/bant/ - International Random

An atom is composed of protons and neutrons located in its nucleus, while electrons orbit around it in the electron cloud.

The more protons an atom has in its nucleus, the smaller its atomic radius (or the "size" of the electron cloud) will be, because the electrons are attracted more strongly.

Atoms in the same period (horizontal row) of the periodic table have a similar atomic radius. The radius increases as one moves down the periods.

Ionization energy is the amount of energy required to remove an electron from an atom.

The smaller the radius, the higher the ionization energy required to remove an electron; similarly, more energy is needed to remove electrons from inner shells.

In lighter atoms, the number of protons and neutrons tends to be equal, up to Calcium (Ca²⁰), which has 20 protons.

Above 20 protons, electrostatic repulsion (like charges repel each other) becomes very strong; therefore, more neutrons are needed to hold the protons together within the nucleus.

An atom's mass is the sum of the protons and neutrons in its nucleus; an isotope occurs when a specific atom has a different number of neutrons.

A molecule results from a chemical bond—a union of two or more atoms (either identical or different) held together by a covalent bond (involving two atoms with similar electronegativity, typically nonmetals).
Examples: O₂ (two oxygen atoms), H₂ (two hydrogen atoms), and CO₂ (one carbon atom and two oxygen atoms).

A compound results from the chemical combination of two or more different elements, regardless of the type of bond involved.

Metals have between 1 and 3 electrons in their valence shell, whereas nonmetals have between 5 and 7 electrons in their valence shell. The octet rule is a theory stating that certain atoms tend to achieve stability when they have 8—or 2 (a duet)—electrons in their valence shell; this is a characteristic property of noble gases (with the exception of helium, which has only a single electron in a single shell).
To achieve this, atoms can gain, lose, or share electrons, forming cations and anions through chemical bonds such as ionic and covalent bonds.
Atoms with 5, 6, or 7 electrons in their outermost shell tend to gain or share electrons (nonmetals such as fluorine and oxygen).
Atoms with 1, 2, or 3 electrons in their valence shell tend to donate those electrons (metals such as sodium, lithium, and magnesium).

Ionic bonding (electron transfer) occurs when a chemical bond forms between two atoms with differing levels of electronegativity—usually between a metal and a nonmetal.
Nonmetals have high electronegativity and thus tend to gain electrons, whereas metals have low electronegativity and tend to lose electrons.
The metal that loses electrons becomes a cation (positive ion), and the nonmetal that gains electrons becomes an anion (negative ion). Since opposite charges attract, they remain bonded together.
Example: Sodium chloride (NaCl). Sodium (Na), an alkali metal, donates electrons to chlorine (Cl), a nonmetal.
Ionic compounds conduct electricity when dissolved in water or in a molten state. They exist as solids at room temperature and exhibit high melting and boiling points.

Covalent bonding (electron sharing) occurs between atoms with high electronegativity (those tending to gain electrons); it generally takes place between nonmetals or between hydrogen and a nonmetal. Since both atoms have high electronegativity, neither tends to donate (lose) electrons; instead, they share electron pairs, with each pair belonging to both atoms simultaneously.
Ex: H2O – Oxygen (O) shares electrons with two hydrogen (H) atoms.
Covalent molecules can exist in all three physical states (liquid, solid, and gas) at room temperature. Generally, they do not conduct electricity, except when they are acids or in aqueous solutions.

Ionic compounds are all solid at room temperature and have high melting points; they are hard yet brittle, like grains of table salt (sodium chloride – NaCl).

The radius of a cation (+) is always smaller than that of an anion (-), because the cation loses electrons and its radius decreases, whereas the anion's radius increases.

Every element in Group 1 has one valence electron.