# Mendeleev Mendeleev developed his table while working on a textbook for his students. He needed a way to show the relationships among more than 60 elements.

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Mendeleev Mendeleev developed his table while working on a textbook for his students. He needed a way to show the relationships among more than 60 elements. He wrote the properties of each element on a separate note card. This approach allowed him to move the cards around until he found an organization that worked. The organization he chose was a periodic table. Elements in a periodic table are arranged into groups based on a set of repeating properties. Mendeleev arranged the elements in his periodic table in order of increasing atomic mass.

This is Mendeleev’s final product

Types of elements Metals Nonmetals Metalloids

These elements are in the upper-right corner of the periodic table. There is a greater variation in physical properties among nonmetals than among metals. Most nonmetals are gases at room temperature, including the main components of air—nitrogen and oxygen. A few are solids, such as sulfur and phosphorus. One nonmetal, bromine, is a dark-red liquid.

Classes Of Elements alkali metals alkaline earth metals Halogens noble gases transition metals inner transition metals

Periodic Trends Atomic Size Ionization Energy electro negativity

Atomic Size This size is expressed as an atomic radius. The atomic radius is one half of the distance between the nuclei of two atoms of the same element when the atoms are joined. The distances between atoms in a molecule are extremely small. So the atomic radius is often measured in picometers. there are one trillion, or 10 12, picometers in a meter. The distance between the nuclei in an iodine molecule is 280 pm. Because the atomic radius is one half the distance between the nuclei, a value of 140 pm (280/2) is assigned as the radius of the iodine atom. In general, atomic size increases from top to bottom within a group and decreases from left to right across a period.

Ionization energy electrons can move to higher energy levels when atoms absorb energy. Sometimes there is enough energy to overcome the attraction of the protons in the nucleus. The energy required to remove an electron from an atom is called ionization energy. The energy required to remove the first electron from an atom is called the first ionization energy. The cation produced has a 1+ charge. First ionization energy tends to decrease from top to bottom within a group and increase from left to right across a period. The second ionization energy is the energy required to remove an electron from an ion with a 1+ charge. The ion produced has a 2+ charge. The third ionization energy is the energy required to remove an electron from an ion with a 2+ charge. The ion produced has a 3+ charge.

ElectroNegativity Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound. Scientists use factors such as ionization energy to calculate values for Electronegativity. Electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right across a period. Metals at the far left of the periodic table have low values. By contrast, nonmetals at the far right (excluding noble gases) have high values.

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