Why does the core charge increase as one moves from left to right across a period in the periodic table - for example, from boron to carbon to oxygen?

The atomic structure is the foundation of understanding how elements behave and interact with each other. An atom consists of a nucleus containing positively charged protons and neutrally charged neutrons, surrounded by negatively charged electrons in orbital shells.

When we look at the periodic table, each element is represented by its atomic symbol and atomic number, which indicates the number of protons in its nucleus. For example, hydrogen, with an atomic number of 1, has one proton, while carbon, with an atomic number of 6, has six protons.

The arrangement of electrons around the nucleus happens in 'shells' or energy levels, and the distribution is governed by rules that dictate how many electrons can occupy a given level. In discussing the periodic table, particularly how the core charge increases across a period, understanding the balance of protons to electrons is crucial because it influences an atom's chemical properties and its interaction with other atoms.

The effective nuclear charge (often symbolized as Z_eff) is a way of understanding the net positive charge experienced by an electron in an atom's outermost shell. This charge is less than the actual charge of the nucleus because the electrons in the inner shells repel the outer electrons, effectively 'shielding' them from the full charge of the nucleus.

The concept is important when considering trends on the periodic table, such as the increase in core charge across a period. Since additional protons are being added to the nucleus as you go from left to right across a period, the force attracting electrons toward the nucleus increases. The increase in Z_eff means that in elements like oxygen, electrons are pulled closer to the nucleus compared to those in boron, which has fewer protons.

Electron shielding is the phenomenon where inner shell electrons act as a 'shield', reducing the effective nuclear charge that outer electrons experience. Each additional electron in a shell or subshell contributes to the shielding effect, lessening the pull from the nucleus on electrons that are further away.

However, across a period of the periodic table, electrons are being added to the same energy level, not to inner levels that could enhance shielding. This means that the increased number of protons in the nucleus has a more direct effect on the outer electrons, leading to increased core charge without significant additional shielding.

For instance, as one moves from boron to carbon and then to oxygen, the core charge felt by valence electrons increases, because even though each element gets one more electron, they do not contribute much to shielding. Thus, the additions primarily increase the effective nuclear charge.