Which of the following increases across a period (from left to right) in the periodic table?Atomic number and atomic radius First ionisation energy and atomic radius First ionisation energy and electronegativityElectronegativity and ionic radius

To answer the question, let's analyze each option step by step:

1. Atomic number and atomic radius: The atomic number refers to the number of protons in an atom's nucleus, while the atomic radius represents the size of the atom. As we move across a period from left to right in the periodic table, the atomic number increases, meaning there are more protons in the nucleus. However, the atomic radius generally decreases because the increased positive charge in the nucleus attracts the electrons more strongly, causing the electron cloud to be pulled closer to the nucleus.

2. First ionization energy and atomic radius: The first ionization energy is the energy required to remove the outermost electron from an atom, while the atomic radius represents the size of the atom. As we move across a period from left to right, the first ionization energy generally increases because the electrons are held more tightly due to the increased nuclear charge. At the same time, the atomic radius generally decreases for the same reasons mentioned earlier.

3. First ionization energy and electronegativity: The first ionization energy, as mentioned before, increases across a period from left to right. Electronegativity, on the other hand, is a measure of an atom's ability to attract electrons in a chemical bond. As we move across a period, electronegativity generally increases because the atoms have a stronger pull on the shared electrons due to the increased nuclear charge.

4. Electronegativity and ionic radius: Electronegativity, as mentioned earlier, increases across a period from left to right. Ionic radius, however, refers to the size of an ion. When an atom loses or gains electrons to form an ion, the size of the ion can change. Generally, as we move across a period, the ionic radius decreases for cations (positively charged ions) because electrons are lost and the remaining electrons are pulled closer to the nucleus. For anions (negatively charged ions), the ionic radius generally increases because electrons are gained and the electron cloud expands.

In conclusion, the option that increases across a period (from left to right) in the periodic table is the first ionization energy and electronegativity.