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Electron in s-orbital is closer to nucleus than electron in p-orbital and electron in p-orbital is closer to nucleus than electron in d-orbital and so on. So,more energy is requried to remove electron from s-orbital than electron in p-orbital in spite of both having same principal quantum number. Similarly, p orbital will require more energy than d-orbital. this is called penetrating effect. it decreases in order s>p>d>f>..