General Chemistry

Horizontal rows on the periodic table are known as periods. Each period corresponds to the highest energy level of the electrons in the atoms of the elements in that row. As we move from left to right across a period, the atomic number of the elements increases, which means that each successive element has one more proton and one more electron than the element before it. The classification and trends exhibited by elements in a period can be understood through the following steps: 1. **Period Number and Electron Shells**: The period number indicates the number of electron shells in the atoms of the elements in that period. For example, elements in the second period have electrons in two shells. 2. **Increasing Atomic Number**: As we move from left to right across a period, the atomic number increases by one for each subsequent element. This is represented by the formula: \[ \text{Atomic number of element (Z)} = \text{Number of protons} = \text{Number of electrons (in a neutral atom)} \] 3. **Valence Electrons**: The number of valence electrons (electrons in the outermost shell) increases as we move across a period. This increase continues until the outer shell is full. 4. **Element Types**: The elements at the beginning of a period are typically metals, and as we move across the period, we encounter metalloids and then nonmetals towards the end of the period. 5. **Atomic Radius**: The atomic radius generally decreases across a period. This is due to the increasing positive charge of the nucleus, which pulls the electrons closer, reducing the size of the atom. The trend can be represented as: \[ \text{Atomic radius} \propto \frac{1}{\text{Effective nuclear charge}} \] 6. **Ionization Energy**: The ionization energy, which is the energy required to remove an electron from an atom, generally increases across a period. This is because the electrons are held more tightly by the increasingly positive nucleus, making them harder to remove. The trend can be summarized by: \[ \text{Ionization energy} \propto \text{Effective nuclear charge} \] 7. **Electronegativity**: Electronegativity, which is a measure of an atom's ability to attract and hold onto electrons, typically increases across a period. This is due to the increasing number of protons attracting the electrons more strongly. The trend can be expressed as: \[ \text{Electronegativity} \propto \text{Effective nuclear charge} \] 8. **Metallic Character**: The metallic character of elements decreases across a period. Metals are typically found on the left side of a period, and as we move to the right, the elements become less metallic and more nonmetallic in character. 9. **Reactivity**: The reactivity of elements varies across a period and depends on whether the element is a metal or a nonmetal. Metals tend to be more reactive at the beginning of a period, while nonmetals are more reactive towards the end of a period. By understanding these trends and classifications, one can predict the chemical properties of elements based on their position in a period on the periodic table. This knowledge is fundamental in the field of chemistry and is essential for understanding the behavior of elements and their compounds.

Question

How do horizontal rows on the periodic table classify elements, and what trends do they exhibit?

Solution

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