The structure of an atom was first modelled by Sir J.J. Thomson, an English Chemist. He proposed a model that described atoms in terms of their shells and electrons. In this model, the electrons orbit the nucleus in a circular path.

## Charge

The charge of an atom is determined by the number of electrons surrounding the nucleus. An atom is electrically neutral with an equal number of protons and electrons. An atom can also be negatively charged if it has only electrons. A neutral atom will have no charges at all.

One way to determine the charge of an atom is by using the periodic table. A single atom has a total charge of -1.6×10-19 Coulomb for its electrons and +1.6×10-19 for its protons. You can also calculate the total charge of an atom by dividing the number of protons by the total number of electrons.

The electrons in an atom are located in the electron cloud, which surrounds the nucleus. They are the smaller and lighter subatomic particles, and their mass is a fraction of the mass of the protons and neutrons. As they are less dense than protons, electrons tend to move around the outer part of an atom like a cloud. The electrons are labile and can transfer from one atom to another.

## Mass

Atomic mass is the sum of the weight of all the particles in an atom, including the protons and the neutrons. Atoms of the same element have the same mass but may have different mass numbers, a condition called isotopy. Electrons contribute only a small portion of the atom’s mass, but they play a critical role in chemical reactions. The mass of an atom can be calculated in different ways.

An atom’s mass is almost completely concentrated in its nucleus, which comprises protons and neutrons. An atom’s mass is measured in mass number, expressed as a superscript left of the symbol for the chemical element. It is possible to get this value by counting the nuclei of an atom and ignoring the electrons to reduce the error.

## Reactivity

The reactivity of an atom depends on its state of electrons. Atoms with a full shell of valence electrons are relatively chemically inert, whereas atoms with a partial shell tend to be more reactive. An atom with two valence electrons is considered stable, while an atom with one valence electron is unstable.

The reactivity of an atom is also affected by the atomic radius. The smaller the atomic radius, the higher the reactivity. A smaller atom’s radius requires more heat energy to undergo chemical reactions. And, as temperature increases, more energy is available for chemical reactions.

The reactivity of an atom is also determined by the number of electrons that are present in the outermost shell of an atom. The outermost shell contains valence electrons and is the first to come into contact with another atom when two atoms come into contact. Because of this, a chemical reaction will be triggered by the presence of electrons in this shell.

## Valency

The valency of an atom is the number of electrons in the outermost shell of an atom. The number of electrons is important because it determines how stable an atom is. The valency of hydrogen, lithium, and sodium atoms is one, which means they can lose an electron when combined. Other elements have two or more outermost electrons, such as magnesium and aluminium.

Valency is also important in predicting the formation of chemical compounds. An atom’s valency is important when mixing with other elements. When atoms have one valence electron, they lose that electron to the combining atom. But, if an atom has two or three valence electrons, it will pick up one electron from another atom. The resulting chemical compound has a valency of three(1 + 1,3), and a high valency indicates a high-strength element.