What trends are observed in chemical properties across a period?

Across a period in the periodic table, elements become less metallic, ionisation energy increases and electronegativity generally increases.

In the periodic table, a period refers to a horizontal row of elements. As you move from left to right across a period, the elements transition from metals to non-metals. This is due to the increase in the number of protons in the nucleus, which increases the positive charge and pulls the electrons closer to the nucleus, reducing the size of the atom. This makes the elements on the right side of the period less metallic as they are less likely to lose electrons and more likely to gain them.

Another trend observed across a period is the increase in ionisation energy. Ionisation energy is the energy required to remove an electron from an atom. As you move across a period, the number of protons in the nucleus increases, which means the positive charge of the nucleus increases. This stronger positive charge pulls the electrons closer to the nucleus, making it harder to remove an electron and thus increasing the ionisation energy.

Electronegativity also generally increases across a period. Electronegativity is a measure of the ability of an atom to attract a bonding pair of electrons. As the atomic radius decreases across a period, the nucleus is closer to the bonding pair of electrons and can attract them more strongly, increasing the atom's electronegativity. However, this trend is not always consistent, as the electronegativity of noble gases is often not considered due to their full electron shells and low reactivity.

In summary, as you move across a period in the periodic table, the elements become less metallic, the ionisation energy increases and the electronegativity generally increases. These trends are due to the increasing number of protons in the nucleus as you move from left to right across a period.